textproduct: Chicago

KEY MESSAGES

- Very cold the rest of the weekend with dangerously cold wind chills expected tonight into Sunday morning.

- Moderating temperatures expected during the upcoming work week.

DISCUSSION

Issued at 245 PM CST Sat Dec 13 2025

Snow across the southern third or so of the CWA is expected to end by 23-00z. Have pushed up the end time of the winter weather advisory to 00z. While the falling snow will end, given the character of the snow that fell today there will probably be enough wind tonight to cause some minor drifting back onto roads in non-wind protected areas.

Otherwise, look for skies to quickly clear out early this evening, though with sfc high remaining well to our west, winds should stay up around 10-15 mph. The low level cold air advection will keep boundary layer rather mixed and limiting radiational cooling, but just straight advection should be sufficient for temps to drop to near zero or below zero most areas. Downtown Chicago and downwind of Lake Michigan in Porter County are the main exceptions, where lows should remain a bit above zero.

With winds not expected to decouple and temps dropping to near/below zero, wind chills of 15-25 below zero still look like a good bet overnight into Sunday morning. Cold wx advisory for tonight/Sunday morning looks to be in good shape and no changes are planned.

Lake effect snow showers are expected to over northeast Porter County tonight into Sunday morning. Expecting multi-band structure to the lake effect and somewhat limited fetch for Porter County, so probably looking at accumulations of 1-3" by midday Sunday when the lake effect is expected to end.

By sunset Sunday evening, Arctic high pressure should be centered over central and southern portions of Illinois and Indiana. There should be a window for strong radiational cooling early Sunday evening, particularly southern CWA in closer proximity to the high center and where there will be fresh snowpack from today's system. Currently have evening lows around 5 below near the Hwy 24 corridor, but wouldn't be surprised to see temps dip down to the double digits below zero. This would also open to the door to some patchy shallow freezing fog in those areas, which may need to be added to later forecast updates for the southern CWA.

The sfc high will quickly move eastward Sunday night, likely reaching West Virginia by sunrise Monday morning. This will allow southerly winds and warm air advection to quickly ramp up through Sunday night. Following evening lows, most areas should see temps rise 10F+ through the night. The rising temps will be accompanied by strengthening winds so any moderation in temps will be offset by the wind chills.

Large scale pattern over North America will quickly transition to zonal for the upcoming work week. The main storm track is expected to remain well north of our area, so we'll be dominated by Pacific origin air mass. Extensive snow cover locally and upstream should temper the moderation some, but still looks like temps should begin climbing above freezing by Tue and Wed afternoons.

The first fast moving system passing to our north should drag a weak cold front across the area Tuesday evening. Very little cooling is expected behind this front, with warm air advection ramping back by later in the day Wednesday. A stronger trough and more vigorous sfc low is progged to track along the Canadian border into the upper Great Lakes by Thursday. Both GFS and ECMWF have a ~980mb sfc low reach Lake Superior, which will result in very tight pressure gradient resulting in strong south winds and warm/moist advection. GFS/ECMWF both have the 50F isodrosotherm close to or into our CWA later Wednesday night into early Thursday, which should this verify would rapidly decimate our snow pack!

Trailing cold front from this system should sweep across the area Thursday, probably accompanied by some rain showers. A quick hitting shot of colder (more seasonable) temps are likely in the wake of this front, but with flow remaining zonal temps look to rebound again to above average levels later Friday into early next weekend. After today's snow in our southern CWA, the overall pattern in the medium range guidance looks to maintain above to much above average temperatures with only brief cool downs through the next 2 weeks. This doesn't bode well for any meaningful snow chances prior to Christmas. Obviously a lot can change and can't rule out a sneaky rouge lower amplitude system producing some snow, but with the upcoming pattern likely to wipe out most or all of our current snow cover, chances for a white Christmas are looking dismal.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 512 PM CST Sat Dec 13 2025

Forecast concern for the terminals...

- Breezy northwest winds tonight gusting 20-25 kts.

The clipper system continues to progress eastward across north- central IL and IN where accumulating snow continues, but snow should be ending within the next 1-2 hours. As for the terminals, current VFR cloud cover will scatter out over the next hour as an arctic surface high moves into the area. While mostly clear skies are expected to prevail through Sunday, some VFR lake effect clouds look to linger across portions of northwest IN (mainly east of GYY) through Sunday afternoon. Otherwise, breezy northwest winds will increase tonight as strong cold advection allows the atmosphere to tap into breezy winds aloft with 20-25 kt gusts expected through mid-morning Sunday. Gusts will wane Sunday afternoon with speeds remaining in the 10-12 kt range through the end of the period, though directions will become more westerly and eventually southwesterly by Sunday evening.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...Cold Weather Advisory until noon CST Sunday for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

Winter Weather Advisory until 6 PM CST this evening for ILZ019- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039.

IN...Cold Weather Advisory until noon CST /1 PM EST/ Sunday for INZ001-INZ010-INZ011-INZ019.

Winter Weather Advisory until 6 PM CST /7 PM EST/ this evening for INZ010-INZ011-INZ019.

LM...Small Craft Advisory until noon CST Sunday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 9 PM CST Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.