Warming trend arrives ahead of the weekend
Ten of the first 12 days in September have been below normal, averaging about 5 degrees cooler than average. Wednesday should be the 11th subnormal day this month, then a significant warming trend should set in, beginning Thursday and continuing through the coming weekend.
The upper-air pattern that has prevailed for much of the time since mid-August shows signs of a major reorientation. The cooling northwest flow that has prevailed here will shift east, as a wide trough of low pressure expands east across the Rockies into the Plains.
The resulting southwest flow will steer warmer air into Illinois and the Great Lake. High temperatures look to approach 80 Thursday, then hover well in the 80s Friday, Saturday and Sunday. The approach of a cold front could trigger showers Sunday.