Mesoscale phenomena affecting the Alabama EF-4 tornadoes during the Super Tuesday Tornado Outbreak of 5-6 February 2008

During the early morning hours of 6 February 2008, 7 tornadoes occurred in northern and central Alabama, including two that were rated EF-4. The purpose of this paper is to examine some of the mesoscale phenomena which appear to have affected the storms, including their overall intensity and efficiency in producing tornadoes.

After a relative “lull” in the tornado activity associated with supercell storms in the southern part of the outbreak area that occurred between 0200 and 0700 UTC, storms re-intensified between 0700 and 1200 UTC, from eastern Mississippi into western Alabama. An upper-level shortwave trough was evident in satellite imagery as it appeared to allow new convective initiation in eastern Louisiana around 0615 UTC. This trough then advanced northeastward, creating upper-level divergence and surface pressure falls near the Alabama/Mississippi border. Tuscaloosa, Alabama recorded a 3-hour pressure fall of 3 hPa between 0700 and 1000 UTC. These pressure falls were associated with rapid increases in 0-1 km storm-relative helicities over northern and central Alabama, from 350-400 m2/s2 at 0600 UTC, to 400-500 m2/s2 by 1000 UTC. These were likely associated, at least in part, with backing of the low-level flow due to isallobaric winds. This increase in SRH, along with destabilization by the upper trough, may have contributed to the development of long-lived supercell storms over Alabama after 06 UTC.

Alabama was also positioned in a somewhat favorable synoptic situation for the generation of gravity waves, with large vertical shear, and a conducive 300 hPa flow pattern. These waves may cause changes in the intensity of mesocyclones they interact with. Radar data indicate that several apparent gravity waves were propagating rapidly NE (at speeds around 28 m/s) in the storm environment. The supercell that eventually produced the EF-4 tornado in Jackson County, Alabama, experienced oscillations in vorticity over a two-hour period, that were at least somewhat correlated to four potential gravity waves interacting with the storm.

Additional mesoscale effects that will be examined include topography and friction. The Lawrence County, Alabama EF-4 tornado, developed and began producing EF-4 damage quickly, around the time the parent storm crossed from the highlands near Bankhead National Forest, into an area of lower topography (by about 100 m) over southern Lawrence County. Vortex stretching, or some other process associated with the topography may have, in part, caused the rapid development of a tornado producing EF-4 damage in a short time period. The storm which produced the Jackson County, Alabama EF-4 tornado moved in close proximity to Lake Guntersville, along the Tennessee River, for several minutes before tornadogenesis eventually occurred. Given the horizontal gradients in roughness length associated with this relatively large body of water, it is possible that an incipient source of vorticity was present, which either delayed tornadogenesis until the effect of the lake diminished, or enhanced its likelihood. The geometry of this interaction will be examined in detail.