The bright sunshine over most of the Midwest today belies the fact that a significant storm system will be moving over the eastern 2/3 of the country during the next three days. Wet snow north, rain central, and severe storms with possible tornadoes over the deep south. The storm the product of another is a series of very strong upper level systems that come out across the north Pacific, dive over the Rockies, and then intensify as they move east across the country.
For the Chicago metro area a real tough forecast for this storm. Just taking a casual look at the computer runs would have a basic thinking of real heavy snow as a closed upper level system and surface low both move south of the area. The surface low from northern Texas this afternoon to northwest Ohio by Wednesday morning where it stalls for a while. This is the favored track for heavy snow for the metro area. As usual though, it is the devil in the details that make the difference.
First we have today's sun which has produced highs in the upper 30s to lower 40s, and even mid 40s far south. There is no snow cover south of roughly I80. The models show snow down there so already their low level temperatures are wrong. This is critical as it looks like most of the metro area will see highs tomorrow around or more likely just above freezing at the surface. The above freezing temperatures obviously will somewhat limit the snow accumulation. Also, heavy wet flakes have more compaction than dry ones.
The snow will be spreading from south to north over the metro area midday Tuesday. South sections late morning and north sections early afternoon. A little rain could mix with the rain at onset. The mixed rain and snow will turn to snow after an hour or two. Heaviest snowfall will be late afternoon to early evening before it turns to just occasional light snow late at night. The evening rush home will be a slow go.
Taking into consideration the high water ratio of the snow, the above freezing temperatures, the system actually be weakening some as it moves through northern Illinois, the lack of frontogenic action aloft, the best vertical velocities are not matched to the snow dendritic growth zone, I come up with 1-3 inches far southeast to a general 3-5 inches the rest of the metro area by Wednesday morning with an isolated 6 inch report possible. The graphic shows the model consensus 12 hour forecast valid midnight Tuesday night. Another 1-2 inches of snow will fall during Wednesday as another piece of energy moves overhead.
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