Thursday, September 1, 2016

New Coastal Flooding Products Help Us Track Hermine's Storm Surge

Tropical Storm Hermine is now a strong tropical storm, with maximum sustained winds of 65 mph, as of the National Hurricane Center advisory at 7:00AM Central time. Hermine is forecast to strengthen a bit more, possibly becoming a hurricane before it makes landfall near Florida's Big Bend/ Apalachee Bay.

Tropical Storm Hermine is forecast to strengthen as it approaches landfall near Florida's Big Bend/ Apalachee Bay. 

New coastal flooding products are helping us track Hermine's storm surge,

The National Hurricane Center's new Potential Storm Surge Flooding Map provides a map of water levels that have around a 10% chance of being exceeded. The map below shows that areas of Jefferson and Taylor counties, along the shore of Apalachee Bay, have around a 10% chance of observing 6 ft (1.83 m) or more of water above ground level.

The National Hurricane Center has released a new storm surge inundation product, which provides users with a map of potential water levels above ground

This tool has brought storm surge forecasting into the realm of geospatial analysis, using the SLOSH modeling expertise of the National Hurricane Center's Storm Surge Unit. This product will be very helpful for decision making as tropical weather systems approach coastlines.

The U-Surge project is also providing historic storm surge analog maps, which enable us to see a map of coastal flooding with previous storms that took similar tracks. This map of storm surge from Tropical Storm Alberto (2006) is available at Tropical Storm Hermine's U-Surge page.

Tropical Storm Alberto tracked into Apalchee Bay as a strong tropical storm with maximum sustained winds 70 mph (60 knots) in 2006. Alberto generated storm tide levels exceeding 8 ft (1.22 m) in Dixie County, and 2-4 ft in the Tampa area.

A map of Tropical Storm Alberto's storm track and maximum coastal flooding levels

With such analogs, it is important not to focus on the exact track or to consider the analog a forecast for the present storm. However, analogs provide us with insights into storm surge patterns that are evident with certain storm tracks.

For example, Alberto's map shows a clear distinction between high water levels to the east of the storm track, and relatively low water levels to the west. Although the storm made landfall more than 150 miles north of Tampa, it still generated a 2-4 foot storm tide in that area. We should expect coastal flooding in these areas with Hermine, as well.

Although several feet of storm surge may not sound threatening for places like Tampa, St. Petersburg and Clearwater, flood impacts are often exacerbated in metro areas as the storm surge impedes the drainage of heavy rain that falls on hard surfaces. Such flooding was evident during Tropical Storm Debby in 2012.

A combination of heavy rain and prolonged storm surge led to flooding in the Tampa metro area during Tropical Storm Debby in 2012. Photo: Jason Behnken

The combined impact of storm surge and heavy rain is a major concern for coastal cities. Thomas Wahl was the lead author on a paper published last year in Nature Climate Change that found an increasing trend in compound rain/ surge events for major U.S. cities.

Such compound flood events will become more severe over time even if storm frequency does not increase, due to rising seas associated with climate change.

Historical inundation graphs are another useful tool if you're interested to track Hermine's water levels relative to past coastal flood events. Hermine's U-Surge page provides historical graphs for Cedar Key and Apalachicola in this new product that has just been launched.

Track Tropical Storm Hermine's water level in the context of historical flood events and sea level rise at Hermine's U-Surge page.

Highest total water levels are likely to occur during late afternoon/ evening in Apalachicola. High tide occurs after 4PM, so total water levels may reach maximums between 4-6PM, as the storm is still approaching at the time of high tide.

Cedar Key has a low tide at 9PM, so expect water levels to remain nearly stationary through the evening, instead of dropping, like normal. Then expect water levels to rise rapidly after 9PM, as increasing tidal levels and storm surge combine to elevate total water level. Maximum water levels should occur after midnight, as strong westerly winds behind the back of Hermine continue to push storm surge as the 3AM high tide approaches.

In locations farther removed from Hermine's track, like Tampa, water levels will not rise so suddenly. Expect a gradual water level rise in the metro area, with flooding most likely associated with heavy rain bands that have trouble draining into the elevated coastal waters.


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