Snowstorm December 20th 1998
1. Introduction
A snowfall plot (figure 1) indicated that the heaviest snow fell across the southwest part of the CWA in Wood Portage, Waupaca and Waushara counties with amounts of 5 to 7 inches reported at many locations. The rest of the CWA received generally 2 to 4 inches. The storm began around daybreak Sunday December 20th and continued to within a couple of hours after midnight Monday December 21st. The heaviest snow fell from early afternoon to mid evening.
2. Upper Level Features
Warm advection was a factor during the first two thirds of the storm as indicated by the 6 hour 850 mb warm advection (figure 2) and the height and temperature forecasts (figure 3) of the Eta model valid at 18 UTC Dec 20th. The right rear quadrant of an upper level jet streak (figure 4) provided lift early in the storm. The nose of another jet streak (figure 5) moved from the central plains into the Upper Mississippi Valley and prolonged the snow through the evening after the warm advection decreased. Time sections for AUW and GRB for the 12 UTC Dec 20 Eta and NGM runs indicated tow well defined periods of lift (figures 6a, 6b, 6c, 6d). Note the second higher area of lift associated with the secondjet streak. The temperature gradient at 850 mb tightened between 12 UTC Dec 20 and 00 UTC Dec 21, indicating some frontogenetical forcing beginning to extend into the southwest part of the CWA. Moisture appeared to be somewhat limited with available mixing ratios in the 850 mb to 700 mb layer of 1.5 to 2.5 g/kg for a 12 hour period (figures 1a and 8b). The apparent low water equivalencies (not just because of ASOS), plus the duration of the snow may have accounted in part for the higher snowfall amounts.
3. Surface features
The surface features were weak with an inverted trough moving east across the state during the day. During the evening a low pressure center developed at the north end of the trough over the Upper Peninsula of Michigan. The movement and evolution of the tough and low pressure center were depicted well by the Eta (figures 9a and 9c). The surface pattern was complicated by a wind shift and associated trough near the shore of Lake Michigan as seen on the 15 UTC surface plot (figure 10). East winds with temperatures in the lower 30s east of the wind shift compared to northeast winds with temperatures in the middle 20s to the west provided evidence for a lake induced warm front. The front drifted to the west of Green Bay, Appleton, and Fond du Lac by 18 UTC.
The synoptic trough eventually merged with the lake induced warm front during the late afternoon and early evening. The trough also intensified during the same period and its merging with the warm front may have aided with its intensification. Most of the heavier snow fell generally along and east of the inverted synoptic trough and west of the front. Overrunning of the warm front from air off Lake Michigan likely aided in the production of the snowfall. Any enhanced convergence associated with the intensification of the synoptic trough also likely contributed to heavier snowfall.
Of separate note, the northeast wind off Lake Michigan and the bay of Green Bay and the associated lake effect snow flurries added moisture to the 12 UTC Dec 20, 1998 GRB sounding (figure 11). As a result, snow reached the ground more quickly. Further to the west, snow fell for several hours before reaching the ground.
4. Additional Upper Features
The storm featured no well defined short wave, just a broad southwest flow from 500 to 300 mb, with several embedded jet streaks. As mentioned earlier two of these jet streaks contributed vertical motion to help produce the snow.
Forecaster References
AK, TK, TH
List of Figures
