Forecasting
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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Debut of the 5-Day Graphical Tropical Weather Outlook
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.
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.
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
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Storm Surge–Plain and Simple (Part 1)
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
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