textproduct: Chicago

KEY MESSAGES

- Isolated to scattered snow showers possible on Wednesday with a point-based 20-40% chance for any measurable snow

- Well below normal temperatures Wednesday night through the weekend, with the coldest period bringing minimum wind chills of about 10 to 20 below Thursday morning and Thursday evening.

- Additional chances for snow Saturday night-Sunday and again early next work week.

DISCUSSION

Issued at 250 PM CST Tue Dec 2 2025

Tonight through Wednesday:

The stubborn low stratus deck kept temps for most locations solidly below guidance (teens to around 20F), except near the lake in Illinois and NW Indiana where a break in the overcast allowed for highs to reach the lower to locally mid 20s.

As high pressure centers over the lower Ohio Valley tonight into Wednesday morning while a cold front (trailing from strong low pressure over Hudson Bay) approaches from the northwest, strengthening low-level southwesterly flow should at least temporarily erode the low clouds. Skies will still remain mostly cloudy from mid and high level clouds though. Expect nearly steady to slowly rising temps in response to southwest winds gusting to 20-25 mph at times. Thinking is that the snowpack should generally limit stronger winds than might have occurred without snow on the ground.

The cold front will move across the area from the late morning- midday across the northwest half or so of the CWA and through the southeast half in the early to mid afternoon on Wednesday. With the strongest cold advection lagging well behind the front and only modest large scale forcing, this is not a typically favorable setup for much, if any snow, near the cold frontal trough. Largely discounted the more aggressive NAM and 15z RAP depictions in favor of the conceptual model and global guidance which is far more paltry with QPF.

In general, with the non-NAM forecast soundings lacking saturation above -10 or -11C, even getting snow showers will likely depend on a sufficiently deep saturated layer (and steep enough lapse rates) reaching cold enough temps aloft. With the stratus layer possibly eroding tonight, it's unclear to what extent this will be the case. With all of this being said, lowered PoPs down to 20-40% for potential isolated to scattered (likely non-accumulating) snow showers and perhaps brief reduced visibility. There may be a short window for some lake enhancement into northwest Indiana, though primarily east of Porter County.

The other item of note on Wednesday is that it offers the best chance to approach or locally exceed the freezing mark (30-34F forecast range) until sometime next week.

Castro

Wednesday Night through Friday Night:

A lobe of the tropospheric polar vortex that's been wobbling around Hudson Bay area of Canada for a few days (and associated with the deep surface low driving Wednesday's cold fro-pa) is expected to remain there for at least the next week. A strong shortwave trough digging down the western flanks of the polar vortex into Wednesday evening will result in an amplifying upper trough, dislodging some pretty brutal early season Arctic air south into the region Wednesday night through Thursday evening.

The duration of the bitterly cold air looks short lived, but could be rather potent with both high and low temps potentially getting within a few degrees of the records for Chicago and Rockford (see climate section below). One interesting forecast element is that the progged 925 mb temps are not exceptionally cold, so the unusually deep snow cover for early December will be doing some work.

Temps Wednesday night should drop below zero across most of interior northern IL assuming skies at least partially clear out, which it looks like they should. Temps may struggle to get much above 10F for the coldest interior areas of northern IL on Thursday, with even "milder" areas close to the lake in NW IN and in the Chicago urban corridor (and near the lake in NW IN) only reaching the 15-20F range for highs. The surface high will slowly shift east Thursday evening, likely supporting pretty ideal radiational cooling conditions, allowing for temps to rapidly drop to below zero (locally well below zero) outside of Chicago. It wouldn't be surprising if typical favored cold spots briefly dip down to 10 below zero or even a bit colder by the early to mid evening.

As the surface high moves farther east in the late evening and overnight, southerly winds will develop and result in temps leveling off and then rising overnight Thursday night. Temperatures will then "warm" up to the lower-mid 20s on Friday ahead of another weaker cold front approaching on Friday night. We'll need to watch for a short period of light snow or snow showers (~20% PoPs) for now as a low-amplitude short-wave traverses the region ahead of the front.

Castro/Izzi

Saturday through Tuesday:

The large scale pattern will remain active but northern stream/Polar jet dominated next week. This could yield a few additional opportunities for snow (~30-50% PoPs), but unlikely to cause any higher end impacts. The first of these windows is with a clipper/hybrid type system passing nearby to our southwest Saturday night-Sunday, with another (possibly more wound up) system Monday night into Tuesday. Temperatures will remain below normal, but not as cold as our late Wednesday night through Thursday stretch this week.

Castro

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 612 PM CST Tue Dec 2 2025

Key Messages:

- IFR/MVFR ceilings are expected through most of the TAF period.

- There is about a 40% chance for snow showers from around midday tomorrow through the afternoon.

- A northwesterly wind shift is expected tomorrow behind a cold front.

The region remains socked beneath a largely IFR stratus deck early this evening. The western terminus of this cloud deck was inching eastward, but it's still not entirely clear whether it will reach the terminals and/or whether some additional scattering of this cloud deck will occur to allow for a period of VFR conditions tonight into tomorrow morning. Either way, IFR or MVFR ceilings will likely be observed tomorrow both ahead and behind a cold front dropping southeastward through the region before the low ceilings likely clear out for good tomorrow evening.

As the cold front passes through the area, it may be accompanied by some snow showers. There is still some uncertainty over how widespread the coverage of these snow showers will be in northern Illinois and northwest Indiana, so the going PROB30 groups still appear to be appropriate for conveying this possibility. If snow showers were to occur, they would likely struggle to accumulate, but the snowflakes would likely be of poor quality and be capable of knocking visibilities down even lower than what is presently advertised in the going TAFs.

Southwesterly winds will turn a bit gusty tonight as the cold front approaches the area, but the existing snowpack and low inversion heights should play a role in tempering the magnitude of gusts. Winds will then shift to a northwesterly direction behind the front with post-frontal winds likely retaining magnitudes similar to those of the pre-frontal winds.

Ogorek

CLIMATE

Issued at 542 PM CST Tue Dec 2 2025

Updated at 250 PM CST Tue Dec 2 2025

Here are the current record low temperatures for Thursday into Friday this week:

Chicago Cold High Low Thursday 12/4 13 (1991) 0 (1893) Friday 12/5 4 (2005)

Rockford Cold High Low Thursday 12/4 7 (1991) -4 (1991) Friday 12/5 -5 (2005)

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 PM this evening to 10 AM CST Wednesday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory from 9 PM Wednesday to 9 AM CST Thursday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory from midnight tonight to 3 PM CST Thursday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.