Winter Forecast Problems Associated with Light to Moderate Snow Events in the Mid-Atlantic States on 14 and 22 February 1986

Abstract

Two cases of light and moderate snow events over the Washington, DC area are presented for 14 and 22 February 1986. These cases were selected since the upper-tropospheric and surface features were quite similar, yet significantly different snowfall patterns occurred in the mid-Atlantic status. The numerical guidance and local forecasters displayed minimal skill in forecasting the amount of snow for each case, especially in the Washington, DC area. Analyses in isobaric and isentropic coordinates are presented which focus on the structure and evolution of the thermal fields and associated temperature advection patterns in the lower troposphere. These analyses reveal that the low-level upslope flow (as derived from vertical motion computations in isentropic coordinates) and the associated warm-air advection patterns (as viewed in isobaric coordinates) were important factors in determining the intensity and spatial distribution of precipitation for each case. Three-hourly soundings from the Genesis of Atlantic Lows Experiment (GALE) are presented for the 22 February 1986 case to demonstrate the effects of evaporative cooling on the temperature changes in the lower troposphere as the precipitation rapidly changed from rain and sleet to moderate snow in the Washington, DC area. The GALE soundings are also used to illustrate the importance of 3-h upper-air data to resolve properly the evolution of the wind and temperature fields during significant weather events.