Storm surge risk for Tampa Bay

By: Dr. Jeff Masters , 1:21 AM GMT on August 18, 2008

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One possible landfall location for Fay is at Tampa Bay. This is of major concern, because over 3 million people live the region, and it is highly vulnerable to storm surge-- particularly for a storm moving northeast or north-northeast at landfall, as Fay is likely to be moving. This vulnerability results from the long stretch of shallow Continental Shelf waters offshore, which allow large surges to pile up. A surge in deeper water can be dispersed down and out away from the hurricane. However, once that surge reaches a shallow, gently sloping shelf, it can no longer be dispersed away from the hurricane. Consequently, water piles up as it is driven ashore by the wind stresses of the hurricane. Even a Category 1 hurricane can create significant surges--up to 7' in Hillsborough County, 6' in Manatee County, 7' in Pinellas County, and 9' in Pasco County. An extreme Category 5 hurricane can create a storm surge of 28' in Hillsborough County. These storm surge heights are computed from NOAA's Sea, Lake and Overland Surges from Hurricane (SLOSH) numerical storm surge prediction model. The data reflects only still water saltwater flooding. Local processes, such as waves, rainfall and flooding from overflowing rivers, are usually included in observations of storm surge height, but are not surge and are not calculated by the SLOSH model.

When compared to observations, the surge computed by the SLOSH model is in error on average, by 1.4 feet, according to a study by Jarvinen and Lawrence (1985). The maximum errors in the study, which looked at 523 sites during 10 hurricanes, were -7.1 feet and +8.8 feet. About 79% of the errors were within 2 feet of the predicted value, though. Errors primarily came from three factors:

1) Maps that are outdated, which may result in inaccuracies of topography or bathymetry
2) Anomalous water heights which can affect the local sea level
3) Local processes, such as waves, astronomical tides, rainfall and flooding from overflowing rivers, which are not calculated by the SLOSH model

The SLOSH model is run hundreds of times, with different storm speeds, directions of motion, and landfall location. The maximum surge height for all of these runs are then compiled into a Maximum of Maximums (MOM), which is then adjusted upward by two feet, for the observed range between average low tide and high tide in the Tampa Bay area (storm surge plus this high tide adjustment is called storm tide). The resulting storm tide map showing the locations likely to be inundated for a mid-strength hurricane of Category 1, 2, 3, 4, 5 is shown in Figure 1.


Figure 1. Worst-probable storm tide inundation (inundation from storm surge plus a 2-foot adjustment for the mean high tide that occurs in the Tampa Bay area). The colors scale to various Category hurricanes--1, 2, 3, 4, and 5. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.

Individual SLOSH runs are also of interest, and these are usually plotted up to show the Maximum Envelope Of Waters (MEOW) for a particular hurricane category, forward speed, and direction of movement. These are not adjusted for high tide, so one should adjust these numbers upwards by 2 feet if the storm were to hit at high tide. The current NHC forecast puts Fay at its closest to Tampa about 2 pm Tuesday afternoon, which also happens to be the time of high tide at the entrance to Tampa Bay. The MEOWs for Tampa Bay for a Category 1 and 2 storm moving NNE at 15 mph are shown in Figures 2 and 3. If Fay hits Tampa, the storm is likely to be moving NNE, but at a slower forward speed. A slower moving storm typically brings a higher surge into bays and inlets, since the slower motion allows more time to pump water into these bays. For the case of Tampa Bay, though, the SLOSH model shows little difference in surge for a Category 2 storm moving at 12 mph versus a Category 2 storm moving at 15 mph.


Figure 2.Maximum storm surge in feet (no adjustment for high tide) expected in the Tampa Bay area for a Category 1 hurricane moving NNE at 15 mph. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.


Figure 3.Maximum storm surge in feet (no adjustment for high tide) expected in the Tampa Bay area for a Category 2 hurricane moving NNE at 15 mph. The map was compiled using data from NOAA's SLOSH model. Image credit: Tampa Bay Regional Planning Council.

The current official NHC forecast says Fay will miss Tampa Bay by about 100 miles on Tuesday afternoon. At Category 1 strength, the predicted storm surge is 3-5 feet in Tampa Bay.

I'll have an update on Fay by 9 am EDT Monday morning.

Jeff Masters

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.