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1. We’re starting into the cloudiest part of the year in Chicago

December and January average only 40% of possible sunshine (based on 1991-2020 data)—the lowest monthly average—percentages start to rise in February (57%). So far, this December is running a little above average sunshine, but it looks like we’re headed into a cloudy stretch ahead.

Each of the past five Decembers have averaged well above the normal sunshine, but there is little correlation after that as to what occurs in January or February, or for the overall winter average percentage of possible sunshine as well. The graph/data does show that only 5 of the past 15 winters have received average or better sunshine.

2. Wednesday set up

  • Upper-level 30,000-foot jet stream flow and low-level/surface temp distribution
  • Polar jet flowing out of the north pushes bitter arctic-source cold air south to near the U.S./Canadian border
  • Leading edge of cold air filters into the northern Great Lakes
  • Chicago is positioned directly under the subtropical jet flowing out of the southwest—the SW jet holding the cold air to the north of Chicago—allowing “transitional,” more seasonably normal/average temps to reside over our area
  • With minor variations, this pattern will stay relatively unchanged for a period of time. Under this upper-air flow pattern, storm systems that affect our area will originate to our west and southwest

3. Daily average temperatures

With a west/southwesterly jet stream aloft, seasonably above-average temps hold over our area until colder air hits, following a large low-pressure system triggered by a buckle in the jet stream the middle of next week

4. Chicago’s next rain producer hits Thursday night into Friday

The center of low pressure will move ENE out of SE Colorado and the Texas Panhandle Thursday. Associated clouds will spread over our area during the day Thursday, with rain beginning here late in the day or during the early evening hours. Rain will continue overnight into Friday, gradually ending from the west as the low pressure pulls away from our area during the day Friday.

There will be snow to the north of this system. The rain/snow line is most likely located along the Illinois/Wisconsin border with some rain, wet snow mix and possibly even minor accumulations in our northernmost counties adjacent to the state line. We’ll know more on the exact storm track, as time passes.