hurricane

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
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After Further Review: Tropical Storm Erika

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Image courtesy of NASA

Tropical Storm Erika, coming as it did so close to the beginning of the new college and professional football seasons, is a reminder that Monday-morning quarterbacking is nearly as popular an activity as the sport itself.  And we at NHC do it too.  After every storm we review our operations with an eye toward improving our products and services.  Erika is no different, though there’s been more questioning and criticism than usual, with few components of the weather enterprise spared.  Some in the media were accused of overinflating the threat, numerical models were bashed, and some public officials were charged with overreacting.   NHC’s forecasts were questioned while others lamented that NHC’s voice wasn’t strong enough amid all the chatter.  So, in the spirit of searching for a tropical storm eureka, in this blog entry we present some of our own post-storm reflections.

How good were NHC’s forecasts for Erika?

The NHC official forecast errors were larger than average.  A preliminary verification shows that the average 72-hr track error for Erika was 153 n mi, about 30% larger than the 5-year average of 113 n mi.   And nearly all of this error was a rightward (northward) bias – that is, Erika moved consistently to the left of the NHC forecast track.   As for intensity, Erika ended up being weaker than forecast; the 72-hr intensity forecasts were off by about 20 kt on average, and the official 5-day forecasts called for a hurricane over or near Florida until as late as 2 AM Friday, when it became clear that Erika was going to have to deal with the high terrain of Hispaniola.

Why was Erika so difficult to get right?

Erika was a weak and disorganized tropical cyclone, and weak systems generally present us (and the computer simulations we consider) with tougher forecast challenges.  In fact, the average 72-hr track error for all tropical depressions and weak tropical storms is around 155 n mi – just about exactly what the errors for Erika were.  So the track issues weren’t really a surprise to us.  Of course, knowing whether such errors were going to occur and how to reduce them in real time wasn’t obvious.  If it had been obvious, we would have called an audible and changed the forecast.

Track errorWhat made this particular situation so challenging was wind shear, mainly due to strong upper-level westerly winds in Erika’s environment.  These winds tended to displace the storm’s thunderstorms away from its low-level circulation, causing the storm to lack a coherent vertical structure.  When this happens, it’s very difficult to tell just how much influence those upper-level winds will have on the storm track.   Sometimes storms hold together against wind shear (Andrew of 1992 is a good example), and there were times when Erika seemed to be winning its battle.  If it had held together better, it would have taken a track more to the north and ended up being a much stronger system.  Obviously, it didn’t play out that way, but that was an outcome far easier to see with the benefit of hindsight.

An additional complication was Puerto Rico and Hispaniola.  If Erika had been able to avoid those land masses, it would have been better able to withstand the disruptive effects of wind shear.  And early on, we expected Erika to mostly avoid land.  In this case, not getting the track right made it much harder to get the intensity right, which made the track forecasts harder yet.

Is there too much reliance on numerical models, and did they fail for Erika?

The improvements in track forecasting over the past few decades are directly attributable to improvements in numerical models, and to the data used to initialize them, to the point where it’s become almost impossible for a human forecaster to consistently outperform the guidance.  The modelling community deserves our praise for the tremendous progress they’ve made, not criticism for missing this one.  While we approach each forecast with an attempt to diagnose when and how the models might fail, it is exceedingly difficult, and it’s not something we do in our public forecasts unless our confidence is very high.

Track errors over timeModel track errors over time

 

 

 

 

 

 

Human forecasters (including those at NHC) are still able to occasionally outperform the intensity models, mainly because satellite depictions of storm structure can often be used by forecasters more effectively than by models, giving us an edge in certain circumstances.  But neither human forecasters nor the models are particularly good at anticipating when thunderstorms in the cyclone core are going to develop and how they’re going to subsequently evolve, especially for weaker cyclones like Erika.

Because the atmosphere is a “chaotic” physical system, small differences in an initial state can lead to very large differences in how that state will evolve with time.  This is why the model guidance for Erika was frustratingly inconsistent – sometimes showing a strong hurricane near Florida, while at other times showing a dissipating system in the northeastern Caribbean.  It’s going to take a large improvement in our ability to observe in and around the tropical cyclone core (among other things), to better forecast cases like Erika for which storm structure is so important to the ultimate outcome.  But our best hope for better forecasts lies in improved modeling–a major goal of the Hurricane Forecast Improvement Program (HFIP).

Given the overall model guidance we received during Erika, it’s hard even now, well after the storm, to see making a substantially different forecast with current capabilities and limitations.  In fact, had our first few advisories called for a track much farther south at a much weaker intensity, or even a forecast of dissipation due to interaction with Hispaniola, our partners and the public might rightly have questioned our rationale to go firmly against many model forecasts of a stronger system farther north.

Was the message from NHC muddled?

We think that there might be some ways for NHC to make key aspects of our message easier to find.  Although NHC’s Tropical Cyclone Discussions (TCDs) repeatedly talked about the uncertainty surrounding Erika’s future beyond the Caribbean, including the possibility that the cyclone could dissipate before reaching Florida, it does not appear that this was a prominent part of the media’s message to Florida residents.   Making key “talking points” more distinctly visible in the TCD and the Public Advisory are options we are considering, as well as enhanced use of NHC’s Twitter and Facebook accounts.  Having said that, consumers and especially re-packagers of tropical cyclone forecast information, like our media partners, should take some responsibility for making use of the information that is already available.  We also invite our media partners to take more advantage of the numerous training sessions we offer, mostly during the hurricane offseason.  Reaching anyone in the television industry with such training, except for on-camera meteorologists, has proven over the years to be very difficult.  We would like to train more reporters, producers, news department staff, executives, etc. so they are more sensitized to forecast uncertainty and how to communicate it with the help of our products, but we realize that a more focused “talking points” approach as described above will probably be needed to assist these busy folks in conveying a consistent message.

An NHC advisory package contains a variety of products, each geared to providing a certain kind of information or to serving a particular set of users.  Some of our media partners have expressed concerns over the increasing number of NHC products, but the various wind and water hazards posed by a tropical cyclone cannot be boiled down to one graphic, one scale, or one index.  We are, in fact, still in the process of intentionally expanding our product and warning suite to focus more on the individual hazards and promote a more consistent message about those hazards.  Even so, two of our products that have been around for many years are still crying out for greater visibility.

We’ve already mentioned the Tropical Cyclone Discussion, a two- to four-paragraph text product that is the window into the forecaster’s thinking and provides the rationale behind the NHC official forecast.  In the TCD we talk about the meteorology of the situation, indicate our level of confidence in the forecast, and when appropriate, discuss alternative scenarios to the one represented by the official forecast.  Anyone whose job it is to communicate the forecast needs to make the TCD mandatory reading on every forecast cycle.

Some users may not understand the amount of uncertainty that is inherent in hurricane forecasts (although we suspect Florida residents now have a greater appreciation of it than they had two weeks ago).  We need to continue to emphasize, and ask our media partners to emphasize, NHC’s Wind Speed Probability Product, available in both text and graphical form, which describes the chances of hurricane- and tropical-storm-force winds occurring at individual locations over the five-day forecast period.  Someone looking at that product would have seen that at no point during Erika’s lifetime did the chance of hurricane-force winds reach even 5% at any individual location in the state of Florida, and that the chances of tropical-storm-force winds remained 50-50 or less.  In addition, in some of the number crunching we did after the storm, we calculated that the chance of hurricane-force winds occurring anywhere along the coast of Florida never got above 21%.

AL052015_PROB64_010_F120AL052015_PROB34_010_F120
Erika TS Winds

We realize that at first it seems counterintuitive that we are forecasting a hurricane near Florida while no one in that state has even a 5% chance of experiencing hurricane-force winds.  That, however, is the reality of uncertainties in 5-day forecasts, especially for weaker systems like Erika, and the wind speed probabilities reliably convey the wind risk in each community.  We did notice a couple of on-air meteorologists referencing the Wind Speed Probabilities, which is great – and the more exposure this product gets, the better.  We would like to work with our television partners to help them take advantage of existing ways for many of them to easily bring the wind speed probabilities into their in-house graphics display systems.

A very nice training module exists for folks interested in learning about how to use the Wind Speed Probabilities: https://www.meted.ucar.edu/training_module.php?id=1190#.VenooLQ2KfQ.  You will need to register for a free COMET/MetEd account in order to access the training module.

Did Floridians over-prepare for Erika?

Anyone who went shopping for water and other supplies once they were in the five-day cone did exactly the right thing (of course, it’s much better to do that at the beginning of hurricane season!).  Being in or near the five-day cone is a useful wake-up call for folks to be prepared in case watches or warnings are needed later.   But as it turned out, no watches or warnings were ever issued for Florida.  In fact, at the time when watches would normally have gone up for South Florida, NHC decided to wait, knowing that there was a significant chance they would never be needed – and that turned out to be the right call.

Watches (which provide ~48 hours notice of the possibility of hurricane or tropical storm conditions) and warnings (~36 hours notice that those conditions are likely) seem to be getting less attention these days, with the focus on a storm ramping up several days in advance of the first watch.   While the additional heads-up is helpful, we worry about focusing in on specific potential targets or impacts that far in advance.  The watch/warning process begins only 48 hours in advance because that’s the time frame when confidence in the forecast finally gets high enough to justify the sometimes costly actions that an approaching tropical cyclone requires.  (Although, we recognize that some especially vulnerable areas sometimes have to begin evacuations prior to the issuance of a watch or a warning, and we and the local Weather Forecast Offices of the National Weather Service directly support emergency management partners as they make such decisions.)

What can NHC do better next time?

While we’d like to make a perfect forecast every time, we know that’s not possible.  Failing that, we’re thinking about ways we can improve the visibility of our key messages, particularly those that will help users better understand forecast uncertainty.  As noted above, we’re considering adding a clearly labeled list of key messages or talking points to either the TCD or the Tropical Cyclone Public Advisory, or both.  We’d also like to try to make increased use of our Twitter accounts (@NHC_Atlantic, @NWSNHC, and @NHCDirector), which so far have been mostly limited to automated tweets or to more administrative or procedural topics.  We’re also looking at whether the size of the cone should change as our confidence in a forecast changes (right now the cone size is only adjusted once at the beginning of each season), and thinking about ways to convey intensity uncertainty on the cone graphic.  But most of all, we need to keep working to educate the media and the public about the nature of hurricane forecasts generally, and how to get the information they need to make smart decisions when storms threaten.

— James Franklin