textproduct: Chicago

KEY MESSAGES

- Winter weather advisory for the entire area this afternoon into tonight for 2-4 inches, with locally higher amounts south of I-80.

- Patchy freezing fog early this morning, mainly west and south of the Chicago metro area.

- Well below normal temperatures will persist through this weekend with wind chills -5 to -15 possible Thursday and Friday mornings.

DISCUSSION

Issued at 239 AM CST Mon Dec 1 2025

Patchy freezing fog will be possible early this morning, mainly west and south of the Chicago metro area. Fog here at LOT has slowly improved over the past hour and with a rather shallow moisture layer, expect the fog to remain patchy and variable and plan to continue SPS mention and will monitor trends through daybreak.

Winds have shifted southeast across the southeast portion of Lake Michigan and this has allowed a narrow band of lake effect snow to develop over the lake and as winds turn southerly through the morning, this lake effect snow is expected to remain over the lake and then shift further northeast today and thus, no mention of light snow or flurries this morning.

Snow is expected to spread and develop across the entire area this afternoon impacting the afternoon/evening commute. Some minor changes this morning included increasing snow/water ratios into the 17:1 range to account for a bit deeper DGZ zone passing across the area for a 2-4 hour period, which may result in a period of moderate to perhaps briefly heavy snow. Still some uncertainty for exactly where the best banding may develop, which could end up being along/south of I-80. Qpf amounts still look reasonable in the 0.2 to 0.25 inch range, though perhaps higher in a narrow band. This would then yield 2 to 4 inches of snow with locally higher amounts possible, mainly south of I-80 as noted above. A shift further south would then lower amounts across northern IL and its possible across far northern IL, amounts may only be in the 1-3 inch range. While amounts remain on the low end of advisory criteria, at least for the northern half or so of the cwa, had to make a decision for an advisory or SPS and given the potential for moderate snow falling during the rush hour, opted to go with an advisory for the entire area, two segmented to account for an earlier start in IL and a later end across northwest IN. Falling snow is expected to end prior to the Tuesday morning rush hour, but any untreated surfaces may be slippery or snow covered.

Tuesday is expected to be dry but cold. After morning lows in the teens for most areas, highs may struggle to reach the mid to upper 20s. Temps may increase back to around or just above freezing on Wednesday ahead of a strong cold front. Precip chances remain uncertain along and ahead of the front and its possible, maybe likely that precip will develop right over the cwa during the afternoon. Previous discussion noted the potential for some freezing drizzle and that is possible along with a mix. For now, have not made any changes to the blended guidance which is high chance pops along with all snow for precip type. Precip would be short in duration and quickly move east of the area Wednesday evening.

Northwest winds will increase behind the front with gusts into the mid 20 mph range during the evening, slowly diminishing into the 10 mph range by Thursday morning. Low temps by this time will likely be in the single digits for most of the area, with subzero single digits across the northwest cwa, with wind chills into the -5 to -15 range. High pressure will move across the cwa on Thursday as high temps struggle into the lower/mid teens. The high moves east Thursday evening with southerly winds by Friday morning but low temps still in the subzero single digit lows for most areas away from Chicago and wind chills in the -5 to -15 range again.

Quite a bit of uncertainty for the end of the week into the weekend. There seems to be some consensus for a system moving across the area, but large differences in timing, strength, temps, precip types, etc. Blended guidance is now just slight chance pops for the weekend and made no changes. cms

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 528 AM CST Mon Dec 1 2025

Key Messages:

- Patchy fog/freezing fog west and south of ORD/MDW early this morning. DPA/RFD have observed this at times overnight and are the most likely to do so early this morning.

- Period of accumulating snow with IFR conditions is expected this afternoon/evening.

- IFR/MVFR cigs may linger into Tuesday morning after snow departs.

Light/variable or calm winds to start the day in association with surface high pressure ridge extending from IA eastward across northern IL/IN. Mainly clear skies earlier in the night had allowed for the development of patchy freezing fog across portions of northern IL west/south of Chicago, though increasing and thickening high cloud cover has largely limited the coverage of fog. DPA and RFD have had brief intermittent VLIFR conditions in patchy fog, and can't rule out some additional impacts early this morning for these two sites.

Attention then turns to the expectation for a period of accumulating snowfall expected across the terminals from this afternoon through this evening. This will likely bring visibilities down into IFR for several hours, before eventually tapering off late this evening. Accumulations are expected to be in the 1-4 inch range. There are indications in current model guidance that IFR stratus and potentially some fog may linger late tonight into early Tuesday behind the departing system, though confidence is somewhat low at this distance.

Aforementioned light/variable winds will become light south- southwest later this morning and may back slightly south- southeast this afternoon/evening. Winds will then shift to the west-northwest tonight, remaining generally under 10 kts.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory from 2 PM this afternoon to 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 from 4 PM CST /5 PM EST/ this afternoon to 3 AM CST /4 AM EST/ Tuesday for INZ001- INZ002-INZ010-INZ011-INZ019.

LM...None.