The latest in a series of storms that came in off of the Pacific will very slowly move east over the next two days. Roughly the eastern 3/4 of the country is covered by a very broad upper level trough. As this trough slowly moves east and fills, the surface low that is stalled over lower Michigan will also slowly move east and fill. As the system moves and fills, areas of flurries and very light snow will from Iowa eastward will slowly come to an end.
The graphic from the NWS shows the snowfall over the last two days. Impressive to say the least over parts of the Texas panhandle and Missouri where local snow amounts reached 18 inches. The snow desperately needed as those areas still in an "exceptional" long term drought.You can get an idea of where the surface low moved, from northern Arkansas into northeast Indiana and then central Michigan. The snow band was to the right of this, mainly just northwest of the 850mb (5000 foot) low.
For the Chicago metro area, snow amounts ranged from only 1-2 inches southeast to a general 4-6 inches over most of the rest of the area. A complete list of all snow amounts and local maps can be found here. The problem area is far northeast Illinois and southeast Wisconsin where local amounts were up to 11 inches. The area shows in the graphic, the bright spot southwest of Kenosha. What made that area so special?
The answer for the above question is a form of lake effect. We are not talking about the normal lake effect that produces lake effect snow. That happens when very cold air aloft moves over the warmer lake waters. If the temperature difference between the lake waters and the air roughly 5000 feet higher is 13C or greater, a very steep unstable lapse rate is produced over the lake. This allows the evaporated lake water to rapidly rise and form the lake effect snow showers.
The case yesterday was due to frictional drag near the lake shore. The very strong low level east winds over the lake blew inland where the land slowed down the wind. The air piled up, rose, and produced the enhanced snowfall over the area. It was interesting that for the previous two days the models showed this effect. The problem was it was shown all the way down through Chicago. If you have driven along the lake in Chicago and immediately north, the land slope inland is pretty darn flat.
The land slope inland really does not pickup until one is well north of the city. It is that area that has the higher land where the convergence took place and enhanced the snow. There were a whole list of meteorological challenges to the Chicago snow forecast which I mentioned on Monday. This was one of them which I did not mention, but really concerned me. NWS Chicago was also very challenged by the computer output. This was the first time I can remember seeing this, so no basis to confidently judge the computer output. It did occur in parts, but not over the immediate Chicago area. Thankfully no forecast I heard went with snow amounts of 10+ inches for the city. The common number was a 3-6 and less southeast, which was pretty darn close for most of the metro area.
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