textproduct: Chicago

KEY MESSAGES

- Locally dense fog may redevelop across interior portions of northern IL tonight into Saturday morning.

- Showers develop Saturday night and continue Sunday.

- There is a chance of thunderstorms, possibly severe, south of I-80 Sunday afternoon/evening.

- Very windy and much colder Monday with some snow showers possible in the morning.

DISCUSSION

Issued at 1248 PM CST Fri Dec 26 2025

Through Saturday:

While the dense fog from early this morning has dissipated, a low-level stratus deck remains persistent across the area, thus making for another gloomy afternoon. With no signs of scattering this deck out anytime soon, temperatures through Saturday are expected to largely remain steady through the daytime and nighttime hours. Accordingly, only about a 5 to 10 degree difference between the daily high and low temperature is anticipated, with lows generally in the 30s and highs on Saturday in the 40s.

The main forecast concern we continue to monitor is the threat for another period of dense fog across portions of the area late tonight into Saturday morning. Radiational cooling looks to be significantly limited (if not non-existant) tonight, owing to the persistent status deck. While this is not typically a good recipe for dense fog development, light and variable winds are expected tonight as a surface high pressure ridge moves overhead. This in combination with continued ample low-level moisture in place (dew point depressions of only a couple of degrees), conditions will be favorable to support the build down of this status deck to surface tonight. Current thinking is that the highest potential for dense fog tonight into Saturday morning will largely be across interior sections of IL (mainly west of Chicago). We thus could be looking at the need for another potential Dense Fog advisory for some areas as we get a better handle on observation trends later this evening.

Similar to today, visibilities are likely to steadily improve into the early afternoon as the fog dissipates. However, with the prospects of continued stratus, another cloudy day is anticipated, with highs generally in the 40s.

KJB

Saturday Night through Friday:

Saturday evening, we'll find a warm front lifting northward across central IL ahead of an impending surface low working across the central Plains. This system will provide our next opportunity for rain, snow, and maybe even a few thunderstorms. Isolated to scattered showers may blossom out ahead of the warm front during the evening. The boundary looks to lift into our CWA toward the end of the evening. Accordingly, we'll see temperatures rising during the night, maybe by as much as 4 or 5 degrees before sunrise Sunday.

This storm will intensify rapidly after the parent upper trough quickly digs south from southern Canada into the northern Plains Saturday night. In fact, the latest GFS and Euro have this thing nearly bombing dropping the central pressure 22-23 mb between early Sunday and early Monday. The peak of these pressure falls will occur just as the storm moves across the region. Precip coverage will expand around the warm front Sunday morning as the storm starts to quickly deepen. Widespread moderate to heavy rainfall is expected along the storm's warm conveyor as it reels tons of layer moisture into the region. This will impact the local area during the afternoon and early evening when surface dewpoints will exceed 60F in at least our southern CWA, but possibly a majority of the area. Previously, guidance had been in pretty solid agreement on the storm tracking immediately across the CWA from SW to NE. This outcome would favor areas roughly south and east of I-55 to receive the most rain. While that still generally looks true, a number of recent camps have trended slightly north and west with the storm track. The latest extended RAP, which looks through 18Z Sunday, is on board with the farther NW track and pulls 60s dewpoints up to the I-88 corridor and west of the Fox Valley. If this NW solution verifies, it would pull the better soaking rain and thunderstorm environments slightly north and west deeper into the CWA (more on storm potential below). Accordingly, higher PoP values and exceedance probs from the blend are also trending slightly north and west. Every piece of latest deterministic guidance resolves over an inch of QPF somewhere around the CWA, most of that found near and east of I-55. Generally less than an inch is favored on a broad scale, though.

Regarding the thunder potential, modest BL destabilization will take place in the afternoon and evening, primarily south of I-80, amid those 60s Tds with as many as a few to several hundred Joules of CAPE being modeled. The lack of buoyancy, especially above the lowest couple of hundred millibars, should keep any convection on the shallower side. But we'll find significant dynamic forcing ahead of the storm and its cold front thanks to those rapid pressure/height falls. Plus, a strong jet will develop atop the tightening baroclinic zone offering 30+ kt of 0-1 km shear and around 50 kt of effective shear. This could lead to an organized line of shallow convection, one that may not even produce much lightning but could possibly bring some strong wind gusts to the surface. Locally damaging gusts even appear within reach, which corresponds well with SPC's day 3 marginal outlook in our south. The thunderstorm potential largely favors southern and southeastern portions of our CWA where the better shear and moisture feed will be found. With this recent trend in the storm track toward the NW, convection now appears more attainable into the southern Chicago metro area, if not even farther NW. Accordingly, piggybacked off the overnight shift to expand slight thunder chances from the blend's solution to include areas roughly south of I-80 and east of I-55.

In the cold advection wing, precip is expected to transition to a wintry mix Sunday evening and eventually to all snow later in the night. The signal for true snow showers vs flurries is growing with time, mainly in our north, as is the potential for accumulations. Snow is expected to continue into Monday morning and taper off toward midday. It's possible that parts of the area will see a coating of snow on roads in time for the Monday morning commute.

Windy conditions are also anticipated around the system Sunday evening through Monday. Westerly gusts between 25 and 30 mph later Sunday evening will build to closer to 40 mph by Monday morning. Winds should gradually ease during the late afternoon and evening.

These winds will make the cooler day on Monday feel even colder. Strong post-frontal cold advection will pull morning lows down into the teens and lower 20s on Monday with only lower and middle 20s forecast in the afternoon. Wind chills are forecast in the single digits above and below zero in the morning, and should fail to exceed single digits above zero throughout the day. Upper 20s and lower 30s are anticipated into midweek before another shot of cold air looks to arrive just in time for the new year.

Doom

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 548 PM CST Fri Dec 26 2025

Aviation forecast concerns:

- Gradual deterioration to IFR conditions likely overnight.

- Areas west of the Chicago metro core more likely to see LIFR/VLIFR conditions by Saturday morning.

- Low confidence in improvement beyond high-end IFR Saturday.

Surface high pressure was developing eastward across the Mississippi Valley early this evening, and will be across the terminals overnight. Light variable winds, subsidence within the ridge axis and lack of thicker cloud cover above existent extensive low stratus deck is expected to result in a gradual build down of current low- MVFR stratus into IFR through late this evening. LIFR/developing VLIFR conditions in stratus/fog across central IA are expected to spread into interior northern IL by morning, with guidance generally highlighting areas west of the core Chicago metro terminals (DPA- west and especially RFD) for the greatest LIFR/VLIFR potential in our forecast area. Have indicated a tempo for 1/2SM and OVC002 for RFD toward daybreak, while confidence for lower conditions decreases with eastward extent into the Chicago area.

The surface high pressure center will pull away from the area to the east Saturday morning, with light/variable winds turning southeast and increasing to around 10 kts. This should help visibilities to improve and the stratus to lift somewhat, though there is quite a spread in available model guidance with respect to the degree of improvement in cig heights into the afternoon. Much of the deterministic high-res guidance would suggest IFR cigs will linger through the day, rather than improve to MVFR or scatter to VFR as depicted by some models. Thinking at this time is that improvement to solid MVFR is is somewhat unlikely, though confidence these trends beyond midday is low. Increasing low level south flow late in the day/evening does look to lead to further moistening ahead of another disturbance Saturday night, which would perhaps support a downward trend in ceiling heights by/during the evening hours.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.