Wet Microburst Activity over the Southeastern United States: Implications for Forecasting

The thermodynamic properties of wet-microburst-producing days, as observed during the 1986 MIST (MIcroburst and Severe Thunderstorm) field project, conducted in northern Alabama, have been examined and are shown to exhibit common characteristics. The parent storms and environment for this microburst type are substantially different than those documented over the High Plains in that the cloud bases are warmer, the subcloud layer is shallower, the radar reflectivities are greater, and the thermal environment is more moist and stable. Analyses of the rawinsonde data, launched in the morning and afternoon, show that low-level moisture is present and is capped by a midlevel dry layer. This midlevel dry air is generally advected from the northwest, where a large area of dry air exists over the central United States. In addition, it appears to be possible to differentiate between microburst days and thunderstorm days producing no wet microbursts by plotting the vertical profile of the equivalent potential temperature (oe). The strong wind-shear days are potentially more unstable. The difference between the surface value of oe and the minimum value aloft (in the afternoon) is greater than 20 K for the microburst days, whereas it is less than 13 K for the thunderstorm days with no microbursts. Consequently, these results suggest that they may be used by the forecaster to issue, in a timely manner (2–12 hours), a “wind-shear alert” to the general population and, more importantly, to the aviation community. Analyses of microburst storm structures indicate that they are vertically deeper than those storms developing during days with no microbursts, and that the precipitation core is largely composed of ice. Convergence, or inflow of environmental air into the microburst storms, was also commonly observed new the level of minimum oe. These wet microburst soundings and oe profiles were compared to other well-documented events. In each case, the soundings and oe profiles were similar to those derived here.