textproduct: Chicago

KEY MESSAGES

- Above average temperatures in the 40s and 50s expected for the upcoming workweek.

- The potential for a period of fog and drizzle is increasing for Monday night into Tuesday.

- A late week system will bring showers to the area followed by falling temperatures and snow chances for the weekend.

DISCUSSION

Issued at 300 PM CST Sun Jan 4 2026

While the potential was never all that high, based on latest model guidance trends paired with the trajectory of upstream observations and radar imagery suggesting precipitation will remain north of the area, have opted to pull any remaining low precipitation chances out of the forecast for far northern Illinois. Still can't fully rule out light mist sneaking down to areas along the Wisconsin state line but no impacts are expected.

Warm and moist air advection is still anticipated across the region tonight into Monday with temperatures overnight holding steady or only falling a few degrees overnight. The increased low-level moisture may allow for some fog development into Monday morning, currently favoring areas northwest of the Chicago metro. Several of the available hi-res guidance then have the fog and low-stratus redeveloping Monday night into Tuesday. This could be paired with a period of rain and drizzle as ascent increases through the low-cloud layer as another disturbance drifts east across the region.

The aforementioned fog/drizzle could limit some of the warming Monday and Tuesday with highs closer to the upper 30s or lower 40s, especially north of I-88, otherwise highs will be solidly in the 40s across the remainder of the area, including the potential for 50s toward central Illinois through the workweek.

There continue to be notable differences in the handling of the late week system, with more northerly solutions (EPS) having rain for the entire area and potentially thunderstorms, and southerly solutions (GEFS) bringing snow accumulations across northwest Illinois. Will continue to monitor trends to see if they consolidate toward one solution or more of a blend of the two. What is perhaps more certain is that in the wake of this system, temperatures will fall back to more seasonable norms for the weekend with highs in the 30s along with lingering snow chances.

Petr

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1058 PM CST Sun Jan 4 2026

Aviation forecast concerns for the 06Z TAFs:

- Marginal LLWS conditions into the predawn hours this morning.

- Development of low-MVFR or IFR ceilings later Monday morning/afternoon.

- Greater potential for IFR ceiling development Monday night, with drizzle and fog potential increasing late.

Surface low pressure was over south-central MN late this evening, and will continue to track east-northeast across central WI to northern LM by daybreak Monday. Surface pressure falls have enhanced the low-level pressure gradient, resulting in southerly winds occasionally gusting into to 20-25 kt range across the terminals. A 45-50 kt west-southwest low level jet at/above 2 kft AGL will linger across the area for the first few hours of the forecast, though surface gustiness should limit the magnitude of low-level shear. Winds will gradually ease and turn southwest toward daybreak as the low pulls away from the region. Winds will turn more westerly after daybreak as a weakening cold front approaches the area. Winds will then diminish and become southeast/east-southeast toward Monday evening as weak surface high pressure drifts across the area.

Cloud-wise, a 10 kft mid-level VFR deck will be exiting the region to the east at the start of the period, with VFR conditions expected to persist into Monday morning. An area of IFR/MVFR stratus along/west of the low track across MN and the eastern Dakotas is forecast to shift east-southeast into the area later Monday morning/midday however, and likely linger at least though the afternoon. Some guidance indicates IFR CIGs making it into northern IL with this, though favorable diurnal timing suggests lower-end MVFR is more likely. It continues to appear that there may be some improvement/scattering of this stratus late afternoon/evening, especially farther east into the Chicago terminals. KRFD, farther to the northwest has a lower likelihood or at least a shorter duration of improvement into early evening. MVFR/IFR probabilities then increase again into the overnight hours of Monday night, as another disturbance approaches and eventually leads to drizzle/fog and lowering cig and vis conditions into Tuesday morning. IFR conditions appear likely especially after midnight, with LIFR not out of the question toward daybreak Tuesday.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 5 AM CST Monday for the IL and IN nearshore waters.