The Role of Mesoscale Features in a Wintertime Great Lakes Cyclone

Abstract

An analysis documenting the role of synoptic and surface mesoscale features in a cyclone has been conducted to determine the physical processes responsible for a large snowfall observed over south-central Wisconsin on 25 January 1988. Synoptic analysis revealed a highly amplified 500 mb flow with a West Coast ridge, an East Coast trough with a cyclone and short wave in northwesterly flow. As the surface cyclone maintained a constant central pressure and propagated southeastward across Wisconsin 1) pronounced surface anticyclogenesis and cold air advection occurred over southern Ontario and the northern and central Great Plains, 2) the 500 mb short wave amplified into a closed circulation over Illinois, and 3) the 1000?500 mb thickness field deformed into a pronounced S-shaped thermal trough. In response to the upper-level development and anticyclogenesis, the surface cyclone became stationary with nearly constant central pressure for 12 h after reaching Lake Michigan and strong surface pressure and isallobazic gradients were produced along the northern and western perimeter of the cyclone. While the surface cyclone was stationary, a gusty mesoscale surface jet formed locally over northeastern Wisconsin and propagated to the south-central part of the state. The surface jet forced a sustained region of surface convergence and ascent that was maximized over south-central Wisconsin during the period of greatest snowfall. The roles of heat and moisture fluxes from the Great Lakes in producing a neutrally buoyant layer of surface northeasterly flow northeast of the surface cyclone and the redistribution of mass around the stationary cyclone are explored to explain the development of the surface jet. Forecasts of the storm by the NMC nested grid model and aviation forecasts are evaluated to illustrate the difficulties operational forecasters faced in trying to forecast the significant snowfall.