A major weather system will gradually spread across the country this week. A very broad upper level trough will initially form over the Rockies the early part of the week and then move eastward later in the week. A warm and moist south flow will develop ahead of the trough. The moist and warm air south will overrun a front that will stall out through the middle of the nation as Canadian high pressure moves across south Canada.
The front will serve as a focusing mechanism for periods of rain and thunderstorms as the system gradually moves east. The graphic shows the expected average rainfall through Friday morning. There will be locally higher amounts where thunderstorms train over an area. This rain is great news for farmers as it will build subsoil moisture. It is tremendous news for those areas in the Plains still in a long term drought.
The Chicago metro area and northern Illinois will see both sides of the front during the next few days. For Monday we will be on the warm side with highs in the 60s. Unfortunately, the front will slide down lake Michigan to put most of the area on the cold northeast wind side. Temperatures will range by Wednesday to only near 40 north of the front near the lake to the low 60s far south.
Showers and thunderstorms will spread over the metro area very late tonight and end by midday Monday. Highs with south winds will jump into the 60s. A front will stall out over far northern Illinois by Tuesday morning and then gradually slide south during the day. By late afternoon it will only be in the upper 40s far northeast near the lake to around 70 far south. Low pressure will then form on the front to increase the very chilly northeast winds for Wednesday and Thursday. Highs by Thursday look to be only 40-45.
There are two positives of the front moving south of the metro area. The first is that the possible severe weather will be south of the area. We will receive the rain without the threat of damaging winds and tornadoes. The other is the heavy rain along the front will end the "abnormally dry" drought condition over northwest Illinois. By the end of the week no areas in Illinois will be classified in any stage of last years drought. That is really good news as we enter another growing season.
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