A Simple Meteorological Synopsis of the 3/31/2011 West Florida Tornado Outbreak
Posted on Friday, February 10, 2012 at 5:00 PM MST
On March 31, 2011, nine tornadoes broke out across West Florida as a strong front moved across the region, leaving heavy damage in its wake. While the Tampa Bay area does see the most tornadoes per square mile than anywhere else in the country, the tornadoes it does see are generally very weak and short-lived, as some struggle to even get to EF-0 status. This outbreak was quite unusual for Florida because of the strength of the tornadoes and because the tornadoes plowed through the most densely populated area in the state.
An unusually warm and humid air mass sat over the Florida peninsula, which actually sparked a preliminary round of severe weather the day before. The two main ingredients needed to produce thunderstorms (heat, moisture, and upward forcing) were plentiful. At the same time, an unusually cold air mass was moving in from the northwest. These conditions made the atmosphere very unstable (warm, moist air being forced into very cold air aloft). If you're interested in how thunderstorms form, be sure to check out my post about the Anatomy and Life Cycle of Thunderstorms.
How Everything Came Together
While fronts of this nature are not uncommon in the early spring on the Florida peninsula, there were a few special parameters in place to trigger the tornado outbreak. There was a boundary (nearly all severe storms initiate along some sort of boundary) spanning the peninsula from Tampa to Melbourne that acted like a beam to focus and magnify nearly all of the severe weather over the greater Tampa Bay area. Coincidentally, the strongest shear was also located in this area as well. Coupled with a particularly strong front there was plenty of fuel for storms to feed off of. This fuel is called Convective Available Potential Energy, or CAPE, which is a measure of how much energy is available in the atmosphere to fuel the storms. Plenty of available fuel coupled with strong shear and a boundary in place made the immediate Tampa Bay area a powder keg for severe weather.
The Front Moves In
There was a pretty dynamic scenario over West-Central Florida as the front moved in. In the map below (from the SPC), the boundary over central Florida is indicated by the thick royal blue dashed line. Any sort of boundary, whether it be a front, a dryline, an outflow boundary, or anything else, will often provide that spark to trigger severe weather, and this one was no different, focusing the severe weather over the western part of the black hatched area across central Florida. The other main feature displayed on the map is wind shear, indicated by the blue barbs. These barbs are interpreted the same way as wind speed barbs, with the short and long lines depicting 5 and 10 knots, respectively, and the pennants (triangles) depicting 50 knots. If you canít decipher the map, the strongest shear was supportive of tornadoes and was located directly over the Tampa Bay area, as was the main boundary. The orange/red contours represent available fuel (CAPE), and the largest amounts of CAPE were located just west of Tampa. This combination merited a Tornado Watch for most of the Florida Peninsula. The boundary acted like a conveyor belt, funneling all of the severe weather right through the Tampa Bay area and east across the peninsula.
As the cells embedded in the front passed through the area of large CAPE just offshore, they quickly strengthened. Then they went right into the strong shear over the Tampa Bay area, which caused them to begin to rapidly rotate while at their peak strength. One of the strongest cells came approached the coast around 11 AM with strengthening rotation after going through the strongest of the shear. A Tornado Warning was issued for Pinellas County.
Tornadoes Break Out
The main tornado came ashore at 11:05 AM at Indian Rocks Beach, packing EF-1 winds around 90 mph. The twister promptly demolished homes on the beach and continued east/northeast. It struck the St. Petersburg/Clearwater airport, collapsing a hangar and forcing the airport to close down for the rest of the day before lifting over Old Tampa Bay.
Under the right conditions, a tornadic supercell will behave like any living being and will begin to cycle, much like a person goes to sleep and wakes up every day. After a tornado has used up most of its available energy, it will lift, reorganize and recharge, and then touch down again. The combination of plenty of available energy (CAPE) and the layout of the terrain were quite supportive of cyclical supercells on March 31st. After tearing through the airport, this cell very quickly cycled and touched down again just west of Interstate 275. It flipped an 18-wheeler and ripped signs out of the ground as it crossed the interstate at the south end of the Howard Frankland Bridge.
It continued to cycle as it progressed east, cycling several more times. As it tracked right along the boundary sitting across the Florida Peninsula, it destroyed a neighborhood in South Tampa and hit the Lakeland Airport, leaving extensive damage in its wake. The strongest winds were estimated to be 105 mph (very strong EF-1). This one cyclical cell accounted for six of the nine tornadoes that day. As the day progressed, the discrete cells dissolved into a squall line (which is common on days like this), eliminating the tornado threat as the front pushed east. The storms did not run out of fuel (CAPE) until after they had crossed the peninsula.
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