NHC

Skill or Luck?: How NHC’s Hurricane Track Forecasts Beat the Models

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Skill or Luck?

There’s one thing that many of us are missing right now while we’re occupying ourselves at home:  sports.  We should have been all set for the playoffs in major league hockey and basketball, and we would be excited about the beginning of the major league baseball and soccer seasons.  We also would have been eagerly anticipating some of this spring and summer’s major sporting events, including the Olympics.  So let’s dream a little…

When we set out to write this blog post for Inside the Eye, we wanted to show how National Hurricane Center (NHC) forecasters use their skill and expertise to predict the future track of a hurricane.  And then it got us thinking, how does luck factor into the equation?  In other words, when meteorologists get a weather forecast right, how much of it is luck, and how much of it is forecasters’ skill in correctly interpreting, or even beating, the weather models available to them?

Investment strategist Michael Mauboussin created a “Skill-Luck Continuum” where individual sports, among other activities in life, are placed on a spectrum somewhere between pure skill and pure luck (Figure 1).   Based on factors such as the number of games in a season, number of players in action, and number of scoring opportunities in a game or match, athletes and their teams in some sports might have to rely on a little more luck than other sports to be successful.  On this spectrum, a sport like basketball would be closest to the skill side (there are a lot of scoring opportunities in a basketball game) whereas a sport like hockey would require a little more luck (there are fewer scoring opportunities in a hockey match, and sometimes you just need the puck to bounce your way).  Fortunately for hockey fans, there are enough games in a season for their favorite team’s “unlucky” games to not matter so much.

The Skill-Luck Continuum for Sports

Figure 1.  The Skill-Luck Continuum in Sports, developed by investment strategist Michael Mauboussin.

 

Where would hurricane forecasting lie on such a continuum?  There’s no doubt that luck plays at least some part in weather forecasting too, particularly in individual forecasts when random or unforeseen circumstances could either play in your favor (and make you look like the best forecaster around) or turn against you (and make you look like you don’t know what you’re doing!).  But luck is much less of a factor when you consider a lot of forecasts over longer periods of time, where the good and bad circumstances should cancel each other out and true skill shines through (just as in sports).  At NHC, we routinely compare our forecasts with weather models over these long periods of time to assess our skill at predicting, for example, the future tracks of hurricanes.

An International Friendly?

From our experience of talking to people about hurricanes and weather models, it seems to be almost common “knowledge” that only two models exist – the U.S. Global Forecast System (GFS) and the European Centre for Medium Range Weather Forecasts (ECMWF) model.  It’s true that those two models are used heavily at NHC and the National Weather Service in general, but there are many more weather models that can simulate a hurricane’s track and general weather across the globe.  (Here’s a comprehensive list showing all of the available weather models that are used at NHC today, if you’re interested:  https://www.nhc.noaa.gov/modelsummary.shtml.)  We’ve also heard and seen people compare the GFS and ECMWF models and talk about which model scenario might be more correct for a given storm.  This blog entry summarizes the performances of those models and discusses how, on the whole, NHC systematically outperforms them on predicting the track of a storm.

Below are the most recent three years of data (2017, 2018, and 2019) of Atlantic basin track forecast skill from NHC and the three best individual track models:  the GFS, ECMWF, and the United Kingdom Meteorological Office model (UKMET) (Figure 2).  Track forecast skill is assessed by comparing NHC’s and each model’s performance to that of a baseline, which in this case is a climatology and persistence model.  This model makes forecasts based on a combination of what past storms with similar characteristics–like location, intensity, forward speed, and the time of year–have done (the climatology part) and a continuation of what the current storm has been doing (the persistence part).  This model contains no information about the current state of the atmosphere and represents a “no-skill” level of accuracy.

Figure 2.  NHC and selected model track forecast skill for the Atlantic basin in 2017, 2018, and 2019.

 

On the skill diagrams above, lines for models or forecasts that are above other lines are considered to be the most skillful.  It can be seen that in each year shown, NHC (black line) outperforms the models and has the greatest skill at most, if not all, forecast times (the black line is above the other colored lines most of the time).  Among the models, the ECMWF (red line) has been the best performer, with the GFS (blue line) and UKMET (green line) trading spots for second place.

Yet another metric to estimate how often NHC outperforms the models is called “frequency of superior performance.”  Based on this metric, over the last 3 years (2017-19), NHC outperformed the GFS 65% of the time, the UKMET 59% of the time, and the ECMWF 56% of the time.   This means that more often than not, NHC is beating these individual models.  So the question is, how do the NHC forecasters beat the models?

Keep Your Eyes on the Ball

Forecasters at NHC are quite skilled at assessing weather models and their associated strengthens and weaknesses.  It is that experience and a methodology of using averages of model solutions (consensus) that typically help NHC perform best.  If you ever read a NHC forecast discussion and see statements like “the track forecast is near the consensus aids,” or “the track forecast is near the middle of the guidance envelope,” the forecaster believed that the best solution was to be near the average of the models.   Although this strategy often works, NHC occasionally abandons this method when something does not seem right in the model solutions.  One recent example of this was Tropical Storm Isaac in 2018.  The figure below (Figure 3) shows the available model guidance, denoted by different colors, at 2 PM EDT (1800 UTC) on September 9 for Isaac, with the red-brown line representing the model consensus (TVCA).

Figure 3. NHC forecast (dashed black line) and selected model tracks at 2 PM EDT (1800 UTC) September 9, 2018 for then-Tropical Storm Isaac.  The solid black line represents the actual track of Isaac and the red-brown line represents the model consensus.

 

Although the models were in fair agreement that the storm would head westward for some time, a few models diverged by the time Isaac was expected to be near the eastern Caribbean Islands, mostly because they disagreed on how fast Isaac would be moving at that time.  Instead of being near the middle of the guidance envelope, NHC placed the forecast on the southern side of the model suite (dashed black line) at the latter forecast times since the forecaster believed that the steering flow would continue to force Isaac westward into the central Caribbean.  Indeed, NHC was correct in this case, and in fact, for the entire storm, NHC had very low track errors.

In some cases all of the models turn out to be wrong, which usually causes the official forecast to suffer as well.  That was the case for a period during Dorian in 2019.  Figure 4 shows many of the available operational models at 8 PM EDT on August 26 (0000 UTC August 27) for then-Tropical Storm Dorian.  As you can see by noting the deviation of the colored lines from the solid black line (Dorian’s actual track), none of the models or the official forecast (colored lines) anticipated that Dorian would turn as sharply as it did over the northeastern Caribbean Sea, and no model showed a direct impact to the Virgin Islands, where Dorian made landfall as a hurricane.

 

Figure 4. NHC forecast (dashed black line) and selected model tracks at 8 PM EDT on August 26 (0000 UTC 27 August), 2019 for then-Tropical Storm Dorian.  The solid black line represents the actual track of Dorian.

 

Figure 5 shows many of the operational models at 2 AM EDT (0600 UTC) on August 30 when Dorian, a major hurricane at the time, was approaching the Bahamas.  You can see that all of the models showed Dorian making landfall in south or central Florida in about four days from the time of the model runs, and none of them captured the catastrophic two-day stall that occurred over Great Abaco and Grand Bahama Islands.  NHC’s forecast followed the consensus of the models in this case and thus did not initially anticipate Dorian’s long, drawn-out battering of the northwestern Bahamas.

Figure 5.  NHC forecast (dashed black line) and selected model tracks at 2 AM EDT (0600 UTC) on August 30, 2019 for Hurricane Dorian.  The sold black line represents the actual track of Dorian.

 

The Undervalued Player?  A Consistently Good Field-Goal Kicker

In American football, probably one of the most undervalued players on the field is the kicker.  They don’t see much action during the majority of the game.  But at the end of close games, who has the best chance to win the game for a team?  A dependably accurate field goal kicker.  In that vein, it’s not just accuracy that can make NHC’s forecasts “better” than the individual models.  Another important factor is how consistent NHC’s predictions are from forecast to forecast compared to those from the models.  We looked at consistency by comparing the average difference in the forecast storm locations between predictions that were made 12 hours apart. For example, by how much did the 96-hour storm position in the current forecast change from the 108-hour position in the forecast that was made 12 hours ago (which was interpolated between the 96- and 120-hour forecast positions)?  Figure 6 shows this 4-day “consistency,” as well as the 4-day error, plotted together for the GFS, ECMWF, UKMET, and NHC forecasts for the Atlantic basin from 2017-19.  It can be seen that NHC is not only more accurate than these models (it’s farthest down on the y-axis), but it is also more consistent (it’s farthest to the left on the x-axis), meaning the official forecast holds steady more than the models do from cycle to cycle.  We like to say that we’re avoiding the model run-to-run “windshield wiper” effect (large shifts in forecast track to the left or right) or “trombone” effect (tracks that speed up or slow down) that are often displayed by even the most accurate models.

Figure 6.  96-hour NHC and model forecast error and consistency for 2017-2019 in the Atlantic basin (change from cycle to cycle).

 

NHC’s emphasis on consistency is so great that there are times when we knowingly accept that we might be sacrificing a little track accuracy to achieve consistency and a better public response to the threat.  An example would be for a hurricane that is forecast to move westward and pose a serious threat to the U.S. southeastern states.  Sometimes, such storms “recurve” to the north and then the northeast and move back out to sea before reaching the coast.  When the models trend toward such a recurvature, the NHC’s forecast will sometimes lag the models’ forecast of a lower threat to land.  In these cases, NHC does not want to prematurely take the southeastern states “off the hook”, sending a potentially erroneous signal that the risk of impacts on land has diminished, only to have later forecasts ratchet the threat back up after the public has turned their attention and energies elsewhere if the models, well, “change their mind”.  That would be the kind of windshield wiper effect NHC wants to prevent in its own forecasts.  Now, there are times where the recurvature does indeed occur.  Then, NHC’s track forecasts, which have hung back a little from the models, could end up having larger errors than the models.  But, NHC can accept having somewhat larger track forecast errors than the models in such circumstances at longer lead times if in doing so it can provide those at risk with a more effective message–achieved in part through consistency.

The superior accuracy and higher levels of consistency of the NHC forecasts are both important characteristics since emergency managers and other decision makers have to make challenging decisions, such as evacuation orders, based on that information.  It is not surprising to us that NHC’s forecasts are more consistent than the global models, since forecasters here take a conservative approach and usually make gradual changes from the forecast they inherited from the previous forecaster.  Conversely, the models often bounce around more and are not constrained by their previous prediction.  And, unlike human forecasters, the models also bear no responsibility or feel remorse when they are wrong!

Filling Out Your Bracket

Accuracy, consistency, and luck are important factors in one particularly favorite sport:  college basketball.  We just passed the time of year when we should have been crowning champions in the men’s and women’s college basketball tournaments.  But before those tournaments would have kicked off, “bracketologists” (no known relation to meteorologists!) would have made predictions on which teams would make it into the tournaments and which teams would have been likely to win.

Think of it this way:  a team can be accurate in that they have a spectacular winning record during the regular season, but does that mean they are guaranteed to win the tournament, or even advance far?  Nope.  As is often said, that’s why they play the game.  An inconsistent team—one whose performance varies wildly from game to game—has a higher risk of having a bad game and losing to an underdog in the first few rounds, even if their regular season record by itself suggests they should have no problem winning.  The problem is, they could have been very lucky in the regular season, winning a lot of close games that could have easily swung the other way.  If that luck runs out, the inconsistent team could have an early exit from the tournament.  With a consistent team, on the other hand, you pretty much know what kind of performance you’re going to get—good or bad—and that increases confidence in knowing how far in the tournament the team would advance.  You’d want to hitch your wagon to a good team that is consistent and hasn’t had to rely on too much luck to get where they are.

The same can be said for hurricane forecasts from NHC and the models.  NHC’s track forecasts are more accurate and more consistent than the individual models in the long run, and that fact should increase overall user confidence in the forecasts put out by NHC.  Even still, there is always room to improve, and it is hoped that forecasts will continue to become more accurate and consistent in the future.  It is always a good idea to read the NHC Forecast Discussion to understand the reasons behind the forecast and to gauge the forecaster’s confidence in the prediction.  For more information on NHC forecast and model verification, click the following link:  https://www.nhc.noaa.gov/verification/

— John Cangialosi, Robbie Berg, and Andrew Penny

National Hurricane Center Decision Support Services for the United States Coast Guard

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United States Coast Guard Cutter (image courtesy of uscg.mil)

Semper Paratus (Always Ready): A Shared Mission of Watching Over a Vast Blue Ocean

The National Hurricane Center (NHC) has the responsibility for issuing weather forecasts and warnings for a wide expanse of the Atlantic and eastern North Pacific Oceans.  Within NHC, the Hurricane Specialist Unit (HSU) issues forecasts for tropical storms and hurricanes in these regions, issues associated U. S. watches and warnings, and provides guidance for the issuance of watches and warnings for international land areas.  NHC’s Tropical Analysis and Forecast Branch (TAFB) makes forecasts of wind speeds and wave heights and issues wind warnings year-round for the eastern North Pacific Ocean north of the equator to 30°N, and for the Atlantic Ocean north of the equator to 31°N and west of 35°W (including the Gulf of Mexico and Caribbean Sea).  These wind warnings include tropical storms and hurricanes as well as winter storms, tradewind gales, and severe gap-wind events (for example, the “Tehuantepecers” south of Mexico).

The United States Coast Guard (USCG) has areas of responsibility (AORs) that extend well beyond those of NHC, with potential weather hazards affecting the fleet and their missions over the ocean, inland U.S. waterways, and flood-prone U.S. land areas. Although the USCG is responsible for search and rescue missions that may occur due to weather hazards, they are also vulnerable to severe weather and must also protect their own fleet and crews from these hazards.

USCG Search and Rescue Regions (SRRs) cover vast ocean areas affected by tropical cyclones. Superimposed on the Pacific SRRs is the NHC tropical cyclone area of responsibility, which overlaps with two eastern Pacific USCG SRRs as well as all Atlantic SRRs. The number of briefings provided by NHC to each USCG district in 2018 are shown. (Map images courtesy of uscg.mil)

One of the USCG’s oldest missions and highest priorities is to render aid to save lives and property in the maritime environment.  To meet these goals, the United States’ area of search-and-rescue responsibility is divided into internationally recognized inland and maritime regions.  There are five Atlantic USCG Search and Rescue Regions (SRRs) (Boston, Norfolk, Miami, New Orleans, and San Juan) and two Pacific USCG SRRs (Alameda and Honolulu) that overlap with NHC’s hurricane and marine areas of responsibility. The other eastern Pacific regions north of the Alameda SRR do not typically, if ever, experience hurricane activity. The multi-million square mile area of the agencies’ overlap allows NHC to provide weather hazard Decision Support Services (DSS) for the USCG.

Building Partnerships with the Districts

The National Weather Service (NWS) signed a Memorandum of Agreement (MOA) with the USCG to provide them with weather support. Over the past couple of years, staff at NHC have had numerous discussions with several of the USCG districts in order to build stronger partnerships. These discussions, primarily involving how NHC can better serve the USCG, established criteria for requiring TAFB to provide weather briefings to key decision makers within the USCG. When criteria are met, TAFB provides the relevant USCG District with once- or twice-a-day briefing packages detailing the weather impacts on their area of responsibility. This information provides the USCG districts with the details necessary to make efficient and effective decisions about potential mobilization of their fleet.

Example of a briefing slide of NHC’s earliest reasonable arrival time of tropical-storm-force winds graphic, which is one of the USCG’s most-desired decision support tools provided by NHC.  This example, from Hurricane Michael, illustrates the timing of the earliest reasonable onset of tropical-storm-force winds at a given location. This information is critical for fleet mobilization, as once these winds arrive preparations become difficult, if not impossible, to complete.

2018 Hurricane Season Briefing Support

During the 2018 hurricane season, TAFB provided 30 briefings to USCG Districts 5 (Norfolk), 7 (Miami), 8 (New Orleans), and 11 (Alameda) for the several tropical storms and hurricanes that affected them. These interactions helped to build the relationships between NHC and the USCG districts and aided the districts in making decisions regarding fleet mobilization, conducting search and rescue missions, and preparation for USCG’s land-based assets and personnel. Some of these briefings occurred during rapidly evolving high impact scenarios, including Hurricane Michael. Michael was forecast to become a hurricane within 72 hours of developing into a tropical depression and was forecast to make landfall within 96 hours of its formation. Ultimately, Michael rapidly intensified into a category 5 hurricane only 3½ days after formation, before making landfall on the Florida Panhandle. Hurricane Michael’s track across the east-central Gulf of Mexico straddled the border of USCG Districts 7 (Miami) and 8 (New Orleans), leading to both Districts taking action in advance of the hurricane.

Support for District 5 (Norfolk)

The NWS’s Ocean Prediction Center, the NHC (through TAFB), and the NWS National Operations Center have worked together to provide weekly high-level coordination briefings to USCG District 5 on upcoming hazards focused on the Atlantic Ocean north of 31°N over the following seven days.  Each Monday (except Tuesday if Monday is a holiday) by noon Eastern Time, the NWS provides a briefing that covers the mid-Atlantic region from New Jersey through North Carolina.  Typically, the briefing covers the area to roughly 65°W, though the exact area covered can vary based on the week’s expected weather hazards.  The USCG, in turn, has been sharing the information with mariners, port partners, and industry groups for situational awareness and critical decision-making.

Future Support

NHC’s TAFB is ready to provide decision support services to the USCG Districts for the 2019 hurricane season. Plans are being developed to continue this type of support for many years to come.

— Andy Latto

It’s Just a Matter of Time

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The hurrier I go, the behinder I get.

— White Rabbit, Alice in Wonderland (Lewis Carroll)


How did it get so late so soon?

— Dr. Seuss

 

 

 

Pressed for Time

Have you ever procrastinated on something for so long that the stress you feel from hurrying at the last minute makes you feel like you’re more and more behind?  We have to factor time into a lot of decisions we make in life, but like the White Rabbit and Dr. Seuss, we often feel as though we didn’t leave ourselves enough time to get everything done before a deadline.

Traffic on Interstate 5 on a rainy day in Seattle, Washington (Wonderlane, flickr)

Imagine this scenario:  you wake up at 7 AM on a Monday morning, and you have a critically important meeting you MUST attend at the office at 9 AM.  If you miss the meeting, it could hurt your career opportunities.  You open your window blinds, and you see it’s raining cats and dogs outside.  On a good day, it takes you 30 minutes to drive from home to work.  “Shoot,” you think to yourself.  “I probably should leave a little early to give myself enough time in case traffic is bad.”  You hurriedly shower, get dressed, eat some breakfast, and arrange for your significant other to get the kids to school on time.  Rushing out the door with your coffee at around 8:15 AM, you pull up your favorite navigation app on your smartphone, select your office as your destination, and dread consumes you.  Estimated arrival:  9:15 AM.  The rain has caused so many accidents and traffic jams on your route to work that it’s going to take at least an hour to get there.  There’s no way you’re going to make it there before the meeting starts.

Now imagine another scenario:  you wake up at 8 AM on a beautiful Saturday morning, and you and your family decide that you’re going to spend the day at the beach.  In no real rush, you get up, eat some breakfast, pack some food and drinks, and gather your towels, chairs, and beach umbrella.  The kids are in even less of a hurry, but you finally get them all in the car.  It normally takes you 30 minutes to drive from home to the beach, although you figure that the nice weather will probably mean a lot of other people will have the same idea today, and traffic could be a little heavy.  You pull up your navigation app on your smartphone, select your favorite beach, and you’re suddenly a bit annoyed.  It’s going to take almost 45 minutes to get there.  “Oh well,” you think.  “We’re in no particular hurry, and the beach will still be there when we get there.”  Forty-five minutes later, you’ve arrived at the beach, you plop yourself on the sand, and time melts away.

As you can imagine, you’d probably approach these two scenarios very differently, and you’d probably have vastly different emotional reactions to the things that make travel time uncertain.  The question is how risky are you willing to be while planning when to leave.  How bad will it be if you don’t get to your destination at the time you want to get there?  Fortunately, navigation programs and apps allow us to account for time uncertainty depending on our tolerance for risk.

Let’s say I will be driving from Miami to Orlando, and my goal is to arrive at 3 PM.  When I get directions from my navigation app, the program allows me to select an “Arrive by” time, telling me it will take between 3 hours 10 minutes and 4 hours to get there by 3 PM.  The program is trying to account for the typical drive time and the uncertainties (like traffic or road construction) that could make that time longer.  So now I have to decide how much risk I want to take on.  If I have a high risk tolerance (it’s not the end of the world if I don’t arrive exactly by 3 PM), then I’ll probably decide to go with the low end of the time range (3 hours 10 minutes) and leave at 11:50 AM.  If, on the other hand, I have a low tolerance of risk and must be in Orlando by 3 PM, then I’ll probably give myself the full 4 hours and leave Miami at 11 AM.  If traffic on Florida’s Turnpike turns out to be light (ha!) and I get there early, no harm, no foul.  I’ve avoided undue stress and may have even left myself some time to grab a coffee before my 3 PM appointment.

Driving directions and estimated driving times from Miami to Orlando, Florida, according to a popular online navigation program.

When Will the Winds Start?

Things aren’t much different when it comes to the arrival or onset of winds associated with a hurricane or tropical storm.  When we make a forecast for a hurricane’s future track and size, we can derive a time at which tropical-storm-force winds would begin in a city, based on that specific forecast.  We call that a deterministic approach because it in no way accounts for uncertainty in the hurricane’s future track or size.  (We attacked the issue of deterministic forecasts in a previous blog post about storm surge forecasting).  It’s like assuming we won’t hit any extra traffic that will slow us down when driving from Point A to Point B.  But what if the storm moves faster than we’re forecasting?  Then the winds will arrive in the city sooner.  What if the storm gets bigger than we’re forecasting?  That, too, will cause the winds to begin in the city earlier than forecast.

During the 2018 hurricane season, we here at the National Hurricane Center in Miami, and fellow forecasters at the Central Pacific Hurricane Center in Honolulu, began producing new products called the “Arrival of Tropical-Storm-Force Winds” graphics for every tropical cyclone within our areas of responsibility.  These graphics serve as a sort of “navigation app,” giving you the times that tropical-storm-force winds are most likely to begin at different locations based on the latest official forecast, as well as “earliest reasonable” times that the winds could begin if the storm speeds up or grows in size.  The times provided by these graphics can help you decide when your preparations for a storm should be complete according to how much risk you’re willing to take that you won’t have them quite done in time.  If you have no tolerance for risk and must be completely prepared before the winds start, then you’d go with the “earliest reasonable” time.  If you have some wiggle room and can afford not to have everything done before the winds start, then maybe you’d be OK going with the “most likely” time.

Let’s look at an example from Hurricane Michael from 2018 to show how these graphics can be useful.  Here’s the first forecast issued by NHC for Potential Tropical Cyclone Fourteen at 4 PM CDT October 6, when the pre-Michael disturbance was located over the northwestern Caribbean Sea.

NHC Forecast Cone for Potential Tropical Cyclone Fourteen (Pre-Michael) Advisory 1 at 4 PM CDT, Saturday, October 6, 2018.

This first official forecast showed the center of eventual-Michael reaching the Florida Panhandle around 1 PM CDT on Wednesday.  But obviously the outer wind field from the storm would reach the coast before that time—you just can’t deduce when that will occur from this particular graphic.  Here’s what each of the “Arrival of Tropical-Storm-Force Winds” graphics showed for that particular forecast:

Most Likely Arrival Time graphic for Advisory 1 of Potential Tropical Cyclone Fourteen in 2018.
Earliest Reasonable Arrival Time graphic of Advisory 1 for Potential Tropical Cyclone Fourteen in 2018.

 

On the left, the Most Likely graphic shows that tropical-storm-force winds were most likely to have begun at locations along the Florida Panhandle between 8 pm Tuesday and 8 am CDT Wednesday, which would have given people about 3 to 3 ½ days to get ready.  On the other hand, the Earliest Reasonable graphic on the right shows that tropical-storm-force winds could have begun at locations along the Florida Panhandle coast as early as 8 am CDT Tuesday, lessening the preparation time to about 2 ½ days.  (Editor’s note:  You’ll note that I’ve used bold red and black coloring of the text in these scenarios to match the bold red and black titles of the two versions of the graphics above).  Not only would these times help people decide when to have their preparations done, but they also help emergency managers decide when to call evacuations, based on how much time it would take to get people out of areas vulnerable to storm surge before tropical-storm-force winds begin.

So when did sustained tropical-storm-force winds actually arrive on the coast of the Florida Panhandle?  According to the Surface Wind Field graphic, they began roughly around 4 am CDT Wednesday, which falls within the “Most Likely” range discussed above.  In the case of Michael, the track forecast turned out to be very good, and the Most Likely Arrival Time product provided an accurate onset time of tropical-storm-force winds.

Surface wind field for Hurricane Michael, Advisory 15, at 4 AM CDT, Wednesday, October 10, 2018.

Not all track forecasts are this accurate, however.  Consider Hurricane Nate, which made landfall along the Gulf Coast about a year earlier in 2017.  The first official forecast issued by NHC for Tropical Depression Sixteen at 11 am EDT Wednesday, October 4 showed the center of eventual-Nate reaching the Gulf Coast Sunday morning (see below).  The corresponding arrival time graphics showed tropical-storm-force winds most likely to begin overnight Saturday, but they could have begun as early as during the day Saturday.

NHC Forecast Cone for Tropical Depression Sixteen (Pre-Nate) Advisory 1 at 11 AM EDT, Wednesday, October 4, 2017.
Most Likely Arrival Time graphic for Advisory 1 of Tropical Depression Sixteen in 2017.
Earliest Reasonable Arrival Time graphic of Advisory 1 for Tropical Depression Sixteen in 2017.

Nate moved faster across the Gulf of Mexico and a little farther west than was originally forecast, and its tropical-storm-force winds first reached the coast during the day on Saturday.  For this particular storm, the times indicated on the Earliest Reasonable graphic (right) ended up being closer to the times when tropical-storm-force winds began in southeastern Louisiana.

Surface wind field for Hurricane Nate, Advisory 14, at 4 PM CDT, Saturday, October 7, 2017.

The problem is that we can never nail arrival times exactly because we can’t know beforehand if a storm will follow the official forecast or deviate in some way that affects when winds will first reach the coast.  That’s why it’s probably prudent to consult both versions of the product and consider what types of decisions you must make before a storm arrives.  But if you want to be sure that you’ll be prepared before the winds start, it’s advisable to go with the “earliest reasonable” version of the graphic.

There’s one caveat to think about:  just because a location is covered by times in the graphics, it doesn’t mean that tropical-storm-force winds will definitely occur at that site.  NHC also provides versions of the graphics that show the arrival times overlaid on top of the overall probability of a location receiving sustained tropical-storm-force winds during the next 5 days.  So, in reality, the arrival times should be thought of as conditional.  They are the possible times that tropical-storm-force winds could begin, assuming that tropical-storm-force winds occur at all.  As an example, look at the Most Likely Arrival Time graphic issued for Hurricane Florence, Advisory 50, at 5 pm Atlantic Standard Time (AST), Tuesday, September 11.  This graphic shows that locations along the southern coast of North Carolina have a near certainty (>90% chance as indicated by the purple shading) of receiving sustained tropical-storm-force winds, which would most likely begin Thursday morning.  Farther north, locations along the coast of Delaware only had a 20-30% chance (as indicated by green shading) of sustained tropical-storm-force winds, but if they happened to occur, they would most likely begin Friday morning.

Most Likely Arrival Time graphic for Advisory 50 of Hurricane Florence issued at 5 PM AST, Tuesday, September 11, 2018. This version of the graphic also includes the cumulative 5-day probability of locations receiving sustained tropical-storm-force winds (colors).

With that, the time has probably arrived to end this particular blog post.  Some may have wanted it to end earlier, which is reasonable, but most likely you are craving more information.  In a second blog post, we’ll cover how the arrival times are derived from the official forecast, how the earliest reasonable and most likely times are calculated, and some of the social science research that went into developing the graphics.  Stay tuned!

— Robbie Berg

 

 

The State of Hurricane Forecasting

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State of the Union given to Congress on January 20, 2015. Image courtesy of NASA.
State of the Union given to Congress on January 20, 2015. Image courtesy of NASA.

The State of Hurricane Forecasting is . . .

The National Hurricane Center (NHC) has the responsibility for issuing advisories and U.S. watches/warnings for tropical cyclones (TCs), which includes tropical depressions, tropical storms, and hurricanes, for the Atlantic and east Pacific basins.   NHC has a long history of issuing advisories for TCs, with the first known recorded forecast being in 1954, when 24-hour predictions of a TC’s track were made.  Since then, we’ve expanded our forecasts out in time and added predictions of TC intensity, size, and associated hazards, such as wind, storm surge, and rainfall.  In addition, the lead times of tropical storm and hurricane watches and warnings have increased to give the public additional time to prepare for these potentially devastating events.  Since we’re at the time of year when the U.S. President and state governors have just given their “State of the Union” or “State of the State” speeches, we thought this might be a good time to give our own “State of Hurricane Forecasting” speech.  This blog entry takes a look at the accuracy of NHC’s forecasts and quantifies how much more accurate they are today compared to decades ago.

Track Forecasting (a.k.a., Where the Storm Will Go)

We are usually more confident in predicting the path of TCs as compared to predicting the strength or size of a TC.  The primary reason for this is because the track of a TC is governed by forces larger than the tropical system itself, since the surrounding steering currents cover a much larger area than the hurricane.  Because these nearby weather patterns are big, we can usually “see” them easily, and the global weather models do a fairly good job in predicting how these steering features might evolve over the course of a few days.

The figure below shows the average NHC track forecast errors for tropical storms and hurricanes by decade beginning in the 1960s.  You can see that there has been a steady reduction in the track errors over time, with the average errors in the current decade about 30-40% smaller than they were in the 2000s and about half of the size (or even smaller) than they were in the 1990s.

NHC official track errors (in nautical miles) for Atlantic tropical storms and hurricanes by decade.

If that doesn’t seem impressive, let’s look at another example.  The next graphic shows two circles centered on a point near Pensacola, Florida, with the blue one representing the average 48-hour track error in 1990 and the red one showing the average 48-hour error today.  What it shows is that if NHC had made a forecast for a storm to be over Pensacola in 48 hours back in 1990, the TC would have ended up, on average, not exactly over Pensacola but somewhere on the blue circle.  If NHC makes the same forecast today, now the storm ends up, on average, somewhere on the red circle.  You can easily see that the NHC forecasts for the path of a TC today are much more accurate, on average, than they were decades ago, and these more accurate forecasts have helped narrow the warning areas, save lives, and make for more efficient and less costly evacuations.

A representation of the average NHC 48-hour track errors for the forecast of a storm centered over Pensacola, Florida, in 1990 (blue) and today (red).

So, you might be wondering why the track forecasts are more accurate today than in the past.  Well, the primary reason is the advancements in technology, specifically the improvements in the observing platforms (satellites, for example) and the various modeling systems we use to make forecasts.  The amount and quality of data available to the models so they can paint an initial picture of the atmosphere have increased dramatically in the last 20 to 30 years.  Also, the resolution and physics in the models we use today are far superior to what forecasters had available in the 1990s or prior decades, in part due to the tremendous improvements in computational capabilities.  In addition, NHC has found ways to even beat the individual dynamical models by using a balance of statistical approaches and experience.

We often hear a lot of questions asking which model is the best one.  Although some models are usually better than others, no model is perfect, and their performance varies from season to season and from storm to storm.  Two of the most well-known models for weather forecasting are the U.S. National Weather Service’s Global Forecast System (GFS) and the European Centre for Medium-Range Weather Forecasts (ECMWF).  The figure below shows a comparison of the NHC forecasts (OFCL, black) and forecasts from the GFS (GFSI, blue) and ECMWF (EMXI, red) models for Hurricanes Harvey, Irma, Maria, and Nate in 2017.  In all of these cases, except for Hurricane Irma, OFCL performed as well as or better than GFSI and EMXI.  Among the two models, EMXI beat GFSI for Harvey, Irma, and Nate, but GFSI beat EMXI for Maria.

72-hour track errors from the NHC official forecast (black), the Global Forecast System model (GFSI, blue), and the European Centre for Medium-Range Weather Forecasting model (EMXI, red) for Hurricanes Harvey, Irma, Maria, and Nate.

Over the past decade, the average track errors of GFSI and EMXI models have been quite close, so even though EMXI was the best-performing model most of the time in 2017, it does not mean that it will always be the best for every storm.  The models that typically have the lowest errors are consensus aids, which blend several models together.  Forecasters construct their own forecasts of how the storm will evolve, aided by model simulations and their knowledge of model strengthens and weaknesses.

Even though our track forecasts are much more accurate today – in fact preliminary estimates are that the 2017 Atlantic track forecasts set record low errors at all time periods – typical track errors currently start off at 37  n mi at 24 hours and then increase by about 35 n mi (40 mi ) per day of the forecast.  This means that our 5-day track error is on average around 180 n mi (210 mi).   So, keep that in mind and be sure to account for forecast uncertainty when using NHC forecasts next hurricane season.

Intensity Forecasting (a.k.a., How Strong the Storm Will Get)

Predicting the intensity of a tropical storm or hurricane is usually more challenging than forecasting its track.  This is because the intensity of these weather systems is affected by factors that are both big and small.  On the large scale, vertical wind shear (the change of wind speed and direction with height) and the amount of moisture in the atmosphere greatly affect the amount or organization of the thunderstorm activity that the TC can produce.  Ocean temperatures also affect the system’s intensity, with temperatures below 80° F usually being too cool to sustain significant thunderstorm activity.  However, smaller-scale features can also be at play.  One of the more complex phenomena that affects a TC’s intensity is an eyewall replacement cycle.   Initially, when two eyewalls, one inside the other, are present, the hurricane’s wind field will begin to expand, and as the inner eyewall dies, the hurricane’s peak winds start to weaken.  However, if the second eyewall contracts, the hurricane can often re-intensify.  The radar image below of Hurricane Irma (2017) was taken at the beginning of an eyewall replacement cycle, when the hurricane had a double eyewall structure.

National Weather Service WSR-88D Doppler radar from San Juan, Puerto Rico, at 5:15 pm EDT September 6, 2017, showing the double eyewall structure of Hurricane Irma, before the occurrence of an eyewall replacement.

Given these complex factors and the fact that errors in the track can also affect the TC’s future intensity, we have not made as much progress in this area as we have for track forecasting.  The next graphic (below) shows NHC average intensity errors for Atlantic tropical storms and hurricanes by decade starting in the 1970s.  Note that only small improvements were made in the intensity predictions from the 1970s through the 2000s.  A much more significant reduction in error has occurred in the current decade, which could mean that the recent investment in new models and techniques is beginning to pay off.  Today’s intensity errors are close to 15 kt (17 mph) from 72 to 120 h.  This number is on the order of one Saffir-Simpson category, so we often encourage those who could be affected by a TC to prepare for a storm one category stronger (on the Saffir-Simpson Hurricane Wind Scale) than what we are forecasting.

NHC official intensity errors (in knots) for Atlantic tropical storms and hurricanes by decade.

Although the GFS and ECMWF models are skillful for track forecasting and help us understand the environment around the TC, did you know that these models are typically inadequate to predict how strong a TC might become?  Both the GFS and ECMWF are global models, and they cannot “see” sufficient detail within the storm to represent and predict the core winds in the hurricane’s eyewall.  Therefore, we use different models to predict intensity, some that are run at high resolution specifically for TCs (e.g., Hurricane Weather Research and Forecasting [HWRF] model, Hurricanes in a Multi-scale Ocean-coupled Non-hydrostatic [HMON] model) and some that are statistical in nature (e.g., Statistical Hurricane Intensity Prediction Scheme [SHIPS], Logistic Growth Equation Model [LGEM]).  The statistical models tell the forecaster what typically occurs for a TC in a specific location and environment based on past storm behavior.   Even though the intensity models are improving, the gains in these models are much smaller than what has occurred in the models we use for track forecasting.

If you want more information on the models, please visit the following page for details:  http://www.nhc.noaa.gov/modelsummary.shtml

Will the errors keep decreasing?

The short answer is they likely won’t forever.  At some point the forecasts made by NHC and other forecasting centers will likely reach the limits of predictability.  No one knows for sure what those limits are or when they will be reached, but researchers are still providing great information that is helping NHC make steady advancements as discussed above.

For more information on the NHC and model verification please visit the following page: http://www.nhc.noaa.gov/verification/

— John Cangialosi

Education: The Best Hurricane Preparedness Tool

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John Cangialosi of NHC and Larry Kelly of the Miami-South Florida Weather Forecast Office discussing their operations during an open house in April 2017.

It’s been an extremely busy hurricane season, and even though there are still two months left, we’re already starting to get ready for the hurricane “off-season.”  It’s no surprise that the National Hurricane Center (NHC) spends the hurricane season issuing forecasts, watches, and warnings for tropical cyclones to protect lives and property, but people often wonder what we do when it’s not hurricane season.  An earlier blog entry discussed some of the main activities during the “off-season”.  However, one item that was not discussed was NHC’s interaction with students and the general public.  To engage and educate students and the general public, NHC organizes and is involved with numerous outreach events.

Local Events

One of the largest public turnouts was in April 2017 for an open house at NHC and the Miami NWS Weather Forecast Office (WFO).  Over 1,000 people showed up to learn more about NHC and WFO operations and how to be hurricane ready.   Similar experiences were provided during several office tours that were open to the public and schools outside of hurricane season. NHC forecasters also make efforts to interact with students and teachers at career days at schools in South Florida, and participate in outreach events at local museums, boat shows, and colleges to help get the message out about weather hazards and preparedness.    For a virtual tour of NHC’s operations, visit http://www.nhc.noaa.gov/nhctour.shtml.

Road Trips

Since many areas in the United States and internationally are affected by hurricanes, the NHC takes their outreach efforts on the road to reach more hurricane vulnerable locations. NHC plays a critical role in two annual Hurricane Awareness Tours (one in the Caribbean/Mexico and one in the United States/Canada).    The Hurricane Awareness Tours are an opportunity for NOAA and its partner agencies to visit locations that are vulnerable to tropical storms and hurricanes.  At each location along the tour, the public can get an up-close look at the NOAA and U.S. Air Force Reserve Hurricane Hunter aircraft and meet some of the crew that fly into hurricanes.  In addition, NHC and other partners discuss the importance of having a personal hurricane plan at each stop of the tour, including knowing whether or not you live in an evacuation zone.  The events provide an opportunity for NHC to spread the hurricane preparedness message through local media and emergency managers, with the main goal being to increase public awareness of hurricane threats and ensure that communities and families in hurricane prone areas are better prepared to face the next storm.  Over 13,000 people showed up for the 2017 U.S./Canada Hurricane Awareness Tour, the most successful turn out in its history.

Line of people waiting to tour the Air Force C130-J aircraft at the Raleigh, North Carolina, stop of the 2017 Hurricane Awareness Tour.
Hurricane Awareness Tour stops from 2005-2017
Caribbean Hurricane Awareness Tour stops from 2005-2017

Virtual World

To take advantage of today’s high-tech society, NHC has moved a portion of our outreach efforts into the virtual world.  To reach students, NHC has teamed up with the University of Rhode Island and NOAA’s Aircraft Operations Center to conduct educational webinars for 4th, 5th, and 6th graders.  These webinars provided an overview of hurricane history and hazards, and we quizzed students on their hurricane knowledge.  The webinars also featured videos of the Hurricane Hunters and allowed students to directly ask questions.  During the past five years, the webinars have reached more than 40,000 students from around the country and beyond.  For more information please visit http://www.hurricanescience.org/resources/nhcwebinar/.

During the past few years, NHC has also partnered with NOAA’s Southeast and Caribbean Regional Collaboration Team to offer a series of webinars that are intended to improve the understanding of NHC and local NWS Weather Forecast Office tropical-cyclone-related products and services.  These webinars are geared toward the general public, emergency managers, and media partners.   Recordings of these webinars can be found at http://www.regions.noaa.gov/secar/index.php/highlights/noaas-2017-hurricane-season-awareness-webinars/.

Former NHC Director Rick Knabb participating in the live broadcast of the 4-6th grade school webinar
Students listening and asking questions during a webinar

 

 

 

 

 

 

 

 

These are just some examples of how meteorologists at NHC interact with the public and students.  It has been one of the most rewarding parts of my job, as I know that I am providing a valuable education to those that live in hurricane vulnerable locations.  In addition, some of these events have inspired students to want to learn more about weather, hopefully encouraging the next generation of meteorologists.  So if you are interested in attending some of these events for the next “off-season”, stayed tuned to hurricanes.gov for updates.

— John Cangialosi

Hurricane Andrew: Working in a Category 5 Storm

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Infrared satellite image of Hurricane Andrew at 5 AM on August 24, 1992, as it was making landfall along the coast of Miami-Dade County, Florida.

The hurricane season had yet to see its first named storm in August 1992, but that changed dramatically with the arrival of Hurricane Andrew.

South Florida and NOAA’s National Hurricane Center in Coral Gables took a direct hit from Andrew.

We’re going to take you behind the scenes for a glimpse of what it was like to be on duty during that fateful week twenty-five years ago.

The disturbance that had rolled off the coast of Africa became Tropical Storm Andrew on Aug. 17. But three days later, it almost disappeared.

Water vapor satellite image of Tropical Storm Andrew on August 19, 1992, while it was struggling to survive northeast of the Lesser Antilles.

“It looked like the storm had dissipated, but we hung onto it just a little bit longer”, said hurricane forecaster Dr. Richard Pasch, who was working the midnight shift. “The aircraft couldn’t locate the center, and Bob Sheets, the NHC director, came in and I told him I don’t think it’s going away. Bob said ’I don’t either‘, so we went ahead and hung onto it.”

It was a wise decision.

Just two days later, the Hurricane Hunter aircraft found a lower air pressure and better organization. On Saturday morning August 22, they found hurricane-force winds.

Hurricane forecaster Dr. Lixion Avila was on the morning shift.  “In the 5 a.m. advisory, I made Andrew a hurricane. I called Director Bob Sheets, and he said ‘OK, I’ll be there in one hour.’”

Public Advisory issued by Hurricane Specialist Lixion Avila at 5 AM on Saturday, August 22, 1992, making Andrew the first hurricane of the 1992 hurricane season.
Forecast discussion issued at 5 AM on Saturday, August 22, when Andrew was designated as a hurricane.

 

 

 

 

 

 

 

 

 

 

 

Later that same day, Hurricane Research Division (HRD) scientists James Franklin and Dr. Mark DeMaria boarded the NOAA Hurricane Hunter WP-3D aircraft in San Juan as part of a dropwindsonde mission to better sample the winds around the hurricane. The mission ended in Miami about midnight and both went to NHC to see if the new data had an impact on the forecast models. “The models now showed Andrew’s path much farther south and a direct threat to Miami-Dade County,” said DeMaria who, along with Franklin, would go on to become part of the NHC management.

Satellite image of Hurricane Andrew on Sunday, August 23, 1992, as it was headed toward the Bahamas and South Florida.

Andrew continued to rapidly strengthen as it approached South Florida.

In the pre-dawn hours of August 24, its eye, with winds screaming at more than 155 mph around it, neared the Dade County coastline.

Hurricane forecaster Dr. Ed Rappaport recalled how tense that night was, but he and many others were oblivious to the chaos outside. They were all focused on the analysis and typing out the 5 a.m. advisory with the updated forecast and warnings.

Hurricane Specialist Dr. Ed Rappaport prepares the 5 AM August 24 advisory for Hurricane Andrew, with the help of Hurricane Specialists Hal Gerrish, Dr. Richard Pasch, and Max Mayfield, as the hurricane rages outside.
Public advisory for Hurricane Andrew issued at 5 AM on Monday, August 24, as the hurricane’s eye was moving onshore in southern Miami-Dade County.

 

 

 

 

 

 

 

 

 

 

 

 

“When the advisory went out, somebody came up to me and said, ‘What do you think?’ I said, ‘about what?’ And he said ‘Well, the building’s swaying!’  A few minutes later, the anemometer measured a wind gust to 164 mph,” Rappaport said.

And then there was the time the whole building shook.

No one knew what it was, but NHC radar meteorologist Martin Nelson noticed the Miami radar, a WSR-57 (Serial #1), quit working.

“We believe another antenna broke loose and hit the fiberglass dome. Once that happened, the actual radar antenna was exposed,” Nelson said.

Part of that dome fell and landed on top of his pickup truck, while many other dome parts were blown down U.S. Highway 1. Radar fixes of Andrew’s trek over South Florida were captured by the new Doppler radar in Melbourne, Florida.

Radar on top of the building in Coral Gables that housed the National Hurricane Center at the time, destroyed by Hurricane Andrew’s winds.

The day before Andrew made landfall, several NHC forecasters were sent to NOAA’s National Meteorological Center in Maryland to back up NHC just in case it was needed.

One of those forecasters was Hugh Cobb. He was taking a break in his hotel room hours after Andrew made landfall and turned on CNN where the newscaster was talking about damage to the hurricane center.

“I happened to glance up and I saw an image of my car. Apparently there was a blue car that was tossed on top of my car.”

Cars at the National Hurricane Center damaged by Hurricane Andrew. The car in the lower right of the image was owned by forecaster Hugh Cobb (now the branch chief of the Tropical Analysis and Forecast Branch), who was deployed to the National Meteorological Center in Maryland in case NHC needed to be backed up.

Several other employees lost their cars. Worse, two dozen NHC and HRD employees had suffered major damage to their homes. Seven of those homes were destroyed.

NHC moved into a new single story facility three years later, well inland on the campus of Florida International University. Herb Saffir, a structural engineer, had a hand in the fortified design of the building. If the name sounds familiar…he is the “Saffir” in the Saffir-Simpson Hurricane Wind Scale.

Construction of the new National Hurricane Center and Miami-South Florida Weather Forecast Office in 1995 on the campus of Florida International University in the western portion of Miami-Dade County.

When another hurricane strikes, especially one with the power of Andrew, things will be different.

“We’ve got a plan in place for a hurricane landfall here in Miami, and we have successfully exercised it a few times, including when 2005’s Hurricane Katrina and Hurricane Wilma hit us,” said Rappaport, who is now the acting NHC Director.  “We’ve got the shutters and other essentials here to provide protection…as we try to impress upon the public to do the same.”

— Dennis Feltgen, NOAA Communications and Public Affairs Officer, National Hurricane Center

Potential Tropical Cyclones – Fitting the “Bill” for More-Timely Warnings

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GOES-East visible image of the Gulf of Mexico on the afternoon of June 19, 2017, when NHC began issuing advisories on Potential Tropical Cyclone Three, which eventually became Tropical Storm Cindy once it developed a well-defined center of circulation.

Two years ago this month, Tropical Storm Bill made landfall along the central Texas coast, just 17 hours after becoming a tropical cyclone only 145 miles offshore.  The precursor disturbance, a broad and ill-defined area of low pressure, had already been producing tropical-storm-force winds, and there was little doubt that the system would soon bring those dangerous winds onshore.  Although NHC’s Tropical Weather Outlooks had been talking about the possibility of those conditions two days in advance, and their likelihood one day in advance, some in the media and emergency management communities lamented the lack of earlier formal tropical storm warnings and full advisory products from NHC.  A few even suggested that NHC classify the disturbance as a tropical storm when it wasn’t one. By policy and tradition, NHC advisories, track and intensity forecasts, and any associated watches and warnings begin only after a disturbance has become a tropical cyclone; in this case a tropical storm warning was issued as soon as Bill formed, about 12 hours before the hazardous winds reached the coast.  For some additional discussion on why warnings couldn’t have been issued any earlier for Bill, please see our blog post written after that event.

This is hardly the only example of a tropical cyclone striking land shortly after genesis, and well within the normal 48-hour watch/warning time frame.  In 2010, Tomas struck Barbados as a tropical storm 27 hours after formation, and St. Vincent and St. Lucia as a hurricane 38 hours after formation.  In September of 2007, Humberto made landfall as a hurricane along the Texas coast a mere 19 hours after becoming a tropical cyclone. This recurring problem has been on our minds for a long time, and this season we’ve introduced a service enhancement to address the issue.

 

 

 

 

 

 

 

Starting this year, NHC has the option to issue advisories, track and intensity forecasts, watches, and warnings for disturbances that are not yet a tropical cyclone, but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours.  This substantial change in policy means that we won’t have to wait for a disturbance to meet the technical requirements of a tropical cyclone (such as having a well-defined center of circulation or sufficiently organized thunderstorm activity) to issue forecasts or post warnings.  And boy, it didn’t take long for us to employ this new option, with both the pre-Bret and pre-Cindy disturbances requiring the initiation of potential tropical cyclone advisories on two consecutive days!  But more on that in a moment.

Although we’ve been working on the technical and administrative changes to bring this about over the past two years, the effort actually began after the Deepwater Horizon disaster in 2010, when NHC was asked to provide enhanced forecast support for the response effort.  Since then, NHC has been practicing making track and intensity forecasts for disturbances, and at the same time we’ve been improving our ability to forecast tropical cyclone genesis.  We now believe that the science has advanced enough to allow the confident prediction of tropical cyclone impacts while these systems are still in the developmental stage.

So for these land-threatening “potential tropical cyclones” (and that’s the term we’re using in our advisories), NHC can now issue the full suite of text, graphical, and watch/warning products that previously has only been used for ongoing tropical cyclones.  This includes the cone graphic, public advisory, discussion, wind speed probabilities – everything – and all the products will look exactly the same as our tropical cyclone products.  The only thing that’s different is what we call the “system type”; we’ve added POTENTIAL TROPICAL CYCLONE to the roster of possible system types.  And since you asked (or at least were thinking about asking), here’s the complete list:

TROPICAL DEPRESSION
TROPICAL STORM
HURRICANE
SUBTROPICAL DEPRESSION
SUBTROPICAL STORM
POST-TROPICAL CYCLONE
REMNANTS OF
POTENTIAL TROPICAL CYCLONE

For those who are interested in the definitions of each of these system types, you can find them in National Weather Service Instruction 10-604, Tropical Cyclone Names and Definitions.

We did consider some alternatives to the term potential tropical cyclone.  “Tropical disturbance” was a fairly obvious option but we knew that some of these precursor disturbances weren’t going to be tropical in nature (such as a frontal cyclone evolving into a subtropical or tropical cyclone), so that eliminated tropical disturbance.  Another option was simply “disturbance”, which aside from evoking Star Wars imagery (I felt a great disturbance in the Gulf), did not in our view adequately convey the appropriate level of threat.  In the end, potential tropical cyclone seemed both accurate and appropriate to the threat, although it’ll take a bit of getting used to for some.

Potential tropical cyclones will share the naming rules currently used for depressions, with depressions and potential tropical cyclones being numbered from a single list (e.g., “One”, “Two”, “Three”, …, “Twenty-Three”, etc.). The assigned number will always match the total number of systems we’ve written advisories on within that basin during the season. For example, if three systems requiring advisories have already occurred within a basin in a given year, the next land-threatening disturbance would be designated “Potential Tropical Cyclone Four”. If a potential tropical cyclone becomes a tropical depression, its numerical designation doesn’t change (i.e., Potential Tropical Cyclone Four becomes Tropical Depression Four).

Potential tropical cyclone advisory packages will be issued at the standard advisory times of 5 AM, 11 AM, 5 PM, and 11 PM EDT, with three-hourly Intermediate Public Advisories being issued at 2 AM, 8 AM, 2 PM, and 8 PM EDT when watches or warnings are in effect. The product suite will include a five-day track and intensity forecast, just as is done for ongoing tropical cyclones. In addition, the Potential Storm Surge Flooding Map and Storm Surge Watch/Warning graphic would be issued for these systems when appropriate.  We’ll continue issuing advisory packages on a potential tropical cyclone until watches or warnings are discontinued or until the threat of tropical-storm-force winds for land areas sufficiently diminishes, at which point advisories would be discontinued. However, if it seems likely that new watches or warnings would be necessary within a short period of time (say 6-12 hours), then advisories could continue during that brief gap in warnings in the interest of service continuity.

Since the primary issuance trigger is the threat of tropical storm conditions over land, there won’t be any specific threshold of formation likelihood for the initiation of advisories.  For example, a fast-moving tropical wave approaching the Lesser Antilles might already have tropical-storm-force winds but no closed wind circulation.  In this case, a genesis forecast of 40% – 50% would likely be enough to trigger advisories and warnings.  In contrast, a genesis forecast of 70% for a system close to shore might not trigger advisories if the system were not expected to reach tropical storm strength before moving inland.

The issuance of NHC products for potential tropical cyclones is very much analogous to the change that occurred after Hurricane Sandy in 2012, when NHC advisories on post-tropical cyclones became possible.  After Sandy, we realized that there was great benefit to users in NHC’s being able to continue writing advisories on systems even after they were no longer a tropical cyclone.  That solved the service continuity problem on the “back end”, and now we’re completing the process by ensuring a steady flow of information on the front end of a tropical cyclone’s life cycle.  In all cases, we’ll be trying to ensure that warning types (tropical vs. non-tropical) don’t have to change in the middle of an event.

There are some things to be aware of with this new capability.  First, potential tropical cyclone advisories will not be issued for systems that threaten only marine areas – largely because this would pose an unmanageable workload/staffing issue for us but also because marine forecast products (the High Seas and Offshore Waters forecasts) already allow the issuance of gale and storm warnings before a tropical cyclone has formed.

Second, because potential tropical cyclones will have a standard five-day forecast track and uncertainty cone, to avoid potential confusion with the cone we’re going to stop drawing potential formation areas for these systems in the Graphical Tropical Weather Outlook.

An example of the Tropical Weather Outlook in which the potential genesis area for Potential Tropical Cyclone Two (pre-Bret) is not indicated east of the southern Windward Islands since advisories and an accompanying cone graphic were being issued by NHC at the time.

We’re also concerned that some users may pay too much attention to the longer-range part of these new forecasts (the part beyond 72 hours).  We know that forecast errors for weaker and developing systems tend to be larger than those for strong storms and hurricanes, and we even considered only going out to 72 hours with the new potential tropical cyclone advisories (since the primary purpose was to support watches and warnings).  But in the end, consistency and technical issues argued for going out to five days, and that’s what we’re doing.  So it’s likely that forecast-to-forecast changes in the longer-range portion of our potential tropical cyclone advisories will be larger than what folks are used to.  And for those of you who like to look at forecast model intensity guidance, be aware that most of these intensity models assume the system is a tropical cyclone.  Since that won’t be the case for these systems, intensity models run on potential tropical cyclones will generally have a high bias.  And lastly, since many potential tropical cyclones will not have well-defined centers, there will likely be large jumps in the reported location of these systems from advisory to advisory.  But even with all these caveats, we think that the ability to post warnings before a cyclone forms is an important service enhancement – one that will help save lives and protect property, while at the same time allowing NHC to analyze and report on tropical systems as accurately and as honestly as possible.

After our experiences with Bret and Cindy, we’re optimistic about the value of this new capability.   Advisories on Potential Tropical Cyclone Two were started 24 hours before Bret officially became a tropical cyclone, giving residents of Trinidad and Tobago, Grenada, and northeastern Venezuela an additional day of warning for tropical storm conditions.  If this were still 2016, places like Trinidad may have only had three to six hours between the time of the first advisory and the time when tropical storm force winds began on the island.  And for Cindy, advisories on Potential Tropical Cyclone Three were initiated roughly 21 hours before Cindy met the criteria of a tropical cyclone.  This allowed Tropical Storm Warnings to be issued for southeastern Louisiana 21 hours earlier than they would have been if the storm had occurred last year.

Just a few weeks into the new season, we’re pretty happy about the way this all worked.  We think we successfully demonstrated the ability to provide more advanced warning than we could have in previous years for these developing tropical cyclones.  But we’d love to hear feedback from our users, customers, and partners.  Were the potential tropical cyclone advisories in advance of Bret and Cindy confusing?  Helpful?  Maybe both?  Or bad puns aside, did the new capability fit the “Bill”?

If you’d like to provide comments on your experiences with the Potential Tropical Cyclone advisories during Bret and Cindy, please feel free to contact Jessica Schauer, the NWS Tropical Cyclone Program Leader, at Jessica.Schauer@noaa.gov.

— James Franklin
Editor’s Note:  This post marks James’s last blog contribution as a member of the NHC family.  After 35 years of service in the federal government (17 years at NOAA’s Hurricane Research Division and 18 years at the National Hurricane Center), James is retiring at the end of this week.  We want to thank James for his contributions to not only the blog, but also for his many contributions to hurricane forecasting and NHC operations over the past several decades.  Although James will no longer be “inside the eye” of the sometimes-hectic NHC scene, we know he won’t be too far away cheering on his beloved Miami Hurricanes, Miami Dolphins, and Florida Panthers.  Congratulations, James, and happy retirement!