Author: Robbie Berg
Develop an Evacuation Plan
Do You Know Your Zone?
Craig Fugate
Former Administrator, Federal Emergency Management Agency (FEMA)
@fema
Hurricane season is almost here. The season officially starts June 1 and ends November 30. During these six months, forecasters watch hurricanes as they develop offshore. While we may see a hurricane coming, we won’t know the exact impact it will have on every community until it makes landfall. To ensure the safety of you and your family, don’t wait until it’s too late to prepare; find out your hurricane evacuation zone today.
It only takes one hurricane to change your life and your community. Now is the time to prepare. When a hurricane hits, it can bring high winds, heavy rainfall, storm surges, coastal and inland flooding, rip currents, and even tornadoes as part of a destructive, hard-hitting package. That’s why if you live in an area where hurricanes are a threat, you need to know where you’d go before the danger arrives and makes evacuation impossible.
Remember these key tips when it comes to hurricane preparedness:
Know your evacuation zone. Evacuations are more common than people realize. Many communities have designated evacuation routes and evacuation zones. Make yourself familiar with these evacuation zones, so if your local authorities issue an evacuation order, you’ll know if you need to leave. It’s also a good idea to know where you’d go if told to evacuate. Be sure to account for your pets, as most local shelters do not permit them. However, by law, public shelters do accept service animals. Remember: if a hurricane threatens your community and local officials say it’s time to evacuate, don’t hesitate — go as soon as you can.
Complete a family communication plan. Plan how you will assemble your family and loved ones, and anticipate where you will go depending on the situation. Choose several destinations in different directions so you have options in an emergency, and know the evacuation routes to get to those destinations. Get together with your family and agree on the ways to contact one another in an emergency, identify meeting locations, and make a Family Emergency Communication Plan.
Sign up for local alerts on your phone. Sign up now so you can stay aware if a storm threatens. Visit https://www.ready.gov/alerts and learn how to search for local alerts and weather apps that are relevant for hazards that affect your area. Download the FEMA app for disaster resources, weather alerts, and safety tips. Earlier this month, FEMA launched a new feature to its free smartphone app that will enable users to receive push notifications to their devices to remind them to take important steps to prepare their homes and families for disasters. The app also provides a customizable checklist of emergency supplies, maps of open shelters and open recovery centers, tips on how to survive natural and manmade disasters, and weather alerts from the National Weather Service for up to five locations across the nation.
Make sure your insurance policies are up to date. Hurricanes have caused eight of the ten costliest disasters in U.S. history, and strong winds or just a few inches of water can cause tens of thousands of dollars in damage to a typical home. Many states have increased deductibles for hurricanes, and not all hurricane-related losses are covered under traditional policies. Also, most homeowner’s insurance policies do not cover damage from flooding. Flood insurance ensures that consumers have adequate financial protection against the devastating effects of flooding, without having to rely on post-disaster loans (usually paid back with interest) or emergency assistance. If you have insurance, review your policy, ensure you’re adequately covered and understand any exclusions, and contact your agent for any needed changes. If you’re not insured against flooding, talk to your agent or visit floodsmart.gov. If you’re not a homeowner, renters insurance policies are also available and should be considered as they’re often low-cost methods of protecting your belongings.
Get prepared now and know what you’re going to do in the event of a hurricane. Planning ahead gives you more options and better control over situations that could become chaotic at the last moment if you’re not ready. To learn more about how to prepare for a hurricane visit ready.gov/hurricanes. Find out about preparedness drills or exercises in your area at ready.gov/prepare.
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Determine Your Risk
“It’ll Never Happen to Me”: Getting Past Barriers to Determining Your Hurricane Risk
Dr. Rick Knabb
Former Director, National Hurricane Center
@NWSNHC
“I’ve lived here for decades, and we’ve never had a hurricane. I figure I’m good.”
“We got hit last year, so this year it’ll be someone else’s turn.”
“I don’t want to evacuate and do all of those other things unless I know for sure it’s going to hit here.”
“I’m staying for a cat 1 or 2, and maybe a 3, but a 4 or 5? I’m outta here!”
“I just hope we don’t get hit this year.”
“It’ll never happen to me.”
These are actual statements people have made to me during my travels around the country over the years. Many of you have probably heard similar things. These are the “before” statements. What do people often say after they’ve actually been hit hard by a hurricane or other weather disaster? You know how it goes…
“In all the years I’ve lived here, I’ve never seen it like this.”
“No one told me it could be this bad.”
And this one really hurts to hear:
“I don’t have enough insurance. I’ve lost nearly everything.”
So many of us seem to be willing to take the gamble, do little or nothing, and hope that a hurricane doesn’t affect us where we live – even though we generally understand that we could lose a lot if a hurricane does come our way. Simply hoping that a hurricane doesn’t strike this year puts us in a weak position, even though we all want our families, homes, businesses and communities to be strong.
The first step for all of us to get past these initial mental barriers is to fully realize all of the hurricane risks we face. Your specific risks depend a lot on exactly where you live, but let’s get a few basics out of the way. Hurricanes are not just coastal events. Damaging and deadly winds, tornadoes, and rain-induced flooding can occur hundreds of miles from the coast and days after landfall. Even storm surges that affect coastal regions can go a lot farther inland – several miles in many locations – than many people realize. Too many unwary swimmers, surfers, and boaters have lost their lives from rip currents and waves near the coast from tropical cyclones that remain well offshore. In the United States over the past half century, nearly 90% of the direct deaths – those attributable to the forces of the storms –have been caused by water, with storm surge taking about half of these lives. Although winds can certainly be damaging and deadly, we must not underestimate how heavy, powerful, damaging, and deadly water can be when determining our hurricane risk.
Some topics get a lot of attention but can actually distract us from properly assessing our hurricane risk, and from taking the right steps to confront it. We hear a lot about what the various seasonal hurricane forecasts say, what El Nino is doing, or how long it’s been since the last hurricane hit our town, or our state, or our country. But none of these things tell you anything helpful about whether you’re going to get up close and personal with a hurricane this year. No one can tell you that months in advance; the five-day forecasts are challenging enough (remember Erika and Joaquin from just last year?). We make it a point to remind folks how little relationship there is between local impacts and seasonal activity; 1992 is the classic example, a “quiet” season with only one major hurricane. Yet that one was Andrew, which struck South Florida at category five intensity. The bottom line is that it only takes one hurricane or tropical storm to make it a bad year for you, and we need to prepare the same way every year for hurricane season, regardless of whatever expectations might be floating about for the season overall.
That’s the logical way to look at preparedness, but it can be even more motivating to consider the emotional aspects of getting ready for a hurricane. How my family and I contend with our risk from hurricanes (or other perils) is a very personal and emotional topic, and I suspect that if you think about it for any length of time, it would be for you as well. The emotional response from envisioning my family and home experiencing a hurricane is a great incentive for me to take steps now to get us ready in advance – long before an actual hurricane is on our doorstep.
I don’t just think about going through the storm itself – I also plan for the potentially nasty, dangerous, and lengthy aftermath. How would I feel if, for example, my home was severely damaged by wind, water, or both – and I didn’t have enough insurance to rebuild the home and replace its contents? How much longer would it take for our lives together to get back to “normal”? What would it do to our family’s financial future to try to recover without enough insurance? This is why we are visiting our insurance agent this month for an annual checkup. Whether you rent or own, live coastal or inland, visit your insurance agent and ask lots of questions to make sure you’re adequately covered. Don’t forget flood insurance, which must be obtained separately because it is not included in standard insurance policies. Along with promising yourself to never drive your car on a water-covered road (Turn Around Don’t Drown!), getting flood insurance is one of the best ways to deal in advance with your risk of inland flooding. Inland flood risk varies by location, but a good starting point is to know that for nearly all of us, if it can rain where you live it can flood where you live. Those who live close enough to the coast to be vulnerable to storm surge have that additional reason to get flood insurance. Update your insurance now, because waiting periods make it difficult or impossible to put new coverage in place when a hurricane is actually out there and you suddenly feel the urge to visit your agent.
The reason I go shopping for disaster supplies before every hurricane season is because the alternatives are extremely unpleasant at best and dangerous or catastrophic at worst. How would I feel if a hurricane was approaching and I decided to go shopping for those supplies at the very same time it dawned on everyone else who hadn’t stocked up in advance? I’d wait in horribly long lines for things my family desperately needs, but in many cases I’d find that stores were out of those items by the time I got there. The possibility of being isolated from emergency responders for days after the storm, with power out, stores closed, and no supplies is pretty frightening. I’ll be posting another blog entry later this week about what’s in my disaster supply kit.
My neighbors might think I’m strange, but they’ll soon see me testing my window shutters to make sure I can properly put them in place. It’s been a while since we’ve had to put them up for a real hurricane threat. But far better to make sure all is in order now, than risk having to tell my wife when a hurricane warning goes up that I can’t protect our home from the debris that might soon be flying around our neighborhood in hurricane-force winds.
There are so many ways that each one of us might identify things – some of them relatively simple and inexpensive – that we can do to make our home stronger before the next hurricane strikes. In addition, when you talk to your insurance agent, ask about what discounts you could get on your premium for making some improvements to your home that might not be as expensive as you expected and that might pay for themselves over time.
Those are just some of the actions I’m taking before hurricane season starts, and these are good topics for a national conversation. Today is the first day of National Hurricane Preparedness Week here in the United States, and for the first time the week coincides with the Hurricane Awareness Tour that this year visits Gulf Coast states. The week truly promises to be bigger, better, and more effective than ever before at helping us all get ready for hurricane season. This will be the second year in a row that we bring to each stop two different types of aircraft that are critical components of our hurricane monitoring and forecasting arsenal – a U. S. Air Force WC-130J from Keesler AFB in Biloxi, Mississippi, and the NOAA G-IV jet stationed at MacDill AFB in Tampa, Florida. In addition, several organizations that have already indicated their partnership with NOAA and the National Weather Service by becoming Weather-Ready Nation Ambassadors will now spring into action in the tour stop cities or in many other locations to promote hurricane preparedness and resilience.
We hope that this tour and other preparedness week activities leave you with a greater confidence in your government at all levels, knowing that federal, state, and local officials are devoting extensive resources to plan together in advance for the next hurricane. On display this week are some of the most advanced technologies and communications tools available to help us issue the most effective hurricane forecasts and warnings, in support of evacuation and other decisions by emergency managers and to promote communication of a consistent message by our media partners.
This post is the first in a series of daily Inside the Eye blog entries that will focus on a chosen theme of the day. Some of the articles have been written by prominent experts from the emergency management community and from our nonprofit partners, and we thank them for their contributions. Later this week we will focus individual days and blog posts more thoroughly on the topics of getting an insurance check-up, stocking up on disaster supplies, and strengthening your home. We will also close out the week by talking about identifying trusted sources of information and then putting it all together in a written hurricane plan for yourself, your family, and, if applicable, your business.
We will first focus tomorrow on planning for evacuation, which is how we can most easily determine and respond to our risk of storm surge. That’s the place to start if you’re feeling overwhelmed by everything involved in hurricane preparedness. The topics of insurance, supplies, and stronger homes have a lot to do with not only safety but also recovering more quickly and fully after the storm. But it’s hard to be resilient if you’re dead, and evacuations are called beforehand by emergency managers to save lives in large numbers, primarily from storm surge that has historically caused more fatalities than any other hurricane hazard. FEMA Administrator Craig Fugate is the author of our guest blog post on this topic, so I invite you back into this space tomorrow to hear it straight from the top of the emergency management community.
I’ll just hit the high points for the moment about evacuation planning. Find out today if you live in a hurricane evacuation zone, by contacting your local emergency management agency. Don’t assume that this is only a problem for people with beachfront property. In many coastal states, the risk of storm surge extends several miles inland in some locations. If you do live in one of these zones, decide today where you’d go and how you’d get there in a real hurricane event if told to evacuate by local emergency managers. Then, when that hurricane actually threatens and those officials instruct you to evacuate, you go! And here is my plea if you live far enough inland that you find out you don’t live in a hurricane evacuation zone: identify today someone you care about that does live in an evacuation zone, and you work it out to be their inland evacuation destination. Those of you who live in a mobile home (or any other structure that is not safe from strong winds) should not plan to host evacuees, as there is a very good chance that emergency managers will also tell you to evacuate to safer shelter.
So let’s go! Let’s go dedicate ourselves to being #HurricaneStrong as our friends from the nonprofit Federal Alliance for Safe Homes (FLASH) are encouraging us to do. You’ll see and hear a lot about that initiative this week, as it focuses on the same themes we’re talking about during National Hurricane Preparedness Week. It’s about realizing that we help decide our own outcome from the next hurricane, and then helping one another to flex our collective muscles and take action now, well in advance. We can’t just let that hurricane decide what happens to us while we sit on the sidelines. Our businesses, homes, families, and friends are worth the effort, and we simply owe it to ourselves. It’s just as personal for you as it is for me.
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The Alphabet Soup of Vertical Datums: Why MHHW is Mmm Mmm Good
Last weekend’s blizzard along the East Coast of the United States caused significant flooding along the coasts of Maryland, Delaware, New Jersey, and New York. Even though this system was not a tropical cyclone, the mechanics of storm surge flooding are essentially the same whether the cause is a hurricane or extratropical storm. The blizzard provides us an excellent opportunity to delve into the topic of vertical datums, which we promised to tackle in a previous blog post anyway!
You Say MLLW, I say MHHW (and undoubtedly someone else says NAVD88)
Simply put, a vertical datum is a reference level. Whenever you talk about water levels related to tides or storm surge, that water level needs to be referenced to some datum to provide essential context. For example, a water surface 2 feet above the floor means something very different than a water surface 2 feet above the roof.
There are many vertical datums out there. Some are based on tide levels (tidal datums), while some are based on the general shape of the Earth (geodetic datums). Savvy and more technical experts generally use geodetic datums such as the North American Vertical Datum of 1988 (NAVD88) because they’re more precise and applicable to a large area, such as an entire continent. For most of us, however, we see water levels referenced to tidal datums such as Mean Lower Low Water (MLLW) or Mean Higher High Water (MHHW).
Some locations along the coast have two high tides and two low tides per day (e.g., the U.S. East Coast), while some areas only have one high tide and one low tide per day (e.g., the U.S. Gulf Coast). Mean Lower Low Water (MLLW) is simply the lowest of the two low tides per day (or the one low tide) averaged over a 19-year period. This 19-year period is called the National Tidal Datum Epoch, which currently runs from 1983 through 2001. So to calculate MLLW for a particular tide station, the National Ocean Service (NOS) took the levels of all the lowest low tides from 1983 to 2001 and averaged them. Similarly, NOS calculates Mean Higher High Water (MHHW) by averaging the highest of the two high tides per day (or the one high tide) over the same 19-year period.
Historically, MLLW has been used for navigational purposes in the marine waters of the United States and its territories. Navigational charts from the NOAA Office of Coast Survey show water depths relative to MLLW, or how far the ocean bottom extends below the MLLW line. If boaters know the tide forecast relative to MLLW, the depth of the ocean bottom relative to MLLW, and the draft of their boat or ship (the distance between the waterline and the bottom of the hull), then they can deduce if the vessel will hit the sea floor. Since this is the most common way that tides have been referenced, the National Weather Service (NWS) has generally used MLLW as a reference for its water level forecasts, and most tide gauge data is referenced to MLLW by default. People who have lived along the same stretch of coastline for many years have become accustomed to knowing what type of coastal flooding will occur when water levels reach specific thresholds above MLLW.
But what about people who don’t know those relationships between MLLW–or any other datum for that matter–and coastal flooding (which change from location to location along the coast, by the way). For this reason, NHC has moved toward providing tropical cyclone related storm surge forecasts in terms of inundation, or how much water will be on normally dry ground. You can go here for more information on the Potential Storm Surge Flooding Map, issued by NHC when tropical cyclones are forecast to affect the East or Gulf Coasts of the United States. For the purposes of using water level observations to get an idea of how much inundation is occurring during a storm, NHC uses MHHW.
Why Does NHC Use MHHW When Looking at Water Level Observations?
To answer this question, it’s probably helpful to look at a cross-section of a typical coastline. Shown below is such a schematic, which depicts both the Mean Lower Low Water line and the Mean Higher High Water line. Anything seaward of the MLLW line is typically submerged under water. The region between the MLLW and MHHW lines is called the intertidal zone, and it is the region that is submerged at high tide and exposed at low tide. Intertidal zones include rocky shorelines, sandy beaches, or wetlands (marshes, mudflats, swamps, and mangroves). Because intertidal zones are submerged during a typical high tide, people don’t generally live here.
NHC and NOS consider anything landward of the MHHW line (marked as the supratidal zone in the graphic) as normally dry ground. Only in the most extreme high tide cycles and during storm surge or tsunami events does that region become submerged under water. Seawater that rises past the MHHW line is considered inundation, and therefore water level measurements relative to MHHW can be considered as proxies for measurements of inundation. NOS has deemed MHHW as the best approximation of the threshold at which inundation can begin to occur. While safe navigation of boats is a downward-looking problem that requires the use of MLLW, coastal flooding is an upward-looking problem that is best communicated using MHHW.
Dr. J. D. Boon, Professor Emeritus of the Virginia Institute of Marine Science, probably puts it best in his book Secrets of the Tide:
…we require the MHHW datum in order to isolate and evaluate storm surge risk
in a conservative way by removing the effect of tidal range – an independent factor
that varies from place to place.…US nautical charts use MLLW to reference charted depths conservatively so that
a mariner will know that the water depths shown on the chart can be counted on for
safe passage even at the lowest levels of the astronomical tide…Reversing direction and looking upward instead of downward, MHHW can be used to
conservatively reference storm tides so that coastal residents will know how much
additional rise to expect above the highest levels of the astronomical tide.These levels are generally familiar to the waterfront resident who witnesses signs of
their presence in wrack lines, marsh vegetation zones and high water marks on
structures.
We should mention that use of other vertical datums is in no way wrong. There are some very good uses for datums such as MLLW or NAVD88, but NHC uses MHHW when referencing storm tide observations to put things into a frame of reference that is understood by the majority of people at risk for coastal flooding. If we see a water level observation of 7 feet above MHHW, there’s a pretty good chance that some location in that area is being or was inundated by as much as 7 feet of water on ground that would normally be dry. This relationship worked quite well during Hurricane Sandy in 2012. Peak water levels measured by NOS tide gauges at the Battery in Manhattan and Sandy Hook, New Jersey, were between 8 and 9 feet above MHHW, and high water marks surveyed by the US Geological Survey after the storm indeed supported inundations of 8 to 9 feet above ground level in places like Sandy Hook and Staten Island.
MHHW and the January 2016 East Coast Blizzard
Since we said the recent blizzard provides a great case for us to explain vertical datums, let’s take a look at some of the water level observations during the event and how they compared to documented flooding.
Some of the worst storm surge flooding from the event occurred in extreme southern New Jersey and Delaware. So let’s look at the area around Cape May, New Jersey. The NOS tide gauge at Cape May measured a peak water level of about 9 feet above MLLW (8.98 feet to be exact). But does that mean that residents of Cape May and surrounding communities had as much as 9 feet of water on their streets? No, it just means that the water surface got about 9 feet higher than the “imaginary” line that marks the average of the lowest of the two low tides per day.
At the Cape May gauge, the difference between MLLW and MHHW is 5.45 feet, which means that the peak water level was only about 3.53 feet above MHHW (8.98 minus 5.45). Nearby, the peak water level observation from the NOS gauge in Atlantic City, New Jersey, was 3.42 feet above MHHW. So does that mean that residents of Cape May, Atlantic City, and surrounding communities had as much as 3 to 4 feet of water on their streets? Actually, yes it does. Pictures obtained via Twitter from West Wildwood, North Wildwood, and Atlantic City appear to support an estimate of 3 to 4 feet of inundation. See below for the evidence.
Incidentally, if you’re ever watching water level observations during a storm from the NOS Center for Operational Oceanographic Products and Services (CO-OPS) website, you can choose which vertical datum you’d like to use. The default will come up as MLLW, but you can change it to MHHW (as we do at NHC) or another datum such as NAVD88 or Mean Sea Level. Alternatively, NOS CO-OPS also provides a real-time “Storm QuickLook” website during coastal flooding events, and the default vertical datum on this page is MHHW. Below is a comparison of the water level data from Lewes, Delaware, during the blizzard using MLLW (top) and MHHW (bottom) as reference levels. Notice that the curves don’t change, only the reference numbers on the left vertical axis.
And finally, if you’re ever looking at storm surge forecast guidance online, make sure you know which vertical datum you’re looking at! For example, the NWS’s Extratropical Storm Surge (ETSS) model is available on the Meteorological Development Laboratory website, and although data shown is relative to Mean Sea Level (MSL), the vertical datum can be changed to MHHW or MLLW.
— Robbie Berg
Thanks go out to Cody Fritz, Shannon Hefferan, and Jamie Rhome from the NHC Storm Surge Unit, as well as the folks over at the National Ocean Service, for their assistance in putting together this blog post.
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Investing for Meteorologists
Teenagers 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.
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
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
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Storm Surge–Plain and Simple (Part 2)
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.
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:
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:
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:
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?
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 
from 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.
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
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