The Role of Dual Cold Fronts Aloft in a Major Tornado and Flash Flooding Event

A cold front aloft (CFA) is a katafront characterized by cold, dry air that advances ahead of the surface cold front by several hundred kilometers, resulting in a warm occlusion structure. These non-classical frontal systems are prolific producers of heavy precipitation and severe thunderstorms. It will be demonstrated that diagnosis of the structure of CFAs is made possible from the synthesis of water vapor channel satellite imagery, isentropic and cross sectional analysis of mesoscale model fields, and thermal retrievals and other products obtained from WSR-88D radar and wind profiler data. The discussion will emphasize a recent case involving a major tornado outbreak, though other examples will also be summarized.

The case of 21-23 January 1999 displayed a dual CFA structure associated with prefrontal lines of tornadic thunderstorms and extensive flash flooding as prolonged heavy rainfall fell over snow-covered ground. Over $1 billion in damage was produced by this system, which was notable for producing the most tornadoes in any state on any day in January and also the most tornadoes in a single day in January. As the cold front in the mid-troposphere separated from the upper-level front to its west, the first CFA ushered in drier air over a very moist lower troposphere in the southern Mississippi Valley region. Tornadic storms erupted during this phase of the system. As this first CFA stalled, a second and much more pronounced CFA approached the first CFA, and copious rains were produced in the Ohio and Tennessee Valleys. Finally, as the two frontal systems aloft merged, an intense squall line developed, which ravaged the entire southeastern U.S. Thus, nearly all of the severe weather in this event was produced in association with the dual CFAs.

It is interesting that, even though the second CFA ushered in extremely dry air associated with a deep tropopause fold, nearly all of the tornadic activity occurred with the first CFA. This happened because the passage of the first CFA through the region decreased both the depth and amount of moisture available for convection. However, the much stronger baroclinicity associated with the second CFA enhanced the low-level wind shear, thereby producing conditions more conducive to squall line development. The presentation will highlight the benefits to be gained from synthesis of mesoscale model and remote sensing systems in the analysis of these important phenomena.