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?
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.
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?
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 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
NHC 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 Meteorologists
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
This 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
Welcome to “Inside the Eye,” the official blog of the National Hurricane Center!
Why have we launched a blog? The staff at the National Hurricane Center (NHC) regularly conducts training and educational workshops during the off season for specific audience groups, including emergency managers and other meteorologists. However, despite our heavy emphasis on outreach, there is always a large segment of the population that does not have the opportunity to hear from us in person on important changes to NHC products or discuss topics related to tropical and marine weather events. Therefore, we’ve launched this blog to be able to keep you, our partners and customers, more informed on a consistent and timely basis.
The National Hurricane Center has long had a vision of being America’s “calm, clear and trusted voice in the eye of the storm.” Much like the NOAA Hurricane Hunter aircraft in the picture to the right, we also want you to have a view from inside the eye. We want you to see what’s happening at the Hurricane Center before, during, and after a storm. We also want to help you understand how to interpret our forecasts so that even when there is a hurricane, you will have a clear understanding of what to expect and plan for.
Stay tuned for blog posts in the coming weeks. We have some big forecast product launches coming up this hurricane season, including an experimental potential surge surge flooding map and an extended 5-day graphical tropical weather outlook, and we plan to use the blog to provide information on the background and interpretation of these products. In addition, our staff has a wide range and diversity of expertise, so you can expect to see blog posts on many different subjects as we go forward. You may even see an occasional guest blog post from experts outside of NHC.
If you’d like to follow our blog and receive updates on blog posts, you can sign up to receive email notifications at the bottom of the column on the right. We also plan to advertise new posts on our various Twitter and Facebook accounts.