“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.
Tropical Storm Colin formed off the coast of the Yucatan Peninsula on Sunday afternoon. The National Hurricane Center (NHC) forecasts the storm to track northeast and make landfall in Florida between Apalachicola and Cedar Key on Monday evening.
Flooding from the combination of heavy rain and storm surge will likely be Colin’s greatest impact. The National Weather Service’s Weather Prediction Center forecasts the potential of four or more inches of rain over the next two days along Florida’s West Coast.
The NHC forecasts water levels from coastal flooding to reach as high as 3 feet above ground level north of Tampa Bay, and 2 feet 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 highest-recorded storm surge, at 4.55 feet, followed by McKay Bay Entrance, in Tampa, which recorded a storm surge of 3.34 feet. These water levels were high enough to inundate land from Tampa to Apalachee Bay.
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 feet above tidal level, a 4.5-foot surge would cause a 1.5-foot 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 higher than other areas.
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 a 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.
Debby’s slow speed also enabled more water to push into the Tampa Bay area. Storm surge levels at McKay Bay Entrance exceeded 4 feet and reached 3.97 feet at Old Port Tampa.
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.
This originally appeared on Hurricane Hal's Storm Surge Blog.