textproduct: Chicago

KEY MESSAGES

- A period of accumulating snow this evening into tonight may lead to hazardous travel conditions, primarily southwest of a Peru, IL to Rensselaer, IN line.

- Another clipper system may produce a few inches of powdery snow accumulation in areas mainly south of Interstate 80 on Saturday.

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

- There is a growing signal for a pattern change toward milder (and less snowy) conditions next week.

DISCUSSION

Issued at 352 AM CST Thu Dec 11 2025

Through Friday:

Expansive stratus early this morning has caused temperatures to stall in the 20s across the area early this morning. Temperatures were accordingly adjusted upward by several degrees through the morning to account for this. Still can't rule out some upper teens in far northeast Illinois near the Wisconsin- Illinois state line where some breaks in the stratus may still occur. Continued cloudy skies today and tonight will likely continue to reduce the influence of the typical diurnal trend, with temperatures remaining mainly in the 20s through tonight.

Attention in the near term then turns to the potential for accumulating snow this evening and overnight for portions of the area. Aloft, broad upper ridging exists across the western CONUS with a closed upper low spinning over Ontario/Quebec. A sheared out clipper system currently over northeast Montana is forecast to dive southeast toward the region between these features within the mid-upper flow and produce a narrow swath of f-gen driven accumulating snow. This is currently favored to occur across Iowa into central Illinois and Indiana but may extend into northeast Illinois and northwest Indiana. Subtle variations in the track will have implications as to where any associated fgen features and accumulating snow axes set up.

While notable differences have existed between the hi-res and global guidance over the past 24+ hours, 6Z hi-res runs have begun to trend toward the farther southwest global solutions which would lead to less snowfall here locally (roughly a trace to locally 3" amounts far southwest CWA). If the forecast remains unchanged, a Winter Weather Advisory will likely be needed for areas along and south of a roughly Peru, IL to Rensselaer, IN line. However, given the potential for guidance to trend even farther southwest (limiting impacts here locally), it felt prudent to hold off one more cycle and let the day shift get one final look at 12Z guidance this morning.

Will plan to issue an SPS for counties mainly southwest of the Chicago metro to highlight the potential for hazardous travel this evening (including part of the commute) and overnight given temperatures in the 20s would support pavement accumulations where any snow falls. There also remains a signal for a weaker secondary fgen band to set up closer to the Chicago metro which could produce locally up to 1" of snow, so felt comfortable maintaining at least some accumulations as far northeast as Chicago.

In the wake of the Thursday night snowfall, some hi-res guidance does develop a lake effect band over southern Lake Michigan which may swing back west into northeast Illinois as the surface flow turns more northeasterly. If this occurs, it could produce additional localized accumulations up to an inch near the lakeshore. Meanwhile a trailing weak disturbance will dive southeast across the area during the day on Friday which could provide enough lift to produce scattered flurries and isolated snow showers late morning through early evening leading to an additional localized dusting.

Petr

Friday night through Wednesday:

The stubborn northwest flow pattern that has been in place for the past several days will persist through this weekend, sending a few additional clipper-type systems and steering much colder temperatures into the Midwest. However, it is appearing increasingly likely that we will finally be able to break free from this pattern next week and transition to milder and less snowy conditions.

Some snow showers or flurries may still be lingering around come Friday night, but the bigger story will be the arrival of a cold front denoting the leading edge of a bitterly cold, modified Arctic air mass. Cold air advection off of brisk west- northwesterly winds behind this front will push single digit air temperatures into much of the area prior to daybreak Saturday. Between these winds not letting up much, cloud cover shrouding the region, and the snowpack in place over much of the area, temperatures will then not be able to rise much, if at all, during the daytime on Saturday. Accordingly, our gridded forecast currently has Saturday's high temperatures remaining in the single digits across our northwestern CWA and in the low- mid teens elsewhere.

The next shortwave trough in the active clipper wave train will also be arriving at our longitude during the daytime on Saturday, bringing along another swath of accumulating snow with it. Global deterministic and ensemble guidance continues to trend towards a more southerly positioning of the sharp low-/mid-level baroclinic zone that this wave will interact with, which focuses the main frontogenetical response across central Illinois and Indiana. This would favor the bulk of the accumulations delivered by the induced snow band remaining south of the Illinois and Kankakee rivers, and possibly even south of our forecast area altogether.

The latest NBM PoPs for Saturday may be a bit too high across our northern CWA based on the latest QPF footprints from global guidance and the expectation that there will be a pocket of dry low- to mid-level air limiting saturation within a sizable chunk of the few kilometers deep dendritic growth zone. However, if this dry pocket ends up being saturated sufficiently via the sublimation of the ice crystals descending into it from a higher mid-/upper-level cloud deck, or if there was even just a slight shift back northward with the shortwave, baroclinic zone, dry air pocket, etc., then it wouldn't be difficult to envision snow (or flurries) occurring as far north as southern Wisconsin. Have thus elected to stick with the PoPs output by the NBM for now. Wherever snow does occur, the cold, dry air mass and deep dendritic growth zone would support high snow-to-liquid ratios and a fluffy/powdery character to the snow.

Northwesterly winds will prolong the push of cold air advection into Saturday night, when it appears that clouds will also begin to clear out from west to east. The end result will be a very cold night across the region, with a large portion of our forecast area likely to see sub-zero low temperatures and minimum wind chills of 20 below zero or lower. Therefore, a Cold Weather Advisory will likely be needed for most or all of the area Saturday night into Sunday morning. The very cold start to the morning will then limit how much temperatures will be able to climb during the daytime on Sunday, even beneath abundant sunshine away from portions of northwest Indiana that may plagued by lake-induced stratocumulus. The current record daily low maximum and record low minimum temperatures for December 14th are 5F and -9F for Chicago and 5F and -11F for Rockford, respectively. Based on our current temperature forecast, Chicago's records probably won't be broken, but it will be a much closer call for Rockford.

Sunday night will be another very cold night across the area. There will be less wind as a result of a strong Canadian/Arctic surface high spreading over the region, so wind chills may not get as low as they'll have gotten the prior night. However, the lighter winds (at least earlier on in the night) may afford better radiational cooling conditions, so low temperatures themselves may end up being similar to what they were Saturday night/Sunday morning.

A long-awaited pattern change then looks to be in store for next week. While one more clipper system should pass to our north on Monday, the northwest flow aloft will begin to be replaced by a more zonal flow pattern early next week. This will result in a warming trend over the first half of the upcoming workweek, with temperatures likely to climb above freezing and possibly even reach the 40s if potential low cloud cover doesn't end up being too much of an inhibiting factor.

Ogorek

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1140 AM CST Thu Dec 11 2025

No significant weather is expected through the TAF cycle. MVFR CIGS will likely continue through tonight and probably into Friday as well. Confidence in MVFR CIGS lingering into Friday is lower, as there is some potential to go VFR later tonight into Friday. Any snow is expected to remain south of the the terminals, though cannot rule out a few flurries tonight or Friday afternoon. If they occur, any flurries would be unlikely to result in any impacts to operations. Light winds are expected through the period.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until noon CST today for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.