Flooding & Severe Weather Outbreak September 17-21, 2007

Flooding and Severe Weather Outbreak in Association With Tropical Depression 10
September 17-21, 2007

Write-up and images by: Melissa Hurlbut, Jason Hess and Peter Wolf
Edited by: Scott Carroll

A stalled frontal boundary and deepening low pressure off the Northeast Florida coast led to heavy rainfall that produced significant flooding across downtown Jacksonville, Riverside, San Marco, Jacksonville Beach, and Ponte Vedra. As this low developed and moved across Northern Florida into the Gulf of Mexico, it then created conditions conducive to severe weather, resulting in 4 tornado reports and numerous reports of wind damage. Several flood warnings and statements were issued from September 17th-21st, in addition to 21 county-based severe thunderstorm and tornado warnings from September 20th-21st.

A weak back-door cold front pushed south across the Jacksonville area late on Sunday, September 16, 2007 (image 1). Low level northeast winds increased behind the front Sunday night as a strong high pressure center built southward and settled over the Carolinas. By early Monday morning, low-level speed convergence over the coastal waters had set up numerous rain bands along the coast and just offshore. Low-level moisture remained ample to produce significant rainfall (image 2). Most of the heavy rain bands remained offshore initially but began to push onshore in the Jacksonville as the morning progressed. The upper level trough over the Florida peninsula was very amplified at this time (image 3). A strong short-wave rotated through the trough and pushed into the Jacksonville area. This, in combination with strong upper divergence, provided the lift and energy which acted to enhance the coastal shower activity, which then back-built into the Jacksonville Metro area during the late morning hours (image 4). Thunderstorms developed and became stationary over the metro area. These storms produced torrential rainfall over the city with 4 to 6 inches in less than 2 hours in some locations. Storm total precipitation over 12 hours reached near 4 to 6 inches over downtown, while areas near the beaches received 6 to 8 inches of rainfall (image 5). This, in combination with higher than normal tides (1 to 2 feet above normal) and previously saturated soils, produced significant flooding. There was a total of 15 water rescues by the fire department as people attempted to drive through the deep waters unsuccessfully. Numerous roads were closed including the major interchange between I-95 and I-10. Additionally, some businesses and houses were inundated with several inches of water. Please see the attached photos courtesy of First Coast News.

Severe Weather Outbreak: Wednesday night & Thursday Morning, September 20-21,2007

By Wednesday night, the area of low pressure that had brought torrential rains to the Jacksonville metro and coastal areas began to track westward across the Florida peninsula (image 6). A warm front and tropical moisture were lifting northward into Northeast Florida, and strong upper divergence enhanced shower and thunderstorm development. The east coast seabreeze had moved inland while a rain band rotated around the gulf low and converged with the old seabreeze boundary. This provided a focus for a northwest to southeast oriented squall line to develop (image 7). Despite the loss of daytime heating, most locations still had marginal instability through the night while some locations, mainly near the coast and across portions of southeast Georgia, maintained higher instability (image 8). Winds had increased just above the surface with 20 to 30 kt speeds from the south increasing to 30 to 40 kt in mid levels. In addition to the aforementioned speed shear, surface flow from the east-southeast created directional shear which would favor rotating storms and possible tornadoes.

As the line tracked northeastward it produced an EF-1 tornado in Eustis, Florida (image 9), with estimated maximum winds of 105 mph, damaging numerous homes. The line progressed overnight producing mesocyclones and wind damage. Flooding was also reported across Marion County. The line regained strength as it reached higher instability along the northeast Florida coast and across southeast Georgia. By 5 am EDT, a low-topped supercell formed ahead of the line, producing an EF-0 tornado that damaged several homes and toppled trees in Ware County, Georgia (image 10 and image 11). Before the line finally weakened, it produced a wall cloud and one-inch sized hail in Riceboro, Georgia.

The area of low pressure tracked northwestward through the day, producing severe weather across the panhandle and later became short-lived Tropical Depression 10. It made landfall on the panhandle of Florida near Pensacola.

Image 2: September 17, 2007 KJAX 12Z (8 am EDT) sounding. Yellow line denotes area of significant low level moisture between 1000 and 700 mb.

Image 3: September 1, 2007, 12Z (8 am EDT) RUC model analysis of upper low, shown in green over North Carolina with the main trough axis extending back through Florida. Blue denotes mean sea level pressure, and a coastal trough is observed offshore of Northeast Florida.

Image 4: Composite reflectivity loop from 1330-1930Z (930am-330pm EDT) on September 17, 2007. Images are every hour.

Image 5: KVAX (Moody AFB) storm total precipitation for September 17, 2007. The highest totals over land were observed over the Jacksonville metro area extending out through the beaches.

Image 6: Surface analysis from 11 am EDT September 20 through 5 am EDT September 21, 2007.

Image 7: Composite reflectivity at 1 am EDT September 21, 2007, showing intensifying squall line across northeast Florida.

Image 8: 00Z KJAX sounding on September 21, 2007, showing moisture and instability.

Image 9: Rotation associated with an EF-1 tornado in Eustis, Florida. Green signifies inbound velocities, while red is outbound relative to the radar (KJAX). Time of image is 11 pm EDT, September 20, 2007.

Image 10: Storm relative velocities in association with an EF-0 tornado in Ware County, Georgia. Image is every 30 minutes from 4 am EDT through 9 am EDT September 21, 2007.

Image 11: Composite reflectivities in association with an EF-0 tornado in Ware County, Georgia. Image is every 30 minutes from 4 am EDT through 9 am EDT, September 21, 2007.

September 17, 2007 flood pictures, courtesy of First Coast News

flooding photo
14th Ave S, Jacksonville Beach

Cassat Ave at Interstate 10

A1A, Jacksonville Beaches

Interchange between I-95 and I-10

S 3rd St, Jacksonville Beach

San Marco