In a dramatic about-face late last week, the persistent cold weather for the first three weeks of December have flipped to well above-normal readings, including 84-plus straight hours above freezing temperatures beginning last Thursday morning.
This mild spate should continue through the week, tapping air now in the central Plains. On Sunday, Denver reached a record high of 69° with westerly Chinook wind. A modified version of this mild air spreads east early in the week, with 45° possible at Chicago on Tuesday.
The catalyst for the change is the breakdown of the upper ridge in the West, which had forced frigid arctic air southward with a northwesterly jet stream. Now, the jet stream flows zonally west-to-east, bringing mild Pacific air with above-normal temperatures to most of the country. Respectable Pacific weather makers slam into the Northwest, but dry considerably before reaching Chicago.
WGN-TV Chief Meteorologist Tom Skilling and the WGN Weather Center staff provide daily coverage of weather in the Chicago area.
