Sunday, June 5, 2016

Tropical Storm Colin to Generate Coastal Flooding along Florida’s West Coast Monday

Tropical Storm Colin formed off the coast of the Yucatan Peninsula this afternoon. The National Hurricane Center forecasts the storm to track northeast and make landfall between Apalachicola and Cedar Key on Monday evening.

National Hurricane Center tracking map for Tropical Storm Colin

Flooding from the combination of heavy rain and storm surge will likely be Colin’s greatest impact. The Quantitative Precipitation Forecast (QPF) from the National Weather Service’s Weather Prediction Center forecasts the potential of 4 or more inches of rain over the next two days along Florida’s West Coast.

The QPF Map forecasts 4"+ of rain near Florida's Gulf Coast between Sunday and Tuesday Evening.

The National Hurricane Center (NHC) forecasts water levels from coastal flooding to reach as high as 3 ft above ground level north of Tampa Bay, and 2 ft above ground level from Tampa Bay south to Florida Bay, if the maximum inundation occurs at high tide.

This prediction relates closely to storm surge observations from Tropical Storm Andrea (2013), a great analog storm for Colin. Andrea formed in the central Gulf of Mexico and tracked northeast, making landfall in Apalachee Bay, north of Cedar Key, about 24 hours after forming. Colin’s forecast track and forward speed are similar, although Colin is moving slightly faster.

The map below shows Andrea’s track and storm surge observations. Cedar Key observed the high-recorded storm surge, at 4.55 ft, followed by McKay Bay Entrance, in Tampa, which recorded a storm surge of 3.34 ft. These water levels were high enough to inundate land from Tampa to Apalachee Bay.

Tropical Storm Andrea's storm surge and track map from the U-Surge Project

Note that the map provides storm surge levels but the NHC forecasts inundation levels. Storm surge is the height above normal astronomical tide, but inundation level is the height above the ground. So if land would normally be 3 ft above tidal level, a 4.5 ft surge would cause a 1.5 ft inundation.

We must keep in mind that such maps are not a prediction of Colin's surge, but rather a historical comparison that provides context and gives us some idea about likely storm surge patterns. For example, note that Andrea's storm surge level inside Tampa Bay was actually higher than on the open coast. Storm surge often reaches localized maximum levels on the inside of bays, where the bay shape forces water into a relatively small area, making it rise.

A year before Andrea, Tropical Storm Debby (2012) approached Florida’s Gulf Coast north of Tampa. Debby’s slow forward speed enabled it to dump torrential rains across a widespread area, totaling more than one foot in some locations. The combination of a prolonged storm surge event and heavy rain from slow-moving Debby, caused widespread flood impacts, including memorable flooding along Tampa’s Bayshore Boulevard.

Tropical Storm Debby's prolonged surge and heavy rain flooded Tampa's Bayshore Blvd.
Photo: Jason Behnken

Debby’s slow speed also enabled more water to push into the Tampa Bay area; storm surge levels at McKay Bay Entrance exceeded 4 ft and reached 3.97 ft at Old Port Tampa.

 Tropical Storm Debby's storm surge and track map from the U-Surge Project

Based on the NHC intensity and track forecast, we should expect Colin to be more like Andrea than Debby, generating minor coastal flooding near Tampa Bay, and minor-moderate coastal flooding near Cedar Key and Apalachee Bay. As Colin is forecast to move through noticeably faster than Debby, rainfall totals should be generally lower and storm surge should not have as much time to build up in Tampa Bay.

Residents in this region should be accustomed to these flood impacts, as Colin is the third tropical storm in five years to take this similar track during the month of June.

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