Cyclones and Warnings and Names, Oh My!

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Visible satellite image of Tropical Storm Bill at 10:15 am CDT on Tuesday, June 16, just before it made landfall along the Texas coast on Matagorda Island.

The development of Tropical Storm Bill so close to the Texas coast, with the posting of a formal tropical storm warning only about 12 hours before winds of that intensity came ashore on Tuesday June 16th, highlighted a long-standing and well-known limitation in the tropical cyclone program of the National Weather Service (NWS).  An ongoing NHC initiative to improve service for such systems is the subject of today’s blog post.

Although there’s nothing new about a tropical cyclone forming on our doorstep, what is new is an increased ability to anticipate it.  NHC has greatly enhanced its forecasts of tropical cyclone formation over the past several years, introducing quantitative 48-hr genesis forecasts to the Tropical Weather Outlook in 2008, and extending those forecasts to 120 hours in 2013.  In 2014, we introduced a graphic showing the locations of tropical disturbances and the areas where they could develop into a depression or storm over the subsequent five days.  Thirty-six hours in advance of Bill’s formation, NHC gave the precursor disturbance a 60% chance of becoming a tropical cyclone, and increased that probability to 80% about 24 hours in advance.  While nothing was guaranteed, we were pretty confident a tropical storm was going to form before the disturbance reached the coast.  And although we weren’t issuing specific track forecasts for the disturbance, NHC’s new graphical Tropical Weather Outlook (example below) showed where the system was generally headed.

Bill_two_atl_5d0

We wouldn’t have had such confidence 20 years ago, or even 5 years ago. And so our tropical cyclone warning system, developed over several decades, doesn’t allow for a watch or warning until a depression or storm actually forms and NHC’s advisories begin; by both policy and software, warning issuances are tied to cyclone advisories.  If we had wanted to issue a tropical storm watch for Bill on the morning of Sunday the 14th (48 hours prior to landfall), or a warning that evening (36 hours ahead of landfall), we would have had to pretend that the disturbance in the Gulf of Mexico was a tropical cyclone.  Even during the day on Monday, data from an Air Force Reserve Hurricane Hunter aircraft showed that the disturbance had not yet become a depression or storm.

Couldn’t NHC have called the disturbance a tropical storm anyway, in the interest of enhanced preparedness?  Yes, but what if the disturbance never becomes a tropical storm – remember, even an 80% chance of formation means it won’t become a tropical storm at least once every five times.  So naming it early risks the credibility of the NWS and NHC, and endangers a trust we’ve worked for decades to establish.  In addition, there are legal and financial consequences to an official designation of a tropical cyclone – consequences that obligate us to call it straight.  And finally, as custodians of the tropical cyclone historical record, we have a responsibility to ensure the integrity of that record.

When systems that have the potential to become tropical cyclones pose hazards to life and property, NHC’s best avenue for highlighting those hazards currently is the Tropical Weather Outlook. Ahead of Bill’s formation, the possibility of tropical storm conditions along the middle and upper Texas coast was included in the Sunday evening Outlook, and by early Monday afternoon the Outlooks were saying those conditions were likely.  Products issued by NWS local forecast offices (WFOs) carried similar statements.

Although most folks seemed to have gotten the message that a tropical storm was coming, it’s widely thought that the Tropical Weather Outlook and local WFO products don’t carry the visibility and weight of an NHC warning, or of an NHC advisory package with its attendant graphics.  In addition, some institutions have preparedness plans that are tied to the presence of warnings.  We agree that warnings during the disturbance stage could improve community response, and we’ve been working toward that goal since 2011.  In that season, NHC initiated an internal experiment in which the Hurricane Specialists prepare track and intensity forecasts for disturbances with a high likelihood of development, and use these forecasts to determine where watches and warnings would have been appropriate.  These internal disturbance forecasts have had some successes and failures, but may now be good enough to make public.

With our colleagues across the NWS, we’re now working through the logistics of expanding the tropical cyclone product and warning suite to accommodate disturbances.  One plan under consideration calls for NHC to produce a five-day track and intensity forecast for those disturbances having a high chance of becoming a tropical cyclone, and which pose the threat of bringing tropical-storm-force winds to land areas.  The forecasts would be publicly issued through the standard NHC advisory products, including the Public Advisory, Discussion, and Wind Speed Probability Product, along with the forecast cone and the other standard graphics. These advisory packages would be issued at the normal advisory times, and continue until the threat of tropical-storm-force winds over land had diminished.  If and when the disturbance became a tropical cyclone, advisory packages would simply continue.

We are still evaluating these and other options for getting tropical cyclone warnings out for potential tropical cyclones.  If we do begin issuing forecasts for these systems, we know from our experimental forecasts that they won’t be as accurate as our current public forecasts for tropical cyclones are – and we’ll want to make sure users know about those uncertainties.  There are many details to iron out and much technical work to do, but we’re hopeful to have this service enhancement in place for the 2017 hurricane season.

— James Franklin

The Ups and Downs of Predicting Tropical Cyclone Formation: The Role of Atmospheric Waves

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A previous blog entry described the new NHC five-day tropical cyclone formation (or genesis) products.  In this blog entry, we discuss the factors that go into these predictions.

The primary tool used at NHC for five-day tropical cyclone genesis forecasts is global numerical modeling.  Global models can predict many of the environmental factors that influence tropical cyclone formation, and the skill of these models has been improving with time.  More tropical cyclone formations are being forecast with longer lead times, and weather prediction models show fewer “false alarms” than in the past.  Recent studies suggest, and forecaster experience seems to confirm, that a consensus of the available model guidance usually outperforms any single model.   This “two heads are better than one” approach works as long as the models (or heads) are somewhat independent of one another.  In addition, NHC is currently evaluating a few statistical techniques that use the global model output to produce objective guidance designed to assist hurricane specialists in developing the probabilities of formation issued in the Tropical Weather Outlook.

Kelvin Waves and the Madden-Julian Oscillation

Global model guidance is not the only tool available to NHC forecasters, however.  Researchers have learned that a majority of lower latitude tropical cyclone formations are associated with waves in the atmosphere moving through the global Tropics from west to east.    Two particularly important wave types are the Convectively Coupled Kelvin Wave (CCKW), which circumnavigates the equator in about 15 to 20 days, and the Madden-Julian Oscillation (MJO), which transits the globe in 30 to 60 days.  These waves are normally initiated by large areas of thunderstorm activity over tropical regions, especially near India and southeastern Asia.  These waves are different in both frequency and direction of motion from the more well-known tropical waves that originate over Africa and often spawn tropical cyclones as they move westward across the Atlantic and eastern North Pacific basins.

Tropical cyclone formation often accompanies the passage of the “active phase” of either the faster-moving CCKWs or the slower-moving MJO.   Figure 1 shows tropical cyclone tracks over a 37-year period in active and inactive phases of the MJO as the wave moves around the globe, along with increased or decreased rainfall anomalies associated with the two phases of the MJO (Zhang 2013).   In the figure, the active phase of the MJO for the Atlantic occurs in panel (a), while for the eastern Pacific the active phase occurs in panel (d).  The less active phases for these two basins fall in panels (c) and (b), respectively.

Figure 1. Tropical cyclone tracks in active and inactive phases of the MJO and increased (green) and decreased (purple) rainfall anomalies associated with the two phases of the MJO (from Zhang 2013). Panel (a) shows the active phase of the MJO for the Atlantic, and (d) shows the active phase for the eastern Pacific. Panels (b) and (c) show the less active phases for both basins.
Figure 1. Tropical cyclone tracks in active and inactive phases of the MJO and increased (green) and decreased (purple) rainfall anomalies associated with the two phases of the MJO (from Zhang 2013). Panel (a) shows the active phase of the MJO for the Atlantic, and (d) shows the active phase for the eastern Pacific. Panels (b) and (c) show the less active phases for both basins.

This concentration of tropical cyclone activity occurs because each type of wave temporarily makes large-scale environmental conditions, such as vertical wind shear or atmospheric moisture, more conducive for tropical cyclone formation.  Although not every wave causes a tropical cyclone to form, pre-existing disturbances have a greater likelihood of developing into tropical cyclones after the passage of a CCKW or the MJO.  High-activity periods can last as long as a week or more with the MJO, but are generally followed by days to possibly weeks of little to no activity during the inactive phases of these waves, when large-scale conditions become unfavorable for tropical cyclone formation.  Forecasters use real-time atmospheric data and other tools to diagnose the location and motion of these important catalysts for tropical cyclone formation.

Here is an example from the 2014 hurricane season of how forecasters used these atmospheric signals.  The graphic below, called a Hovmöeller diagram, shows where large areas of rising air (cool colors) and sinking air (warm colors) exist near the equator as a function of time.  The dashed black contours depict the active phase of successive CCKWs, and the solid red contours show the inactive phases.    In this particular case, forecasters noted that there was a strong CCKW moving through the eastern Pacific in the middle part of October.  Extrapolating the wave forward in time, along with numerical models forecasts of the wave’s location and strength, suggested that a tropical cyclone could form within a few days over the far eastern Pacific from a disturbance that was already in the area.  The green dot indicates where Tropical Storm Trudy formed, a day or two after the CCKW passed the disturbance.  Although CCKW tracking is only a secondary factor in determining a Tropical Weather Outlook forecast, a basic knowledge of this atmospheric phenomenon is an important part of the process.

active inactive phase
Figure 2. Hovmoeller diagram showing large areas of rising air (cool colors) and sinking air (warm colors) near the equator as a function of time

Forecasters consider many factors when preparing the five-day genesis probabilities for the Tropical Weather Outlook, including explicit forecasts from the global models and knowledge of any ongoing CCKWs or the MJO.   In addition, the final NHC forecast also reflects the current trends of the disturbance, which are weighted much more heavily in the two-day outlook, but also can affect the five-day forecast as well.  There are several ongoing research projects that will hopefully yield objective probabilities and other tools designed to help better predict tropical cyclone formation.  These tools, in combination with the dynamical guidance from numerical models, should improve the quality of genesis forecasts and perhaps in the next five years extend reliable tropical cyclone formation forecasts from five days to one week.

— Eric Blake and Todd Kimberlain

 

Reference:

Zhang, Chidong 2013: Madden–Julian Oscillation: Bridging Weather and Climate. Bull. Amer. Meteor. Soc.94, 1849–1870.

Acknowledgments:

Thanks to Chidong Zhang and David Zermeno, University of Miami RSMAS, for Figure 1.

Investing for Meteorologists

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InvestTeenagers today seem to enjoy taking words and employing them as a new part of speech, especially if it results in the use of fewer syllables.  Thus we have the verb fail  used as a noun in place of failure, the verb invite used as a replacement noun for invitation, and so on.  This has given us such linguistic classics as what an epic fail  or where’s my invite.  My teenage daughter has managed the inverse transformation, telling me that she has “no time to piano today”.  Texting and Twitter can be blamed for much of this, of course, but the hurricane community’s gift to the Lexicon of Lazy Locutions originated nearly two decades ago.  The noun that represents our particular role in the decline and fall of the English language is the subject of today’s blog post.

If you lurk around the dark meteorological corners of the Internet, or even if you just watch weather broadcasts during hurricane season, you’ve probably come across expressions like Invest AL94.  With the accent on the first syllable (IN-vest rather than in-VEST), this is not an insider’s instruction to sell your AAPL stock at $100, but rather it’s a reference to a specific “investigative area” – a weather system for which a tropical cyclone forecast center is interested in collecting specialized data sets or running model guidance.

Accounting for Invests

NHC has responsibility for identifying these invests, or disturbances of interest, in the Atlantic basin.  NHC and the Joint Typhoon Warning Center (JTWC) have shared responsibility for designating invests in the eastern Pacific, while the Central Pacific Hurricane Center (CPHC) has this responsibility in the central Pacific.  The rest of the globe (for this purpose at least) belongs to JTWC.

NHC, CPHC, and JTWC prepare their forecasts and advisories on a computer platform known as the Automated Tropical Cyclone Forecast system (or ATCF).  Tropical cyclones are followed on the ATCF using identifiers such as AL032014; this particular identifier would decode as the Atlantic basin’s third tropical cyclone of the 2014 season.  Invests are given identifiers using the numbers 90 through 99 in place of the cyclone number, so the first Atlantic invest of 2014 was AL902014, or AL90 for short.  After AL992014 is used, we would cycle around and reuse AL902014, so unlike the ATCF identifiers for true tropical cyclones, invest identifiers are not unique.

NRL Page
Example of the Naval Research Laboratory Tropical Cyclone web page from June 21, 2014.

Once NHC or one of the other forecast centers “opens an invest”, data collection and processing is initiated on several government and academic websites, including those at the Naval Research Laboratory (see example to the right) and the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin.   Information on these sites, along with the standard suite of models run on the invest, then helps NHC prepare the genesis forecasts that appear in the Tropical Weather Outlook.

It’s important to recognize that the designation of a disturbance as an invest does not correspond to any particular likelihood of development of that system into a tropical cyclone.  Indeed, we will open an invest in part to help us determine what that likelihood is.  Also, and particularly near the beginning of the season, it’s not uncommon for NHC to create one or more invests solely to test data flow or model processing scripts. The Tropical Weather Outlook should always be consulted to determine the significance or potential threat of an invest disturbance.

No Insider Trading

ATCF databases have traditionally been posted to NHC’s public FTP server to facilitate the exchange of information with modelers and other quasi-operational groups, and to make model guidance available to the private sector.   Unfortunately, posting of the ATCF data to the FTP site allowed some pre-decisional information to leak out to those who knew exactly where to look.  For example, one could find the command that renumbers the invest system AL902013 to the tropical cyclone AL012013.   Renumbering an invest is a process the NHC Hurricane Specialist needs to invoke in order to prepare the first advisory on a tropical cyclone, even though the final decision to release that advisory might not be made for another hour or two.  In the early years of the FTP site, these leaks seemed to fly under the community’s radar, but over time had become increasingly known.  Anticipation of new cyclones began to cause problems for us and for our partners in emergency management and the media, some of whom would infer or even prematurely announce that we were going to start advisories.  (And indeed, occasionally we’ve had to change our minds and not initiate advisories on a renumbered invest).

In 2014, NHC has made some changes to how data from the ATCF get publicly posted.  The most significant of these is the establishment of a blackout period, during which changes made to the ATCF storm ID and some other parameters will not flow to the FTP server.  The blackout period begins 90 minutes prior to the nominal advisory release time (e.g., 9:30 a.m.) and ends at the nominal advisory release time (e.g., 11:00 a.m.).  Quasi-operational websites that make use of ATCF data will now draw their data from the FTP server rather than NHC’s internal databases.  In this way, everyone will be able to learn about an NHC advisory, and know for sure that it’s coming, all at the same time when that advisory is released.  We want to emphasize that while the blackout period will restrict the release of pre-decisional information, it will not restrict the distribution of model guidance used by private-sector forecasters.

The Closing Bell

TS Lee ATCF
Track models from the ATCF for Tropical Storm Lee in 2011.

One final thought on invests.  NHC knows that lots of folks, including non-meteorologists, look at the tropical cyclone models.  While we make model data available on our FTP site for use by the meteorological community, and the public can find these data displayed all over the Internet, we deliberately avoid enhancing their visibility or prominence by posting model plots on our own website.  This is particularly important for invests, where the model guidance is notoriously poor and erratic, partly because many of these models were never designed to be run on disturbances.  NHC’s Hurricane Specialists work hard, based on their knowledge and experience, to interpret all the available models and other data in the formulation of their official forecasts and warnings, and in so doing help NHC continue to be America’s calm, clear, and trusted voice in the eye of the storm.  And hopefully stay out of Weird Al Yankovic’s sequel to “Word Crimes.”

— James Franklin

Storm Surge–Plain and Simple (Part 2)

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In our last storm surge post, we talked about the need for a storm surge graphic and why we use “above ground level” to communicate storm surge forecasts.  Now we’ll discuss how we create the new storm surge graphic.
But first, we need to touch on how forecast uncertainty relates to storm surge forecasting.eggs

Putting All Your Eggs in One Basket

The exact amount of storm surge that any one particular location will get from a storm is dependent on a number of factors, including storm track, storm intensity, storm size, forward speed, shape of the coastline, and depth of the ocean bottom just offshore.  Needless to say, it’s a complex phenomenon.  Although we’re getting better on some aspects of hurricane forecasting, we still aren’t able to nail down the exact landfall of the storm or exactly how strong and big the storm will be when it reaches the coast.  This means that there is a lot of uncertainty involved in storm surge forecasting.  Here’s an illustration showing why all of this is important.

Here’s the forecast track for a Category 4 hurricane located southeast of Louisiana and only about 12 hours away from reaching the northern Gulf Coast:

Ivan forecast

Here’s the question:  how much storm surge could this hurricane produce in Mobile, Alabama, and Pensacola, Florida (marked on the map)?  If we take this forecast and run it through SLOSH (the National Weather Service’s operational storm surge model), here’s what you get:

Mobile surge

The forecast has this hurricane making landfall near Dauphin Island, with the center moving northward just west of Mobile Bay along the black line.  You can see from this map that water levels will rise to at least 14 ft. above NGVD29 (the particular reference level we are using in this scenario) in the upper reaches of Mobile Bay while they will rise to about 2 ft. above NGVD29 in the Pensacola area.  What’s the problem with this storm surge forecast?  It assumes that the track, intensity, and size forecasts of the hurricane will all be perfect.  This is rarely, if ever, the case.

Here’s what actually happened with this hurricane.  The storm turned ever so slightly toward the east and made landfall about 30 miles east of where the earlier forecast had shown it moving inland.  Despite the shift, this was a good track forecast–30 miles is more or less typical for a 12-hour error.  So, what kind of storm surge resulted from the actual track of this hurricane?  If we take the actual track of the storm and run it through SLOSH, here’s what we get:

Ivan actual       Pensacola surge

Since the center of the hurricane actually moved east of Mobile Bay, winds were pushing water out of the bay, and the water was only able to rise about 4-5 ft. above NGVD29 near Mobile.  On the other hand, significantly more water was pushed toward the Pensacola area, with values as high as 12 ft. above NGVD29 in the upper reaches of Pensacola Bay.

This scenario was an actual storm–Hurricane Ivan in 2004.  If emergency managers in Pensacola at the time had relied on that single SLOSH map that was based on a perfect forecast (or, put all their eggs in one basket), they would have been woefully unprepared and may not have evacuated enough people away from the coast.  Granted, such decisions would have been made more than 12 hours away from landfall, but at that time, forecast errors are even larger and make storm surge forecasting even more difficult.

Scrambled Eggs?Irene track

If you’re going to put all your eggs in one basket, you might as well scramble them beforehand so that they don’t break when you drop the basket.  In a sense, that’s what we do when trying to assess an area’s storm surge risk before a tropical cyclone.  Instead of assuming one perfect forecast, we generate many simulated storms weighted around the official forecast–some to the left, some to the right; some faster, some slower; some bigger, some smaller–and then run each of those storms through SLOSH.  We then “scramble” the SLOSH output sizefrom all storms together and derive statistics that tell us the probability of certain storm surge heights at given locations along the coast.

If we go back to our example from Hurricane Ivan, we can see the value of this method in assessing storm surge risk.  The image below shows the probability that the storm surge would reach at least 8 ft. above the reference level (NGVD29) for Ivan from the NHC Tropical Cyclone Storm Surge Probability product.  The first thing that should jump out at you is that the probability of at least 8 ft. of surge was just about equal in Mobile Bay (60-70% chance) and Pensacola Bay (50-60% chance).  The probabilistic approach indicates that both areas were at a significant risk of storm surge, and both areas should have been preparing similarly for the arrival of the storm.  Because we accounted for the uncertainty in the official forecast, we were able to assess the true storm surge risk for all areas near the coast.

Ivan psurge

The Tropical Cyclone Storm Surge Probability product provides the data that are used to create the Potential Storm Surge Flooding map that will be available experimentally beginning in the 2014 hurricane season.  In other words, the Potential Storm Surge Flooding map accounts for the uncertainties associated with NHC’s tropical cyclone forecasts.  In Part 3 of this storm surge series, we’ll talk more about the map itself and how it should be interpreted.

— Robbie Berg and Jamie Rhome

Debut of the 5-Day Graphical Tropical Weather Outlook

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For many years NHC’s forecasts of tropical cyclone formation extended only 36-48 hours into the future. Recent advances in numerical modeling, however, as well as improved understanding of some of the physical triggers for genesis, prompted NHC to begin an in-house experiment to see whether its genesis forecasts could be extended.  The four-year experiment showed, somewhat surprisingly, that a five-day tropical cyclone forecast could be made with nearly the reliability of the existing 48-hour forecasts, and NHC publicly extended the range of its Tropical Weather Outlook (TWO) text product to five days on August 1st of 2013.

In its present form, the text TWO describes areas of disturbed weather and their potential for development into a tropical or subtropical cyclone.  This description normally includes discussion of the large-scale factors that could influence development, the general motion of the disturbance and any hazards that might affect land areas, and concludes with a quantitative forecast of formation likelihood for both the next 48 hours and the next five days.

The New 5-Day Graphic Explained

Beginning today, July 1, 2014, at 2 PM EDT (11 AM PDT), the text TWO will be accompanied by an experimental graphical depiction of the five-day potential cyclone genesis areas.  These areas will appear as color-coded hatched areas (yellow, orange and red representing low, medium, and high risks of tropical cyclone formation, respectively).  Although the areas or swaths don’t explicitly represent a track forecast, they do provide a general indication of where these systems are headed whenever the formation potential extends over several days.

5Day Graphic

If a hatched formation area is associated with a currently existing disturbance, the location of the disturbance is marked with an ‘X’ on the graphic. Arrows are used to link the location of a disturbance with its potential genesis area if the formation area is displaced from the current location of the disturbance.  The overview graphic (above) can occasionally become crowded with disturbances, especially during the peak of the hurricane season, so separate graphics for each disturbance are created to ensure legibility.

The introduction of the five-day graphic on July 1st will be accompanied by an important change to the existing 48-hour graphic.  Disturbances on this graphic will no longer be identified with circles or ovals; instead the location of current disturbances will be marked with an “X” for consistency with the five-day graphic.

48 Hour Graphic

In a future blog post we’ll be talking about how NHC’s Hurricane Specialists arrive at the formation probabilities appearing in the TWO, as well as some experimental guidance and ongoing research projects that might allow us to extend these genesis forecasts even further in time.  In the meantime, we welcome user feedback on the new graphic, which can be provided at http://www.nws.noaa.gov/survey/nws-survey.php?code=FDGTWO

The following video also provides a description of the new 5-Day Graphical Tropical Weather Outlook:

 

 — Todd Kimberlain, Eric Blake, and James Franklin

Storm Surge–Plain and Simple (Part 1)

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lake2

You may have heard that NHC is unveiling an experimental storm surge graphic this hurricane season.  We mentioned in our first blog post on May 29 that we would be discussing the background and interpretation of this graphic.  There’s a lot to cover, so instead of throwing it all at you in one shot, we are going to do a three-part series on the new graphic and communication on storm surge in general.  Here’s what we plan on covering:

Part 1:  Why do we need a storm surge graphic?

Part 2:  How is the storm surge graphic created?

Part 3:  How should you interpret the storm surge graphic?

High Water Memo

So, let’s get on with Part 1.  First, let’s look back at a little history.  Way back in 1955, the U.S. Weather Bureau issued a memo (figure to the right) to weather offices along the coast, directing them to refer to any water rise produced by a hurricane or tropical storm in terms of “above normal tide levels,” and those rises were to be specified in ranges to account for uncertainty.  Believe it or not, that policy went unchanged for over 50 years!  In 2008, the NHC Public Advisories for Hurricane Ike referred to storm surge like this:

“COASTAL STORM SURGE FLOODING OF UP TO 20 FEET…WITH A FEW SPOTS TO NEAR 25 FEET…ABOVE NORMAL TIDES ALONG WITH LARGE AND DANGEROUS BATTERING WAVES…CAN BE EXPECTED NEAR AND TO THE EAST OF WHERE THE CENTER OF IKE MAKES LANDFALL. THE SURGE EXTENDS A GREATER THAN USUAL DISTANCE FROM THE CENTER DUE TO THE LARGE SIZE OF THE CYCLONE. WATER LEVELS HAVE ALREADY RISEN BY MORE THAN 5 FEET ALONG MUCH OF THE NORTHWESTERN GULF COAST.”

For many years, we didn’t have the technology, nor sufficient accuracy in our track forecasts, to be any more specific in our Public Advisories.  The best we could do was give an estimate of the highest storm surge expected with a general description of where that surge could occur relative to the center of the storm.  Unfortunately, many times these statements were too vague for emergency managers and other decision makers to make sound decisions before a storm.  One question a statement like this could not answer:  “How far inland could the storm surge go?”

Another issue had to do with what are called vertical datums.  We’ll leave the more technical discussion of vertical datums for another blog post, but what you need to know for this discussion is that a vertical datum is simply a reference point.  The water level height caused by the combination of storm surge and the tide must be attached to some point of reference.  The operative question is “the height of the water level is 6 feet above what?”  The problem was that many people either weren’t specifying what the datum was, or they were confusing one datum with another.

Here’s an example, again using Ike, where confusion set in.  The figure below shows output from the National Weather Service SLOSH model indicating simulated water level heights from Hurricane Ike along the Texas and Louisiana coasts.  What’s the first thing that jumps out at you?  The first question many people have is why do the values increase (go from 15 feet to over 21 feet) as you move inland from the coast into Chambers and Jefferson Counties in Texas?  Shouldn’t the deepest water have occurred at the immediate coast?  The subtlety here is that the water level in this picture is depicted relative to a datum called NAVD88.  So, the water levels in Chambers and Jefferson Counties were more than 21 feet above NAVD88, not 21 feet above the actual ground at those locations.

Ike Surge NAVD

Luckily, there’s a way to display how much water was sitting on normally dry ground, which is what most people typically envision when given storm surge heights.  Since we know what the elevation of the land is at each location, relative to the same vertical datum used for the surge data itself, we can subtract the land elevation from the surge heights to get a good idea of how high the water was above the ground at each location.  The next figure is the same simulation for Ike but instead shows this subtraction at play.  Notice any differences from the previous image?

Ike Surge Ground

Now it should all make sense.  The highest values (about 15 feet above ground level) are located along the immediate coast and decrease as you move inland.

Recent hurricanes like Katrina, Rita, and Ike showed that we needed to make some changes in the ways that we communicate storm surge information.  And thankfully, we now have the technologies and capabilities to go beyond simplified text statements in the Public Advisory.  In Part 2 of this series, we’ll talk about the Probabilistic Storm Surge product, how it accounts for uncertainties in the storm surge forecast, and how it is being used to create the Experimental Potential Storm Surge Flooding Map for this hurricane season.

— Robbie Berg and Jamie Rhome

The “Outreach and Education” Season

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2014 NHC Outreach Slide

The staff at the National Hurricane Center is often asked about what they do during the “off-season.”   The off-season (December 1st thru May 15th) is a very busy time for employees of the Center.  Meteorologists in the Tropical Analysis and Forecast Branch continue their year-round forecast responsibilities, and staff in the Technology and Science Branch develop new forecast tools, upgrade user interfaces, and maintain NHC’s computers.  During the off-season, the Hurricane Specialist Unit’s around-the-clock forecasting role ceases; however, the staff take on other important functions that help improve forecasts and better prepare the public for the next hurricane event.  The Hurricane Specialist Unit’s off-season activities fall generally within the following areas:

  • Complete Tropical Cyclone Reports, seasonal review articles, and forecast verification of the previous season’s tropical cyclone forecasts
  • Work on tools to make the forecast process more efficient
  • Incorporate new scientific techniques and modeling to improve forecast accuracy
  • Develop enhancements to NHC tropical cyclone products and services
  • Provide outreach and education to key partners and customers

Each area of off-season focus is an important aspect in NHC’s ability to improve its services.  The outreach and educational component increases emergency manager and media understanding of NHC products, and public awareness of hurricane hazards and risk.

Training for Emergency Managers and Decision Makers

FEMA logoNHC staff facilitated nearly 10 weeks of training for emergency managers and fellow meteorologists throughout the United States and Caribbean during this past off-season.  Each year, the outreach and education period begins in earnest in January, when three one-week FEMA Hurricane Preparedness for Decision Maker courses are conducted at NHC.  Local and state emergency managers from the gulf, southeastern, and northeastern U.S. coastal areas learn about the NHC forecast process, products, and forecast uncertainty.  One day of the course is devoted to the storm surge hazard.  Partners from the Federal Emergency Management Agency and U.S. Army Corps of Engineers provide information on the tools available that assist emergency managers in evacuation decision making.  Since the course is held at the NHC, it also allows an opportunity for the NHC staff to meet and interact with emergency managers that help protect local communities during tropical cyclone threats.  The course began in 1992 and continues to be refined today.  A one-day version of the course is taught at some state and/or national hurricane conferences, and a three-day version of the course is offered to one state each year.  This past off-season the three-day course was taught at the New Jersey Office of Emergency Management in West Trenton.

Training for International MeteorologistsWMO logo

In March, the NHC hosts a two-week World Meteorological Organization RA-IV Workshop on Hurricane Forecasting and Warning.  Forecasters from national meteorological agencies from 15 to 20 countries in the Caribbean, North and South America, and Asia participate.  The course is conducted in both English and Spanish and the visiting forecasters learn details about tropical analysis, satellite observing tools, and how NHC constructs tropical cyclone forecasts.

Training for National Weather Service Meteorologists

408491This past outreach and education season also featured two National Weather Service (NWS) Effective Hurricane Messaging Courses.  These workshops provided local NWS forecasters the opportunity to more thoroughly understand how NHC forecasts are made and how best to communicate potential tropical cyclone hazards to emergency managers, the media, and the public.  The workshop also allowed NHC staff and NWS forecasters to become more familiar with each other’s responsibilities during hurricane events.  The workshop will help strengthen the NWS tropical cyclone warning coordination process and ensure a consistent message is communicated throughout the agency.

Discussions during these gatherings often focus on how best to communicate the tropical cyclone threat and potential hazards.  These discussions sometimes result in ideas for new products or enhancements to existing NHC products and services.

NHC’s mission to save lives and mitigate property loss begins with a better public understanding of the hazards posed by tropical cyclones.  Next time you think of the NHC “off-season”, remember it as the “Outreach and Education” season.  As former NHC Director Max Mayfield said, “the battle against hurricanes is won outside the hurricane season.”  Take the time to educate yourself before the next tropical cyclone threat by learning about hurricane and storm surge risk in your community.  If you live in an evacuation zone, have a plan and a designated place to go to ride out the storm.  Become hurricane prepared!  For more information on hurricane preparedness see http://www.hurricanes.gov/prepare or http://www.ready.gov/hurricanes.

— Dan Brown and Robbie Berg