National Weather Service United States Department of Commerce


Event Summary


The first significant and widespread snowfall of the 2021-2022 winter season occurred across southeast Michigan on November 27, 2021. Much of the snowfall occurred between 10 AM and 11 PM. Leading up to the event, a Winter Weather Advisory was issued for areas between the I-69 corridor and the Ohio border, where generally 2-4 inches of snowfall was expected to fall. Further north into the Saginaw Valley/Tri-Cities region and the Thumb, snowfall totals were expected to be more in the 1-2 inch range. Actual snowfall totals fell closely within forecasted amounts, with advisory locations seeing the forecasted 2-4 inches of snow. The highest snowfall locations occurred in a swath between the I-96 and I-94 corridors (including Ann Arbor and portions of western and northwestern Metro Detroit), where 3 to a little over 4 inches of snow fell. The first snow of the season resulted in hazardous travel conditions across the region due to reduced visibilites, snow-covered roadways, and slick conditions.

The snowfall occurred as a result of a clipper low pressure system moving southeast across the lower Great Lakes. This system was able to utilize forcing resulting from frontogenetical (FGEN) forcing in the low and midlevels which provided a synoptic source of lift throughout the event. Additionally, mesoscale forcing during the late afternoon and evening hours allowed for enhanced convergence between the I-96 and I-94 corridors, which helped locally enhance snowfall totals closer to the 4 inch range. While not an otherwise unique aspect compared to other snow-producing systems, snowfall occurred approximately 2-3 hours earlier than anticipated as a result of the earlier arrival of the lead edge of the FGEN forcing. More notably, however, was the earlier snow accumulation on area roadways which led to numerous traffic snarls and accidents as drivers readjusted to wintertime driving for the first widespread snow event of the season. One reason for this was the wide surface temperature and dewpoint spread at the onset of the event. Despite road temperatures initially being above freezing at the start of the event, quick saturation in the low-levels of the atmosphere due to falling snow led to rapid evaporational cooling near the surface, which enabled ground surfaces to effectively cool below freezing, thus being capable of accumulating snow vs. remaining wet. 



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