textproduct: Chicago

KEY MESSAGES

- Light snow with likely some travel impacts tonight, mainly this evening, southwestern CWA

- Additional rounds of accumulating snow expected Thursday night and again Saturday, heaviest amounts likely south of I-80 Thursday night.

- Bitterly cold temperatures and potentially dangerously cold wind chills are expected this weekend.

DISCUSSION

Issued at 228 PM CST Wed Dec 10 2025

Through Thursday Night:

Active winter weather pattern will continue with a couple more shots of snow just in the next 24-36 hours.

First up, sheared/channelized shortwave is noted on water vapor imagery over western MN early this afternoon. Fairly widespread snow is accompanying this feature over eastern MN, with 1-3SM VSBY pretty common and a few less than 1SM snow obs. Most guidance is really not handling this feature particularly well, though RAP/NAMNest appear to be handling it the best. Have increased pops to likely this evening over southwestern CWA where RAP/NAMNest are favoring the best, albeit still quite light QPF (just a hundredth or two).

Thicker stratus associated with this snow shows up quite nicely on visible satellite imagery, with recent satellite trends leaving me a little concerned that this light snow activity could end up a bit farther east and deeper into our CWA than guidance would suggest. For now stretched the chance pops farther east, but oncoming evening shift will need to closely monitor satellite and radar trends and may need to tweak forecast accordingly. Thinking accumulation should end up an inch or less, but with temps below freezing and lingering road treatment probably mostly washed away, even a coating of snow on roads could cause hazardous travel conditions.

Stratus deck blankets most of the western Great Lakes and Midwest and is also not be handled particularly well by most guidance. Forecast soundings maintain a frontal inversion through the night and into tomorrow morning with this stratus deck trapped beneath it. Lacking any particularly strong subsidence, the stratus should hang on through the night, so have bumped up cloud cover and temps accordingly in the grids for tonight.

Shortwave on the nose of a 160kt 300mb jet have plowed onshore into southern British Columbia this morning. This feature is progged to zip across the southern Canadian Rockies tonight before diving southeast around the western flanks of the deep longwave trough over the Great Lakes and eastern Canada. There remain split solutions in handling this feature, with most highres/CAM guidance favoring a farther north track with our CWA ground zero for the heaviest snowfall. Global models on the other hand are farther south, with the axis of highest snowfall totals across central IL. In fact, GFS and ECMWF would keep accumulating snow south of the city of Chicago with a couple of inches of accumulation across our far southern and southwestern CWA.

Given this shortwave was still offshore at 12z this morning, I tend to lean a bit more toward the global model's handling of this feature. Of the 3 big global models, the GFS is the farther north and have largely followed that solution as a bit of compromise. NBM pops have trended downward northern CWA and this seems reasonable, so made no adjustments to NBM pops. Which conveys the uncertainly with farther north CAM/highres guidance. If global models don't shift back north, then pops over northern CWA will need to be lowered in subsequent forecast updates.

- Izzi

Friday through Wednesday:

With good agreement in all accumulating snow likely having shifted out of the area prior to daybreak Friday, PoPs nudged down to slight chance in the morning will probably be able to be removed with later forecasts. A short-wave associated with weak surface low pressure over Lake Superior will push across the western Lakes in the afternoon and early evening. This wave may bring a glancing blow of flurries and perhaps a few true snow showers (~20% PoPs) to approximately the northeastern 1/2 or 1/3 of the CWA towards and after sunset. The cold front trailing from the surface low will bring the first shot of modified Arctic air for the bitterly cold weekend in store (more on that below).

Expansive 1040+ mb Arctic high pressure will gradually spread southeastward Friday through the weekend, from the Canadian Prairies Friday, to the mid-upper MO River Valley Saturday night, and to central Illinois and Indiana Sunday night. The initial cold front passage Friday night will bring lows in the single digits and lower teens and wind chills of about 0 to 15 below. The final clipper-like system in the parade of these systems over the past 7-10 days will likely bring a swath of accumulating snow to portions of the region during the day on Saturday, interacting with a tight lower to mid-level baroclinic zone in place (implied frontogenesis for enhanced mesoscale banding).

Similar to the Thursday night system, the southward shift of the focus for a potentially plowable fluffy snowfall in recent model cycles continued with the 12z global ensemble guidance. With this in mind, the highest PoPs of nearly 70% are focused near/south of the Illinois and Kankakee River Valleys. That said, felt comfortable with likely threshold (55-60%) PoPs still up to the I-88 corridor for a few key reasons. Low amplitude features like Saturday's are prone to larger shifts in relatively short lead times in their meaningful QPF and accumulating snow footprints. In addition, there's still a respectable % of ensemble members (rough estimate of about 30%) with the favored axis as far north as the Chicago metro.

Another important item to keep in mind is that with surface temps in the single digits and teens, much of the column will reside in the DGZ, and thus it won't take much for light snow to be wrung out. Also, notably for this aspect, with air temps so cold, road treatments are typically rendered less effective and often make for worse travel impacts with only light amounts of snow vs. with temps closer to freezing (such as today). Wherever the enhanced mesoscale banding sets up, the deep DGZ and more favorably aligned ascent through it could certainly present a chance for well above climo 15-20:1 ratios and only modest QPF fluffing up to several inches of snow. Experimental probabilities based on ensemble membership currently would suggest the highest chances of this being somewhere across central Illinois and Indiana.

Regardless of how the Saturday daytime snow plays out (or if some areas are even missed altogether), the snow cover across the region, approaching Arctic high, and 850 mb temps of -15 to -20C will set the stage for the coldest night of the month thus far. Sustained northwest winds of 10-15 mph and temps plunging below zero for most of the area (possibly near -10F in spots west of the Fox Valley) will yield wind chills of 15 to 30 below zero. A Cold Weather Advisory will likely be needed for most of, if not the entire CWA into Sunday morning.

The surface high will slide east overhead on Sunday, limiting mixing heights but also bringing lighter winds as highs reach the positive single digits and lower teens. The surface high position over central IL Sunday evening will be close enough for temps to tank everywhere except probably Chicago despite onset of warm air advection aloft. Expect recovery into the 20s on Monday and then lows in the teens Monday night. This will be merely a prelude to notably milder conditions expected the rest of next week with a change to a zonal jet stream pattern. Temperatures may approach 40F next Wednesday barring substantial effects from low clouds, which is possible this time of year. Current signs point to the overall milder pattern lingering towards Christmas. Precip chances will remain low through the current day 7.

Castro

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1139 AM CST Wed Dec 10 2025

Snow is coming to quick end from northwest to southeast across the terminal airspace late this morning. In spite of this, MVFR stratus remains rather expansive late this morning, and is currently encompassing the Upper Midwest and western Great Lakes region. There thus is no near term hope of scattering the clouds out, so expect MVFR CIGs to persist, possibly even into the day on Thursday. Otherwise, winds will remain gusty (25-30kt) from the northwest into this afternoon, but will abate with sunset late this afternoon.

Another quick shot of light snow and flurries is expected this evening as an impulse over MN late this morning shifts into our area. For the latest set of TAFs we have maintained just a flurry mention with this activity. However, we will have to keep an eye on the potential for some brief IFR VSBYs with any heavier pockets of snow that occur this evening, particularly at KRFD. This activity will end prior to midnight.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Gale Warning until 3 PM CST this afternoon for Winthrop Harbor IL to Gary IN.

Gale Warning until 6 PM CST this evening for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.