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

 

 

Why Some of the Nation’s Top Hurricane Experts Bought Flood Insurance

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Editor’s Note:  Today, May 9, is Day 4 of National Hurricane Preparedness Week.  Today’s theme is “Get an Insurance Checkup,” and two hurricane experts talk in this blog post about why they decided to get flood insurance for their homes.  For more information on flood insurance, you can visit http://www.floodsmart.gov or re-read our previous blog post on securing an insurance checkup.

By now, billion-dollar flood disasters in the U.S. are something of an overlooked seasonal rite-of-passage. The Midwest flooding of April 2017 feels like a distant memory, as does the California flooding two months before it, Hurricane Matthew’s historic flooding the fall before that, the devastating floods in Louisiana the summer before that, and Texas and Louisiana  — again — the spring before that. Some 500,000 homes damaged or destroyed at a cost of more than $150 billion in two years from those events alone. No one expected that kind of flooding to affect them. They never do.

For Hurricane Harvey, its Category 4 winds at landfall were just a prelude of things to come.  Harvey wasn’t even a hurricane by the time its heaviest rains reached Houston. Though the tropical storm’s still-high winds hampered rescue efforts, the winds were not at the forefront of the minds’ of residents living through the unimaginable. The magnitude of the flood nightmare caught even Houston, no stranger to big floods, by surprise. Three to five feet of rainwater poured down from the skies above in what would become the worst freshwater flood in United States history. The residences of nearly one-in-three people in America’s fourth-largest city were under water.

The Addicks neighborhood of Houston, Texas, sits in floodwaters after the heavy rains from Hurricane Harvey. (Image courtesy of the Harris County Flood Control District)

In Harvey’s wake lay a dizzying disbelief of devastation. More than 120,000 homes in Harris County, where Houston is located, were damaged by floodwaters.[1] What’s more, an estimated 70 percent of those homes were uninsured for floods.[1],[2] That meant the majority of Houstonians, not the insurance companies, were on the hook for the bill. The best-case scenario for uninsured survivors was Federal Emergency Management Agency (FEMA) disaster assistance (typically ranging $3,000–$8,000) or a U.S. Small Business Administration (SBA) low-interest federal loan (up to $200,000 for home repair). In reality, covering the cost of repairs for most came from a combination of federal assistance and personal finances, which for many meant adding to or incurring new consumer debt.

It was never intended for the federal government to bail out the uninsured after a disaster— in fact, quite the opposite. When the National Flood Insurance Program (NFIP) was established 50 years ago, its goal was to help insure the uninsured before a disaster. Flooding is the most common and expensive type of disaster, and insuring high-risk flood areas often demands an astronomical price tag. Back in the 1960s, private market flood insurance simply wasn’t available. This is where the NFIP came in. Through the NFIP, the federal government began offering largely affordable policies to the residents of participating communities who adopted and enforced floodplain management ordinances in high-risk flood areas to reduce future flood risk. In theory, securing and insuring high-risk communities reduces the reliance on federal post-disaster assistance and saves the government (and U.S. taxpayers) money, which is a good thing.

Flood insurance, once voluntary, is today required for all properties with federally-backed mortgages in high-risk flood areas. To define these high-risk flood areas, FEMA routinely conducts flood hazard analyses to identify land areas at risk of being inundated by a flood that has a 1 percent chance of being equaled or exceeded in any given year. Since 1 percent is interchangeable with 1-in-100, these high-risk floods have become known as 1-in-100 year (or 100-year) floods. FEMA designates these so-called 100-year floodplains as Special Flood Hazard Areas (SFHAs).

Which brings us back to Houston. When Harvey’s floodwaters receded, some 2-in-3 survivors had no insurance to cover their flood losses.1,2  The vast majority of flooding during Harvey happened outside of the SFHA. The storm widened rivers and reservoirs to a point where roads became riverbeds. When everything’s flooded, flood zones feel a little meaningless. But one of the lessons Harvey taught is that flood zones, and our understanding of them, do matter — more so today than ever.

The so-called 100-year floodplain can be understandably confusing.  A 1 percent chance of anything happening in a given year feels remote, but most of us don’t live in our houses for a single year. Consider, for example, a 30-year mortgage. The odds of one of those 100-year floods happening over the period of a 30-year mortgage is about 1 in 4. A 25 percent chance of a devastating flood over the period of your mortgage are higher than the odds of a devastating house fire, and you probably wouldn’t go 30 years without installing smoke detectors in your home.

It’s easy to see the need for flood insurance when it’s required; it’s not as clear when it isn’t. So what about those living in high-risk areas without a federally-backed mortgage? Or what about those living outside the high-risk, 100-year floodplain? After all, a moderate chance of flooding hardly implies you’re safe. In fact, FEMA estimates that nearly 1 in 4 of all federal flood claims occur outside of high-risk areas. As every billion-dollar flood disaster shows, floods can be some of the most egregious rule breakers.

Financial decisions, including whether to insure your home and belongings from a flood, are deeply personal family issues. They often aren’t easy decisions, even for those most familiar with the threat. Below, two of the nation’s leading hurricane experts discuss their own experiences living in places where water is a stark reality. Though the aim of their livelihoods is piecing together the clues of Mother Nature’s next step, they’ve each lived through the unpredictable moments. The billion-dollar floods don’t get any easier, and as coastal populations soar, neither will the decisions that shield us from Mother Nature’s most unpredictable moments.

Bill Read, Former Director, National Hurricane Center

I think a big problem people have is differentiating between “requiring” and “needing” flood insurance.  My first exposure to the issue of “needing” flood insurance occurred when we moved from Maryland to League City, Texas, in 1992.  The house we decided to purchase was (barely) outside the floodplain indicating the 100-year elevation.  However, I was concerned about the risk mainly due to storm surge, as data indicated a Category 3 hurricane or above would bring water levels above the level of our home’s foundation.  Our realtor and our lender were both adamant in telling us we didn’t “need” flood insurance. I asked our insurance agent at USAA and he was spot on in differentiating between “required” and “needed”.   We chose to follow our insurer’s advice.  When we decided to move to a new house in 2005, I found a development on the lowest flood risk land in League City, a parcel that sat outside the 500-year elevation.  While no longer in a storm surge risk area, I was concerned about flooding from an “off the charts” rain event. Interestingly, my realtor for this move was savvy about flooding along the Gulf coast and advised us to keep flood insurance, as did USAA, which I had already decided I would do based on the many floods I had witnessed outside the 100-year risk areas in Houston since 1992.  Along came Harvey, and although we did not flood from the 45 inches of rain we received, the water reached our porch and was one inch from entering the house.  Six of my friends were not as fortunate, and two of them did not have flood insurance.  Needless to say, when my policy came up for renewal in February, I quickly did so!

Flood waters from Harvey creeping up onto the porch of the residence of former NHC Director Bill Read in League City, Texas.
A golf buddy of Bill Read and the man’s son move four of the son’s dogs to higher ground after Harvey flooded League City, Texas.
Debris lies the street of one of Bill Read’s friends in Dickinson, Texas, two months after Harvey flooded the area.

 

Jamie Rhome, Storm Surge Specialist, National Hurricane Center

I recently purchased a home in South Florida, and while going through the mortgage approval process, was informed that the home was outside of the high-risk area (aka the 100-year flood zone) and thus flood insurance wasn’t “required.”  I’m also far enough inland to prevent storm surge (saltwater) inundation.  However, the home is situated near a freshwater lake, and South Florida often experiences very heavy rains, sometimes exceeding 10 inches in a day.  One can easily envision a scenario where debris, from heavy rains or winds, clogs the storm water drains and water pools in the street, eventually coming up the driveway and ultimately wetting the bottom floor of my home.  Indeed, South Floridians are very accustomed to this very issue as it frequently occurs during our rainy season.  Without flood insurance, I as the homeowner would be responsible for all the damage, which can easily climb into the thousands of dollars.  Imagine replacing floors, walls, furniture, possessions, etc., and then taking steps to prevent mold.   Given the small cost of flood insurance, the decision was an easy one and I determined that flood insurance was “needed” even if it wasn’t “required.”   I was also lucky enough to have a realtor who was well-informed on flood insurance and overall flood risk.   He encouraged the purchase of flood insurance citing his experience living in Florida and personal experience with freshwater flooding.  Not all home buyers benefit from such well-informed or well-intentioned realtors and home buyers often navigate these complicated waters on their own.  If in doubt, it’s better to be safe than sorry.

Street flooding from a heavy rain event in Jamie’s former South Florida neighborhood in October 2017. When drains are clogged by debris, streets easily flood with water creeping up driveways.
View from the back porch of Jamie’s current South Florida residence. Even though Jamie is not located in the 100-year flood zone, it’s not hard to envision that water might still some day be a problem.

 

— Michael Lowry (UCAR Visiting Scientist), Jamie Rhome and Robbie Berg (NHC)

 

[1]  Preliminary analysis of Hurricane Harvey flooding in Harris County, Texas.  California WaterBlog.  Available at https://californiawaterblog.com/2017/09/01/preliminary-analysis-of-hurricane-harvey-flooding-in-harris-county-texas/

[2] Hurricane Harvey:  70% of home damage costs aren’t covered by insurance.  CNN Money.  Available at http://money.cnn.com/2017/09/01/news/hurricane-harvey-cost-damage-homes-flood/index.html

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