Nocturnal Locally Heavy Rainfall Event: Sept. 16-17, 2011
Angie Enyedi, NWS Jacksonville
A backdoor cold front pushed slowly SSW across the Jacksonville forecast area during the late afternoon and evening of Sept. 16, 2011. This boundary converged with the west coast sea breeze across inland northeast Florida during the evening, generally from northern Columbia county southward across eastern Alachua county. The airmass was unstable which aided in persist updrafts. High moisture content in the atmosphere and slow storm motion favored slow moving, heavy rainfall producing cells. A line of nearly stationary convection produced excessive rainfall amounts over several hours across inland northeast Florida.
As this initial line of rainfall developed, another line of convection started to intensify across eastern Duval county and northern St. Johns county where strong NE flow was convergening along the coast. Over the next few hours, both bands of convection produced very heavy rainfall amounts, with widespread values of 4 to 6 inches. Rainfall across eastern Duval and northern St. Johns counties was excessive, with radar estimated 8 to 10 inches from Ft. Caroline to Jacksonville Beach. These amounts caused major flooding in parking lots and along roadways in Jacksonville Beach. There was also some flooding of businesses in east Arlington. The heavy rainfall started to end around 04Z to 05Z as convergence weakend.