textproduct: Chicago

KEY MESSAGES

- Winter Weather Advisory for the entire area into tonight for 1-3 inches of snow, with isolated 4 inch totals possible, especially south of the Kankakee River.

- Quick shot of Arctic air with near record cold temperatures possible late Wed night through Thursday night.

DISCUSSION

Issued at 225 PM CST Mon Dec 1 2025

Through Tonight:

Once again, no changes were made to the Winter Weather Advisory in effect through this evening/early overnight. Will let the evening shift assess the need for any changes to the planned end times.

The short-wave bringing the accumulating snow through this evening is positively tilted, albeit packing fairly robust forcing for large scale ascent. This will be augmented on the mesoscale by lower to mid-level frontogenesis that will lead to transient enhanced banding with associated higher snowfall rates, primarily south of I-80 per extrapolation of most recent regional radar and latest guidance trends.

Despite a deep DGZ for this event, forecast soundings depict some limiting factors for widespread, more efficient fluffy dendrite type snowflakes. Not seeing complete saturation through the DGZ (lacking supersaturation with respect to ice) and the strongest ascent is primarily focused above the DGZ. In these scenarios, it's common for relatively small snowflakes that are still effective at reducing visibility but accumulate less efficiently (ratios as low as 10:1). The wildcard here is wherever f-gen associated banding sets up, which again should be into the southern half or third of the CWA. These areas will be the most likely to have a more sustained period of 15-20:1 type ratios and temporary snowfall rates between 1/2" to 1"/hr.

As alluded to above, once the accumulating snow starts, the smaller flake size (for most of the CWA) will actually be quite effective in knocking down visibility. Upstream sites over the past couple hours have featured plenty of 1/2 to 3/4 mile observations. With temps in the 20s during the snow, untreated roadways will become snow covered and slippery, making for hazardous travel conditions during with the evening commute.

Updated event total snowfall amounts again came down a bit particularly for the northern 1/2 of the CWA. Most locations north should end up in the 1-3" range by the time the snow ends tonight, lowest near the Wisconsin border, with the best chance for amounts in the ~3-4" range south of the Kankakee River Valley. We'll continue to message a general north to south range of 1-4". Temperatures will drop into the teens to lower 20s after the snow ends, though with west-northwest winds only 5-10 mph by then, wind chills won't be much lower than the air temps.

Castro

Tuesday through Monday:

Low confidence sky cover forecast for Tuesday with big spread in guidance. The typically more reliable ECMWF and HRRR with handling post-frotal CAA stratus both would suggest we stay BKN-OVC during the day Tuesday. Would normally fairly confidently follow the lead of these models this time of year, but interestingly, satellite imagery doesn't show all that much stratus. Have trended slightly higher for sky cover tomorrow, but it wouldn't be surprising if it were completely sunny nor would it be shocking if we stayed OVC all day. If skies do clear out, then high temps could end up a couple of degrees warmer than our current forecast which leans more toward a cloudier solution.

If skies clear out during the day Tuesday, it would be short lived as mid-high level OVC should quickly arrive Tuesday evening in advance of the next system. The expected cloud cover and strengthening southerly flow should result in minimal drop in temps Tuesday night, with temps possibly even rising a couple few degrees late.

Tropospheric lobe of the Polar Vortex has been wobbling around Hudson Bay area of Canada for a few days and is expected to remain there for at least the next week. A strong shortwave trough digging down the western flanks of the Polar Vortex should Wednesday should result in an amplifying upper trough, which will dislodge some pretty brutal early season Arctic air south into the Midwest and Great Lakes 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).

Forcing associated with that digging trough should bring some snow showers to the area Wednesday. While cold fronts aren't typically overly efficient snow producers, strong low-mid level frontogenesis associated deep vertical circulation paired with coupled upper level jet streaks should result in strong forcing. It doesn't look like a big snow, but if coupling of jets pans out as progged, then some areas could end up with a coating to an inch of snow and some travel impacts.

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, with even "milder" areas close to the lake in NW IN and in the Chicago urban corridor only looking to get just barely into the teens for highs on Thursday. Depending on how quickly the surface high moves east Thursday evening, we could have pretty ideal radiational cooling conditions allowing for a rapid drop in temps to below zero readings Thursday evening. As the sfc high moves farther east, southerly winds should develop and probably result in temps leveling off or even rising overnight Thursday night.

A moderation in temps is expected Friday into the weekend when another system could bring snow to somewhere in the region.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1155 AM CST Mon Dec 1 2025

* A period of snow and IFR conditions expected late this afternoon through mid-late evening

* Lingering IFR CIGS are possible overnight into Tuesday morning

A large area of snow over Missouri and southern Iowa, currently approaching the Mississippi River, will overspread the terminals late this afternoon into this evening. Recent aircraft observations from MDW depict a deep layer of dry air, roughly 6000-7000 ft deep, that will need to saturate before snow can begin. The trend in guidance has been to slow the onset timing of the snow and have made a light adjustment in this direction for the 18z TAFs. Upstream observations have shown VSBY rapidly dropping from 10SM to IFR/LIFR as the snow overcomes the dry layer and begins reaching the ground. Snow should taper off and end late this evening or just after midnight. Total accumulations of 1-3" are expected at the terminals.

Mixed signals in guidance regarding the potential for lingering CIGS overnight into Tuesday, which is leading to lower than average confidence forecast. Have opted to maintain IFR CIGS overnight into Tuesday morning with a trend upward to MVFR late Tuesday morning into Tuesday afternoon. Quite plausible that low cloudiness scatters out and we lose CIGS as early as late tonight, so this portion of the forecast may need to significant adjustments made in later TAF updates.

- Izzi

CLIMATE

Issued at 225 PM CST Mon Dec 1 2025

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

IN...Winter Weather Advisory until 3 AM CST /4 AM EST/ Tuesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.