18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
24-48h predictability of lake-enhanced snowbands in the 2-3 January 1999 Midwest blizzard
James E. Sieveking, Univ. of Wisconsin, Milwaukee, WI; and P. J. Roebber
Regions downwind of the Great Lakes frequently experience lake effect snowfall in the cold season. However, eastern Wisconsin and Northern Illinois also experience lake-enhancement of snowfall in association with the passage of synoptic-scale cyclones equatorward of the Great Lakes basin. In the resulting east-to-northeast flow, fluxes of heat and moisture can lead to localized bands of heavy snowfall embedded within the synoptic-scale precipitation shield.

On 2-3 January 1999, a blizzard passed to the southeast of Lake Michigan. Lake effect snow bands were observed prior to the arrival of the main precipitation shield and numerous snowbands were detected throughout the event, with total snowfall varying from 25-60 cm throughout the region.

Once daily forecasts from a quasi-operational version of the Pennsylvania State University – National Center for Atmospheric Research (PSU-NCAR) fifth-generation Mesoscale Model (MM5) with interior nesting to 5 km grid spacing are used to examine the predictability of the snowbands in this event. Subjective comparisons of model performance are combined with quantitative measures of synoptic scale forecasts of pressure, wind and temperature for 00-24h and 24-48h ranges. These comparisons indicate reliable synoptic-scale forecasts of the cyclone event at both time ranges. A comprehensive comparison of model output with high-resolution, WSR-88D derived precipitation data, ground-truthed using all available gage data, shows useful QPF skill for this event.

Using the baseline model forecast as a control, additional model runs were conducted to assess the direct impact of the lakes on the precipitation bands in this event. This assessment includes not only precipitation, but the mesoscale structures of pressure, temperature, and wind and the associated dynamics. The connection between synoptic-scale structure and the in-situ mesoscale forcing provided by the lakes is addressed with respect to the general problem of mesoscale predictability.

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