Heavy Snow in Wisconsin, December 11, 2000

Heavy snow, accompanied by northeast to north winds gusting to around 30 mph, affected all of south-central and parts of southeast Wisconsin. This system was the first in a series of winter disturbances that accumulatively would lead to new, monthly snowfall records for December, and new record snow depths at any one time. Accumulations were generally 7 to 10 inches, although a band of 10 to 11 inches was found from northwest Dane County to south-central Columbia County, and across the southern parts of Green, Rock, and Walworth Counties. The greatest reported value was 11.1 inches in Delavan (Walworth Co.). Lesser amounts (7 to 9 inches) were noted in Dodge and Jefferson Counties. Dozens of vehicle accidents resulted due to slippery roads and blowing snow. The fresh snowcover, combined with clear skies during the early morning hours of the 12th, resulted in a new record low of -10 at Janesville (Rock Co.). However, other parts of the Janesville area dropped to -17 and every other location in south-central Wisconsin sunk to the -10 to -15 range for morning lows.Synoptically, a surface low moved from central Colorado Sunday night December 10th, to north of Dallas, TX, by the end of the 10th, to east-cental Illinois by noon CST on the 11th, and then to near Toledo, Ohio by 2100CST. Strong upper level forcing due to a coupled jet structure over Wisconsin, along with strong mid-level differential positive vorticity advection, led to significant upward vertical motions over southern Wisconsin and northern Illinois, thus deepening and slowing the surface low.This system produced a heavy band of snow, ranging from 8 to 12 inches, across northeast Iowa, extreme northern Illinois, and southern Wisconsin. As the surface low moved into western Indiana, gusty northeast winds to 30 knots, in the northwest quadrant of the circulation around the low, drew moisture from Lake Michigan. This enhanced the snowfall in the counties adjoining the lake from north of Chicago, to northern Milwaukee county. Snowfall totals in these lakeshore locations were between 12 and 15 inches.