Today was a great day to see the power of the snowpack, the tremendous influence it has on the temperature. The snowpack is obviously going to add a chill to the air. The energy from the atmosphere is going into melting the snow vs warming the ground which warms the air. The deeper the snowpack the greater will be the chill. This becomes especially noticable on a day where the surface winds are relatively light. With stronger winds there is more mixing of the atmosphere near the ground to bring down the warmer air vs lighter winds.
The graphic from Stormeyes.org shows the roughly 4:00 p.m. CDT Sunday surface map. Notice the area south of Chicago where the surface temperatures quickly drop from the lower 50s to the upper 40s. That is the approximate edge of the snowpack, a map of which can be found here. The farther north you go where the snow is deeper, the lower the temperatures become. This was a good day to observe this affect since all of Illinois and Wisconsin were under low clouds and rain so basically equal big picture conditions. Over Indiana where temperature were in the 60s there was no rain, higher clouds, and stronger winds so not an equal comparison.
For the Chicago metro area, the rain will taper off the a few showers later tonight and then some rain or wet snow showers on Monday. Highs will only be in the 35-40 range the next few days. Warmer weather with highs in the upper 40s will return by the end of the week as the upper level flow brings in milder Pacific air. The normal high for Chicago is now up to 44 and rising.
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