Tropical Storm Debby --- I'm Baaaack

As I blogged last time, Tropical Storm Debby defied catalog odds. She broke apart and, after buckets of rain, faded away. Suppose however, that things turned out differently.  

Imagine instead that starting from conditions at 6/24 15 GMT  [Maximum sustained wind  50 kts,  Storm speed 5 kts N40E, Central pressure 980mb] Tropical Storm Debby intensifies over four hours into a Category 3 Hurricane [Maximum sustained wind 105 kts, Storm speed 16 kts N40E, Central pressure 925mb].

Any bet on the size of the storm surge when this not-so-faded Debby strikes Florida?  Let’s run a computer simulation. 

These movies show 18 hours of Debby’s progress with storm information given in the yellow box. Black arrows plot surface wind direction and speed in kts. Colors contour storm surge --  yellows and reds are positive,  blues and purples are negative. Numbers in the yellow circles list peak surge height in decimeters. Lower box shows cross section of the coast and sea along the dashed line.

As you might expect, onshore winds tend to drive water onto the land and offshore winds tend to drive water out to sea.  The surge height depends on how strong the winds blow, which direction they come from, how long they persist, the configuration of the shoreline, and the depth of water offshore.

In the first simulation the storm nearly parallels the coast along Franklin County east of Apalachicola. (Get out those Florida road maps if you don’t know these places!). The strong onshore wind, low pressure (this draws up the ocean by about a meter) and the interior right angle of the coast contribute to peak storm surge of over 9 meters in Wakulla and Taylor Counties.

True, not so many folks live in those parts of Florida, but suppose instead, that hurricane Debby headed straight west toward urban St. Petersburg.  You can see the consequences for yourself in the second movie and in the YouTube below.

Sure, these are “What If”  Debby scenarios, but does anyone doubt that sooner or later, similar real Dollys, Ellas, Ferns, or Gingers will be at the doorstep? If you live at the coast, storm surge predictions are something that you’d like to have. Armed with computer simulators like these, forecasters should be ready.

Steven N. Ward   Santa Cruz