North Gainesville Tornado - December 16, 2007

During the early morning hours on Sunday, December 16th, 2007, an approaching strong cold front and associated squall line combined with the remnant wind circulation of former Tropical Storm Olga to produce severe weather across portions of north central and central Florida. A short-lived tornado was spawned over a rural area in the northwestern section of Gainesville in Alachua County. Additionally, wind damage was reported west of Gainesville around the Meadowbrook Golf Course.

Synopsis
On Saturday, December 15th, 2007, a sharp upper trough dug into the plains states (Fig. 1). At the surface, a low pressure system began to rapidly develop near the Texas/Arkansas state line (Fig. 3). At this time, the associated warm sector already covered most of the Jacksonville county warning area (CWA) with a quasi-stationary front over the extreme northern portion of the area. In fact, by Saturday evening, surface dew points ranged from the mid 60's to the lower 70's across the CWA. Another feature that ended up being a major player in the severe weather across central Florida was the remnant low that was once Tropical Storm Olga (last position and forecast track for Olga from the National Hurricane Center is shown in Fig. 5). The low is located just off the 12Z December 15th map (Fig. 3), but the associated inverted trough is depicted. By 12Z December 16th, this low had merged with the frontal boundary and continued to move to near the Georgia/South Carolina state line (Fig. 4). The effect of the circulation around this low will be discussed in the "Soundings" section below.

By Sunday, December 16th, the upper trough had deepened and become negatively-tilted, and by 12Z the best area of upper diffluence had already shifted northeast of the area (Fig. 2). By this time, the surface low had rapidly intensified and moved into the Ohio Valley, and the frontal system had occluded with a second low developing along the upper mid-Atlantic coast (Fig. 4). The remnant Olga low had merged with the frontal boundary and continued to move to near the Georgia/South Carolina state line. The effect of the circulation around this low will be discussed in the "Soundings" section below. The Storm Prediction Center issued a tornado watch for the entire Jacksonville CWA at 9:40 PM the night of December 15th effective until 6:00 AM the following morning.


Fig. 1 - 500 mb heights, 12Z December 15th, 2007.	Fig. 2 - 500 mb heights, 12Z December 16th, 2007.


Fig. 3 - Surface weather map, 12Z December 15th, 2007.	Fig. 4 - Surface weather map, 12Z December 16th, 2007.


Fig. 5 - Final NHC advisory and forecast track for tropical cyclone Olga, 03Z December 13th, 2007.

Soundings
The remnant Olga circulation had a marked influence on low-level wind flow as it interacted with the southwesterly flow ahead of the squall line and the lighter westerly flow behind the squall line. Prior to the circulation, winds had become more uni-directional SSW to SW across most of the area ahead of the squall line (Fig. 6a). Although wind speed shear increased through the night with an increasing low-level jet, directional shear decreased. As a result, helicity values of around 250 m²/s² were noted on both the 00Z and 12Z KJAX soundings from December 16th (although values may have increased somewhat after midnight). As the low moved along the squall line, wind flow backed to a more south to south-southeasterly direction ahead of the squall line, locally increasing helicity values in the light blue shaded area of Fig. 6b affecting central and north central Florida. Soundings taken during the late night and early morning hours across central Florida show significantly higher helicity values- 468 m²/s² at Tampa from their 05Z special balloon release (Fig. 9) and a whopping 804 m2/s2 at Cape Canaveral from their 12Z release (Fig. 10, Frame 1). This increase in helicity was due both to the increasing low-level jet and the local backing of winds due to the remnant low.

Overnight, convective available potential energy (CAPE) values above the level of free convection dropped over land across extreme northeast Florida, from around 900 J/Kg over Jacksonville at 00Z (Fig. 7) to around 300 J/Kg at 12Z (Fig. 8). However, plenty of CAPE was available across central Florida, with over 1700 J/Kg over Cape Canaveral at 12Z. In fact, the energy helicity index (EHI) computed at the Cape at 12Z was 8.65 units. This was more than enough energy and helicity to provide favorable conditions for tornado development. The backing of the wind in the blue-shaded area also provided enhanced low-level convergence, particularly north of the low along the squall line.


Fig. 6a - Surface wind flow associated with squall line and cold front prior to influence by Olga remnant low.	Fig. 6b - Surface wind flow with influence from circulation associated with Olga remnant low. Motion of low is indicated by yellow arrow. Approximate region of enhanced helicities indicated by blue-shaded area.


Fig. 7 - Jacksonville sounding, 00Z December 16th, 2007. (click for larger image)	Fig. 8 - Jacksonville sounding, 12Z December 16th, 2007. (click for larger image)


Fig. 9 - Tampa sounding, 05Z December 16th, 2007. (click for larger image)	Fig. 10 - Cape Canaveral sounding loop, 12-15Z December 16th, 2007. (click for larger images)

Radar Imagery


Fig. 11 - Jacksonville radar loop, December 16th, 2007, 0726-0752Z.	Fig. 12 - Jacksonville radar velocity image, December 16th, 2007, 0741Z.

Text Products

Area Forecast Discussions	Hazardous Weather Outlooks	Warnings, Watches & Statements
12/14 at 310 PM 12/15 at 321 AM 12/15 at 127 PM 12/15 at 930 PM 12/16 at 153 AM	12/15 at 521 AM 12/15 at 236 PM 12/15 at 905 PM 12/15 at 1005 PM 12/16 at 155 AM 12/16/07 at 458 AM	Tornado Watch 12/15 942 PM-12/16 6 AM: SE Georgia and NE Florida Severe Thunderstorm Warning 212 AM: W Alachua, W Bradford, SW Union, S Columbia Tornado Warning 242 AM: NE Alachua, Bradford, S Union, W Clay, W Putnam Severe Weather Statements 246 AM: Severe Thunderstorm Warning follow-up 259 AM: Tornado Warning follow-up #1 (cancelled S Union) 316 AM: Tornado Warning follow-up #2 (expiration)
Zone Forecast Products	Local Storm Reports
12/15 at 229 PM 12/15 at 1010 PM 12/16 at 207 AM	12/16 at 1009 AM (wind damage 7 mi W of Gainesville) 12/16 at 1024 AM (preliminary damage report from N Gainesville) 12/18 at 444 PM (final damage report from N Gainesville)

Damage Survey & Photos
National Weather Service Jacksonville conducted a damage survey, examining areas in northwest Gainesville near NW 59th Street and NW 93rd Avenue. Scattered debris in addition to trees and branches scattered in different directions were indicative of rotation and not straight line winds. The tornado only briefly touched down, with a storm path of approximately 3/4 of a mile and a width of 200 yards.

The tornado initially produced damage on the EF-0 scale, with winds approximated at 80 mph. A few pine trees were snapped or uprooted in different directions. Bushes and grass in the path of the storm were lying flat. As the tornado continued, wind speeds increased to 90 to 95 mph, bringing it to a low end EF-1 rating on the enhanced Fujita scale. At this point, the tornado pushed over a large truck in its path. A farm outbuilding had one concrete wall destroyed, and the metal roof was ripped apart and strewn across a field. Pieces of metal from the outbuilding were also wrapped around fences and tree branches.

The tornado then crossed NW 93rd Avenue, damaging a house in its path and removing a significant portion of the shingles on the roof. A wall on the house was pushed inward and a carport was damaged. More trees in this area were snapped or uprooted.

From here, the tornado significantly weakened, producing only minor damage to trees in its path and lifting shortly after. Due to the quick movement of the storm and the short path, it is estimated that the tornado was only on the ground for one minute before dissipating.