Monday, June 6, 2016

The U-Surge Project Launches Website for Cedar Key, Florida, as Colin Approaches

The U-Surge Project launches a community storm surge website for Cedar Key, Florida, today, as Tropical Cyclone Colin approaches. The website contains the first comprehensive data archive of coastal flood events at Cedar Key, a small village 80 miles north of Tampa.

Cedar Key, Florida, is a historic community around 80 miles north of Tampa

U-Surge selected Cedar Key as a partner community thanks to extensive weather records in this historic location and its strategic location near Apalachee Bay. The physical features of this region, like shallow water and the concave shape of the bay, efficiently create large surges. Fortunately, this region infrequently observes hurricane landfalls, at least compared to locations farther west and south along Florida's Gulf Coast. 

The U-Surge Project launches website for Cedar Key, Florida as Tropical Storm Colin approaches.

The historic record for Cedar Key contains 30 tropical surges, the highest of which reached 8.5 feet in Hurricane Alma of 1966. When these observations are converted to a common datum, or reference line, Alma’s water level drops to 8.11 ft above the North American Vertical Datum (NAVD88) of 1988, but still retains first place.

The U-Surge Project has built the first comprehensive coastal flood record for Cedar Key, Florida, which includes 30 events since 1920. Hurricane Alma of 1966 generated the highest recorded storm tide, with water levels reaching 8.11 ft above the NAVD88 datum.

Tropical Storm Colin

Tropical Storm Colin is forecast to track northeast today before making landfall near Apalachee Bay this evening. Although Colin has strengthened slightly overnight, the storm is rather disorganized, as the most intense convection is offset from the center of circulation. This is good news for residents of coastal Florida, as disorganized storms lack the broad shield of powerful winds for generating large surges.

Colin’s timing, however, makes things a bit more interesting. The storm is arriving just one day after the new moon, in a time of the lunar cycle in which tidal ranges increase. Fortunately, Colin is forecast to make landfall this evening, several hours after high tide. Therefore, expect the worst flooding impacts to occur around the time of this afternoon’s high tide, which is around 3:00PM local time at Cedar Key.

Predicted (blue), preliminary (red) and verified (green) water levels for Cedar Key, Florida, from May 27 - June 16. The red box shows the time of Colin's impact, when tidal ranges are high.
Base graph: NOAA Tides and Currents.

Colin's flood impacts should be most severe around the time of high tide this afternoon. In Cedar Key, high tide occurs around 3PM local time, so expect the worst flooding from around 2-4PM.
Base graph: NOAA Tides and Currents

I’m predicting that storm surge levels at that time will be around 2.5 ft above normal astronomical tides, creating a total water level of around 4.72 ft above NAVD88. Due to the high astronomical tides at that time, Colin’s total water level would rank 9th in the list of historic storm tides from tropical systems. Sea level rise helps boost Colin into the top 10, as storm tides are measures of total water levels, and sea levels have risen around 8 inches over the last century near Cedar Key.

Storm tide levels should exceed 4.7 feet above the NAVD88 datum at Cedar Key this afternoon, placing Colin's storm tide 9th in the historical records.

A storm tide of 4.72 ft above NAVD88 would top the water level produced by Tropical Storm Debby in 2012 by several inches near Cedar Key. Debby splashed water over flood defenses, flooded roads, and inflicted some property damage, as seen in the video below, which is also posted on the U-Surge webpage for Cedar Key. The U-Surge project helps residents compare current and past events, as water levels are converted to the same datum.

Tropical Storm Debby's storm tide inflicted some property damage near Cedar Key in 2012. Colin's storm tide may surpass Debby's, partly because Colin is striking near the time of the new moon, when the tidal range is higher than normal.

In general, Colin’s biggest impact should be heavy rain, as the heaviest convection is displaced to the east of the center of circulation and is already impacting Florida. Heavy rainfall compounded with higher-than-normal storm tides will cause minor flooding in coastal communities from Tampa north to Apalachee Bay.

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.