Author: Robbie Berg
Get a Plan!
A lot of the advice you hear in advance of hurricane season includes some variation on “Get a Plan”. And you may have thought, “Why? How difficult can it be?” The truth is, it’s really not difficult. Most of the things that I want you to consider when dealing with a natural disaster such as a hurricane are basic, common-sense issues that you deal with every day of your life. The problem is that when a hurricane or other severe weather is bearing down on you and your family, the things that you normally take for granted may be in short supply, and you’ve got to get a lot done in a very short period of time. In those circumstances, having gone through the planning process with your loved ones, and developing a written plan to help guide your actions, could save your life.
Simply considering developing a plan is a very good first step. It will help you think about the hazards where you live. Are you close to the ocean and potentially in a storm surge zone? Do you live in a flood plain? Do you live in a house likely to be damaged in high winds? Are the roads you rely on to get to the grocery store or the pharmacy prone to flooding? Knowing how a storm can affect you will help determine what kind of plan you should develop.
Once you know how a hurricane will affect your physical environment, consider what that means for your loved ones. Do you care for young children or elderly relatives? Do you have pets or livestock that you need to care for? Think about how a prolonged period without power or water will affect them. What would you do if you couldn’t get to the grocery store for a few days, or if the gas station ran out of fuel?
Once you’ve thought about these issues, though, take the next step and write down your answers. When a hurricane is coming, having a written plan will help ensure that you’re taking care of the most important things. And by going through the planning process together, you and your family will be more confident that you’ll be able to survive the storm and get your life back to normal as quickly as possible. Having a guide to make sure you’ve taken care of the essentials will give you peace of mind and allow you to focus on keeping your family safe.
Will everything go exactly as you planned it? Probably not. There are so many variables in a hurricane and so many unknown factors that you’ll likely have to improvise. But it’s much easier to deviate from a well-considered plan than it is to try to develop a course of action from scratch during the middle of the event.
There are a lot of resources to help you. In Florida, try www.flgetaplan.com. In South Carolina go to http://www.scemd.org/planandprepare/preparedness/famdiasterplan, and in Texas a good resource is http://www.texasprepares.org/. Other great sites include The Red Cross and FEMA’s Ready.gov site. And there are many more; simply search “disaster plan” on the internet and you’ll find plenty of options. All of them are very good, and any one of them will help you accomplish your goal of getting you ready. Kids can even get involved by helping their parents fill out a family communication plan.
Hurricanes are a fact of life, and their impacts can be catastrophic. If you are well prepared for them however, the likelihood that you will survive and be able to recover quickly are tremendously improved. That preparation starts with developing a plan to guide you through the situation. An hour or two spent now on writing your plan could save your life!
Identifying Trusted Sources for a Hurricane Event
Dr. Gina Eosco
Senior Social Scientist/Risk Communication Specialist, Eastern Research Group
@WxComm / @ERGupdate
Hurricanes are dangerous! Extreme winds, storm surge, inland flooding from heavy rains, and tornadoes can all be life threatening. Understanding your risk from these threats is important—and a trusted, credible source for hurricane information can help.
- The National Weather Service, including the National Hurricane Center
- TV stations
- Social media
- And others… like friends and family
But, are all sources of weather information the same? Are all sources trustworthy?
To answer that question, you first have to define what is meant by “trustworthy.” As it turns out, several factors contribute to people’s perceptions of trust, including the source’s knowledge and expertise, openness and honesty, and concern and care.[i] Using these factors of trust, here are some tips to identify trusted sources of hurricane information:
Does your source have hurricane knowledge and expertise? Knowledge and expertise are gained through a combination of study and practical experience. Does your source hire staff with degrees in meteorology? In addition to formal degrees, the American Meteorological Society certifies individuals who have “achieved a high level of competency in communicating complex weather.” The National Weather Association also offers a certification program. These certifications and corresponding logos are shown on TV next to a broadcaster’s name, online, or in someone’s biography.
Of equal importance is how many years of experience does your source have? Hurricanes are like fingerprints. No two storms are the same. Forecasters must observe, collect, and analyze complex data to determine the hurricane track, intensity, and potential impacts, like storm surge. It stands to reason that the more experience a source has with different storms, the more knowledge and expertise that individual has gained over time.
Determining if a source is open and honest is a bit more subjective. Here are a few guiding thoughts.
Does your source convey what he or she knows and doesn’t know? There is always some uncertainty with a storm. Communicating uncertainty is vital to an open dialogue about the risk of hurricanes. For example, during Hurricane Joaquin in September and October of 2015, the National Hurricane Center issued a new “key messages” section in their Tropical Cyclone Discussion and also sent it out on Twitter, at one point stating, “Because landfall, if it occurs, is still more than three days away, it’s too early to talk about specific wind, rain or surge impacts from Joaquin in the United States.” NHC openly shared what it did and did not know about the storm.
The last factor that leads to trust is perceiving that a source has concern and care for you. You might get the impression that a source feels concern and care by his or her tone of voice or word choice—but how do you really know? One way to gauge concern and care is by understanding the source’s mission. For instance, the National Hurricane Center explicitly states that its mission is
“To save lives, mitigate property loss, and improve economic efficiency by issuing the best watches, warnings, forecasts and analyses of hazardous tropical weather, and by increasing understanding of these hazards.”
Hurricanes can be fearful experiences to live through, and evacuation decisions can be equally as stressful. Hurricane risk information from a trusted source can calm those fears and provide the details you need to make an informed decision that is right for you.
Hurricanes may vary in their strength, but with trusted sources by your side, you can always remain #HurricaneStrong.
[i] Peters, R. G., Covello, V. T., & McCallum, D. B. (1997). The determinants of trust and credibility in environmental risk communication: An empirical study. Risk analysis, 17(1), 43-54.
President and CEO, Federal Alliance For Safe Homes (FLASH)©
@LCHenderson / @FederalAlliance
If you’ve been following along with us this National Hurricane Preparedness Week and joining us to get #HurricaneStrong, you’ve already moved through the first four steps on your path to the ultimate state of readiness. You’ve determined your risk, developed an evacuation plan for your family, secured an insurance check-up, and assembled your disaster supplies.
So, now it’s time to focus on our message for Day Five–Strengthen Your Home. This may be the most challenging of the critical steps to preparedness, but it is also one of the most rewarding. Having a strong home will give you the peace of mind that comes with feeling safe and sound.
If you have a strong home, and reside outside of a storm surge evacuation zone, you should be able to take shelter in your home. This will keep you, your family, and your pets together, off the roads, and out of the shelters. This is especially important as shelter space is limited, and should be reserved for those that must evacuate.
Also, if a hurricane strikes, your strong home can be the key ingredient for a swift recovery after the storm because strong, high-performing homes resist the wind and water that come with a hurricane. They suffer less damage, save you money, and reduce or eliminate repair time.
So how can you get a strong home?
First, it is important to understand the different ways that hurricanes can damage a home.
Hurricanes can cause damage to buildings and homes in a variety of ways:
- High winds put pressure on the home and its connections
- Windborne debris batters and breaks windows, doors and garage doors
- Wind-driven rain enters through openings like windows and doors
- Rising waters flood inside homes, damaging appliances, electrical systems, flooring, and more
- Waves and storm surge batter a home, causing it to break apart or even wash away
High winds exert extreme pressure on your home, and cause four types of building failure. Uplift occurs when the wind moves over the structure pulling upward, especially on the roof. Racking happens when horizontal pressure causes the house to tilt. Sliding occurs when the horizontal wind pressure pushes a house off its foundation. And, overturning occurs when a house resists horizontal pressure, and won’t rack or slide.
These high wind failures can be prevented or effectively lessened when a home is well-connected with the right amount of nails and metal connectors. The key connections to reinforce include the roof-to-upper story; upper story to first floor; and house to foundation. When you make these connections the right way, you will have a continuous load path that ties your home together from the roof to the foundation.
If you are getting ready to build a new home, remember, it is possible to build a hurricane-resistant home from a variety of materials, including wood, engineered wood, and concrete products like concrete block, insulated concrete forms, cast-in-place concrete, and more. Many materials provide the additional benefits of durability, energy savings, and sustainability, so it’s important to research all of your options before you begin.
Windborne debris batters and breaks windows, doors, and garage doors, but this can be prevented by using impact-resistant windows and doors, or tested and approved hurricane shutters. Shutters can be permanently installed, or temporarily. Even the proper thickness of 5/8” plywood cut to fit each window and opening is effective if properly mounted with the right fasteners. Click here to view our videos about hurricane shutters, impact-resistant windows, and how to make temporary, emergency panels.
Wind-driven rain can be kept out of your home if you refresh the caulk around your windows and the flashing around doors at least once per year. Hurricane shutters will also help prevent this damage.
Rising waters flood inside homes, damaging appliances, electrical systems, flooring, and more. Click here to use this calculator and see the potential damage to your home from flooding. What you’ll learn is that only four inches of water can cause nearly $30,000 in damage.
The best financial protection from flood damage is flood insurance, but you must purchase a separate policy as it is not part of your homeowner’s insurance coverage. Also, flood policies carry a 30-day waiting period, so it is important to secure your protection now before the hurricane season begins.
In addition to flood insurance, there are many steps you can take to minimize flood damage. This animation provides a comprehensive overview, and here are some examples:
- Elevate electrical outlets
- Anchor fuel tanks
- Install a floating drain
- Elevate appliances inside and outside
It is also important to keep your gutters and downspouts around the home clear of debris, so they can carry the water away from your foundation.
If flooding is imminent, you should secure sandbags. If they are properly filled, placed, and maintained, sandbags can redirect storm water and debris flows away from a home and other structures. Follow the guidance below to make sure you use and then dispose of them correctly.
- Fill sandbags one-half full.
- Use sand if readily available, otherwise use local soil.
- Fold top of sandbag down and rest bag on its folded top.
- Take care in stacking sandbags.
- Limit placement to three layers unless a building is used as a backing or sandbags are placed in a pyramid.
- Tamp each sandbag into place, completing each layer prior to starting the next layer.
- Clear a path between buildings for debris flow.
- Lay a plastic sheet in between the building and the bags to control the flow and prevent water from seeping through openings like sliding glass doors.
- Sandbags will not seal out water.
- Sandbags deteriorate when exposed to continued wetting and drying for several months. If bags are placed too early, they may be ineffective when needed.
- Sandbags are for small water flow protection up to two feet. Protection from larger flow requires a more permanent flood prevention system.
- Wet sandbags are very heavy and caution should be used to avoid injury.
It is important to consult your local environmental protection department before disposing of used sandbags. Sandbags exposed to contaminated floodwaters may pose an environmental hazard and require special handling.
Storm surge and waves are often the most destructive threats to homes in the path of a hurricane. So, whether you live near the coast where storm surge is possible, or you are inland where rising waters from heavy rains threaten, it is best to build or buy a home that is elevated above the Base Flood Elevation (BFE) or the expected level of flooding established by the FEMA flood maps.
No matter where you live, the best protection from hurricanes or any natural disaster is to ensure that your home is constructed to meet or exceed current building codes. Post-disaster investigations have proven again and again that homes built to code have the best chance of surviving. And when you have a strong home that survives the wind and the water from hurricanes, you will become resilient in the face of the storm.
Put an Insurance Review on Your Hurricane Prep To-do List
Jeanne M. Salvatore
Senior Vice President and Chief Communications Officer, Insurance Information Institute
@JeanneSalvatore / @iiiorg
Most people don’t put insurance on their hurricane preparedness checklist. But they should! Having the right amount and type of insurance is a crucial component of disaster preparedness. And the good news is that it only takes one simple step – a call to your insurance professional.
Making sure you understand all your insurance options and updating insurance coverage should be as routine as an annual physical. Think of it as a way to maintain a healthy financial future if a natural disaster were to strike.
Prior to a conversation with your insurance professional, take a few minutes to write down all of your insurance questions. Or even better, pull out and read your current insurance policy. This is bound to lead you to more questions, and that’s a good thing.
An important partner in hurricane preparedness is an insurance agent or company representative who is willing to answer all of your questions and clearly explain what is covered and what is not. If he or she can’t answer questions to your satisfaction, find another insurance professional. Insurance is a very competitive business, and there are many insurers who would like to deliver top-notch service to you and your family.
Know which disasters are covered in a homeowners policy and which are not. Wind damage is covered, no matter if the cause is a tornado, catastrophic hurricane or just a summer storm. When a wind blows, it is going to be covered by standard home and business insurance policies. Keep in mind that in most coastal states there is both a standard deductible and a hurricane deductible. The former is a flat dollar amount, such as $1,000, while the latter is typically a percentage of the insured value of your home.
The one big hurricane-related disaster that is not covered by standard policies is flooding. Fortunately, flood insurance is available from the National Flood Insurance Program (NFIP) and from a few private insurance companies.
Did you know that 90 percent of all natural disasters involve some form of flooding? That is reason enough for everyone to consider flood insurance. And don’t be lulled into the false notion that only areas bordering large bodies of water get flooded. Floods can occur wherever it rains, and at least 20 percent of flood claims are paid to people who live in low- to moderate-risk flood zones. In general, the lower your flood risk, the less you pay for protection. Learn more about your risk of flooding and the estimated cost of an NFIP flood insurance policy by going to www.floodsmart.gov.
I always remind people that in addition to the right kind of coverage, they also need the right amount of insurance. What that means is having enough insurance to completely rebuild your home and replace all its contents. Rebuilding costs are often a lot higher than a home’s real estate or market value, because the costs of building materials and labor continue to rise.
Be smart. Be ready. Ask questions. Give yourself confidence that your insurance is up to date and ready to protect your financial future – before hurricane season starts. For more information on insurance, visit the Insurance Information Institute’s website at www.iii.org.
Do You Know Your Zone?
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.
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
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.
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.
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.