![]() ![]() Sleet was in fact observed for a time Monday afternoon. If the elevated warm layer is shallow enough and the surface refreezing layer is deep enough, falling snowflakes melt slightly and then refreeze into ice pellets (sleet). However, with an elevated warm layer above a subfreezing layer, sleet and freezing rain start to become a concern. When all levels of the atmosphere are below freezing, snow is the expected outcome. In terms of precipitation type, this elevated warm layer makes a big difference. It is common to see milder air like this along a warm front riding up and over colder and denser air in the surface layer. However, at Midway, around the 5000 ft level, the vertical temperature profile showed a layer of warmer above-freezing air. In fact, this vertical temperature profile looked very similar to that measured at the exact same time by a plane departing from O’Hare Airport north of Midway. This was similar to the cold air being seen to the north of this location. In between these two locations, roughly from northern Indiana along the I-80 corridor of Illinois, the situation was more complicated.Īround 5:00 PM on Monday, weather instruments on a plane taking off from Midway Airport recorded a subfreezing layer of air from near the surface up to around 5000 feet. In warmer locations well south of the boundary toward central Illinois, above-freezing temperatures from the surface up to around 5000 feet were supporting widespread rainfall. In the cold air well head of the advancing boundary, subfreezing temperatures at all levels of the atmosphere were supporting areas of moderate snowfall across Iowa, Wisconsin, and far northern Illinois. During the afternoon this boundary, along with the precipitation, began lifting north as a warm front across Illinois and Indiana. ![]() Ahead of this low pressure system, a broad area of precipitation stretched from Texas into the central Atlantic coast along a frontal boundary. ![]() Early on Monday afternoon, February 11, a center of low pressure was developing across the central United States. ![]()
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