textproduct: Chicago

KEY MESSAGES

- Period of dense fog possible this morning, particularly across interior sections of northern IL.

- Periods of rain are expected on Thursday. Southwest winds may become gusty Thursday night

- A storm system may bring a mix of rain and snow this weekend.

DISCUSSION

Issued at 334 AM CST Wed Jan 7 2026

Through Tonight:

Low stratus and mist (VSBYs 3-5 miles) remain persistent across the entire area this morning. The main concern over the next few hours that we will continue to monitor closely is whether or not this deck of stratus will act to build down to the surface as dense fog. There are signs that this may already be in the process of occurring across parts of northwestern IL as slight colder temperatures (upper 20s to low 30s) across eastern IA are beginning to slowly ooze eastward across the Mississippi on light westerly winds. We suspect as this continues to interact with the very moist low-level airmass across northwestern IL that VSBYs will be on a downward trend the remainder of the overnight period, thus increasing the threat for a period of dense fog for at least portions of the LOT CWA this morning. We certainly could see the need to hoist a dense fog advisory within the next few hours to cover through the mid to late morning period for a good chunk of my interior northern IL counties (especially west of Chicago). However, before locking into an advisory, we plan to continue monitoring trends over the next couple of hours.

The low clouds and fog are likely to be slow to improve later this morning. In fact, most of this cloud cover will likely not erode until sometime this afternoon as the lower-level begins to shift southerly. Northeastern IL is expected to be the slowest to improve this afternoon. As a result, temperatures in this area may struggle to warm much above the lower 40s, particularly near the WI state line. However, farther south, scattering afternoon cloud cover should help conditions warm into the mid 40s in and around the I-80 corridor. Some areas well south of I-80 could even end up around 50 this afternoon.

Southerly winds and warm air advection will persist tonight in advance of our next approaching weather system taking shape across the Plains. This is expected to result in another mild night, with overnight low temperatures expected to remain in the mid and upper 30s. Higher level cloud cover will also be on a quick increase overnight in advance of the Plains weather system.

KJB

Thursday through Tuesday:

At the start of Thursday, we should find a warm front quickly lifting north across the lower Midwest in advance of an impending surface low. This storm will strengthen sharply over the south- central Plains during the earlier part of Thursday before jetting northeast into the region. Widespread rain should overspread the CWA during the afternoon and remain through the day. There's now solid model agreement that the low center will pass through north central or northwestern IL which would favor the axis of steadiest rain and highest QPF staying just out of reach to our northwest. Highest exceedance probs from the global ensembles run along eastern IA, far northwestern IL, and into southern WI. A pretty good consensus among models for around our CWA is a few to several tenths of an inch through the night. A newer signal though, especially among some of the higher res camps coming into play, is for localized swaths of higher QPF across the CWA beneath the storm's low level conveyor and along/ahead of the cold front. We'll also have to keep any eye on the potential for low-topped convection during the evening and night. After the warm front lifts into the area, models have upwards of a couple of hundred Joules of instability working into mainly the southern half or so of the CWA. This environment does not look supportive of anything more than an isolated strike or two at this point, but with 50 to 60 kt of 925mb flow ahead of the front, any convectively-driven showers may be able to transport strong to even damaging wind gusts to the surface. The potential for convection is greater with southern extent and best convective chances exist across central and southern IL where dewpoints are favored near and just above 60F. An SPC Day 2 marginal risk for severe wind currently extends as far north as about Lincoln, IL. The widespread soaking rain should lift to the north of the area earlier in the evening while the convective potential will last into the overnight.

Efficient warm advection ahead of this storm will pull Thursday afternoon temperatures into the lower 50s for many around the area. However, it looks as though daily highs on Thursday may occur late in the evening as the warm front surges across the CWA. Temperatures at the start of Friday are forecast in the 40s to near 50, and probably won't move much during the day as cold advection may offset diurnal heating. Friday looks rather gray stuck beneath low hanging stratus and perhaps period of drizzle.

Another very dynamic system is slated to move across the region on Saturday. There's still quite the spread in possible outcomes, so won't get into too many details at this point. But guidance has been favoring a push of rain or a wintry mix early Saturday before transitioning to a potential for periods of snow showers during the afternoon and evening. The passing surface low will move well ahead of the sprawling upper vort max which will promote additional snow showers through the night and into Sunday. There isn't really any support for anything more than a couple of inches of accumulation, but uncertainty is very high at the moment and would not put any stock into snow totals right now. But late Saturday and Sunday are also shaping up to be rather windy, which does add some blowing/drifting snow concerns depending on the nature of the snow showers. Stay tuned while more is ironed out with this system over the coming days.

Doom

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1102 PM CST Tue Jan 6 2026

Aviation forecast concerns for the 06Z TAFs:

- IFR/LIFR conditions expected through Midday Wednesday. Period of VLIFR possible with dense fog toward/after sunrise, especially west of ORD/MDW.

- Improvement to VFR likely by early-mid Wednesday afternoon.

Late evening satellite imagery depicts an extensive area of low stratus across the forecast area, extending upstream into eastern IA/southeastern MN. A mid level disturbance currently transiting northern IL from west to east has brought a period of drizzle to the region during the evening, resulting in IFR vis and patchy LIFR cigs. Some slight improvement is expected at the start of the 06Z TAF period as the drizzle ends, though stratus is expected to gradually build down again during the predawn hours beneath a lowering subsidence inversion. Upstream conditions across central IA (LIFR/VLIFR with some dense fog development) appear to support model guidance trends of deteriorating cigs/vis into northern IL after 09-10Z, with development of LIFR/VLIFR cigs with dense fog possible into Wednesday morning especially from the Fox River Valley westward (particularly KRFD/KDPA). LIFR cigs are likely into the Chicago terminals as well, though guidance suggests primarily IFR vsbys there. This is coincident with weak surface high pressure and lighter low-level wind fields moving across the area.

Guidance is in good agreement in relatively quick improvement to VFR during the early-mid afternoon hours however, as the surface high moves off to the east and winds turn southerly. Can't rule out some patchy fog or low stratus (more likely due to increasing flow above the surface) redevelopment Wednesday night, though at this time models do not hit this scenario very hard. Eventually some increase in high-level VFR clouds is likely toward Thursday morning.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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