A late winter storm system is lifting into the Midwest and is likely to produce the Chicago area’s heaviest rainfall in more than a month beginning late tonight and running into Thursday—while producing a snowstorm from Iowa into Wisconsin—with the axis of heaviest snow in the Badger State likely to be centered along and either side of a Prairie du Chien in far southwest Wisconsin
northeastward to Wisconsin Dells (including the Madison area) to the Door Peninsula line in northeast Wisconsin (including the Green Bay area).

The rains, which may include embedded t-storms here in Chicago as we move into Thursday morning, are to reach Chicago proper from the south between 10pm and midnight—sooner to the southwest—and continue into Thursday afternoon—likely heaviest with the greatest thunder threat in the 5 to 8 or 9am time frame.  

This would put the storm’s heaviest rainfall into a good part of the Thursday morning commute period.

Sub-freezing dew points mean evaporative cooling on the front end of tonight’s rain may lead to a period of sleet in the city on the front end of the rain area. A transition to liquid-form precip follows fairly expeditiously.

That we’re to sit beneath the nose of a powerful upper air wind max places the Chicago area in the area of greatest vertical motion Thursday morning which is the basis for the expectation of heaviest precip and possible thunder in some locations in the 5 to 8 or 9am time frame.

Rains will continue—but at a lighter and possibly more occasional pace later Thursday morning and afternoon before exiting the area.

Rain may shift to snow Thursday afternoon, mainly over far northwest sections of the metro area toward McHenry, Boone and Winnebago areas. And snowflakes aren’t out of the question in parts of the Fox Valley—even a few as far east as Chicago—before precip winds down.

Blending the predictions of multiple computer model projections leads to the expectation much of the Chicago area may see 0.60″ to as much as 1.40″ of rain—with average tallies coming in around one inch. That would make this system the heaviest precip producer here since 1. 1.12″ fell back on Jan 3rd—more than a month ago!

Brief and modest cooling Friday (with daytime highs holding to the upper 30s) gives way so a return to the milder temps over the weekend which have dominated recent days.

Highs surge to 40 Saturday and 44 Sunday at a time of year that highs from 34 to 35-deg are “normal”. (“Normals”, as defined meteorologically by the World Meteorological Organization, are smoothed averages of the most recent 30-year period). 

Interesting notes on home the active pattern we’re entering differs from February’s opening 7 days comes from an analysis by my WGN meteorological colleague Mark Carroll who notes the “TRACE” of precip which has occurred Feb 1-7 marks the driest February open in Chicago in 32 years (since 1991). And the fact there’s been NO SNOW from Feb 1 to 7 is the first time a February’s opening week has opened snow-free here in 18 years.

Longer range—above normal temps continue until a downturn hits later next week. Wednesday marks the 35th of the past 39 days ABOVE NORMAL. This week will finish with an avg temp nearly 8-deg ABOVE NORMAL and next week is currently predicted to finish 4-deg above normal, even factoring in colder late week temps.

Perhaps most important is the fact the Chicago weather pattern is to be an active one—with not only the incoming storm to produce precip but two systems being monitored next week—the first in the Tue/Wed time frame and potentially a second one in the Thu/Fri period. If one of those is going to produce snow here or nearby—the best chance is with the second one, though NOT a slam dunk yet. Just a system to monitor—and we’ll be doing that.

Full forecast details at the WGN Weather Center

Strong winds possible Thursday afternoon and evening with gusts building to
40 to 50 mph likely

TONIGHT: Turning cloudy. Rain developing—reaching the city in the 10 to 12 midnight time frame where it may begin as sleet—and becoming heavy at times with possible t-storms in the hours approaching daybreak. Turning fairly windy from the east.  Low 33—then rising slowly. 

THURSDAY: Rain, possibly heavy with thunder in some areas through mid morning. Then showery, turning quite windy with gusts building to 40 mph and higher. Colder in the afternoon and evening when rain may change to a period of snow, mainly in northwest sections of the Greater Chicago area (toward McHenry and Boone counties west to Rockford in Winnebago county with some mixed snowflakes possible in parts of the Fox Valley).  High 44 falling to the
30s in the afternoon.

THURSDAY NIGHT: Windy and modestly colder. Brief light showers or rain or snow possible. Low 25.

FRIDAY: Some cloud breaks develop allowing mixed sun, breezy and a little colder. A few flurries possible mainly northwest Indiana. High 36.

SATURDAY: Mostly sunny, milder by afternoon. High 40.

SUNDAY: Sunshine yields to increasing cloudiness. Still milder. High 44.

MONDAY:  Partly sunny, mild. High 45.

TUESDAY: Increasing cloudiness. Growing rain prospects at night. Mild. High 47.

WEDNESDAY: Some possible rain or sprinkles wind down—but a good deal of cloudiness is likely to linger. Remaining mild for the season. High 44.