textproduct: Chicago

KEY MESSAGES

- Periods of dense fog may impact portions of the area tonight through Friday morning.

- Waves of showers and drizzle are expected tonight into Thursday. A few thunderstorms will be possible as well.

- Friday will be windy and unseasonably warm, with a period of showers and embedded thunderstorms during the daytime hours and then more widespread showers and scattered storms late Friday night into early Saturday. Some of these storms may be strong to even severe, though confidence is low.

DISCUSSION

Issued at 206 PM CST Wed Mar 4 2026

Through Thursday night...

Following a foggy, dreary start to the day, much of the area north of I-80 managed to scatter out enough to let in a little bit of sunshine. But low clouds are filling back in from the south as a warm front lifts into central IL. We're also seeing some fog thicken up and drizzle develop across central IL early this afternoon, including in our southern CWA, and that too is expected to trend northward over the next several hours.

As of this writing, the warm front has lifted to just north of the I- 70 corridor in central IL. Fog and pockets of drizzle will propagate northward ahead of the front and spread into northern IL later this afternoon. There's been a growing signal among a handful of high res camps for a plume of true light rain showers to move across the area late this afternoon into mid- evening ahead of a shortwave impulse and surface reflection currently working across MO. We can't rule out some heavier showers or maybe even an isolated thunderstorm or two in our far south late this afternoon with a few hundred Joules of MUCAPE extending into the area. The warm front will stall across central IL tonight keeping the higher/heavier precip coverage south of the CWA this evening and through most of the night.

Overnight into Thursday morning, a surface low will lift through central IL into northwestern IN dragging the warm front up into the local area. A push of densely scattered to widespread showers is anticipated along and southeast of the low track. Latest guidance suggests this may equate to areas roughly near and southeast of the I-55 corridor from the predawn hours into Thursday morning with isolated, lighter precip coverage anticipated farther northwest. Near and southeast of the low track will also be the area to watch for elevated convection late tonight and into Thursday as a few to several hundred Joules of elevated CAPE will be found in the storm's warm sector. Some light showers may linger in our south and drizzle into northern IL into early afternoon, but conditions should generally be drying out by the end of the morning behind the departing low.

Regarding the fog, visibilities around the area will trend lower through the rest of today with area-wide fog likely tonight into Thursday. It's very possible that widespread areas of dense fog develop, especially where drizzle is found. Confidence in any given area or time window is too low to consider a headline anywhere, but wouldn't be at all surprised if targeted advisories or SPSs are in our future. Conditions should largely improve beyond Thursday morning, but visibility reductions may fester through the day, especially around the lakeshore.

Doom

Friday through Wednesday:

Forecast attention continues to focus on the threat for potentially two periods of showers and thunderstorms Friday and Friday night.

A mid/upper-level trough is forecast to traverse the central and southern Rockies early Friday morning. Thereafter, a sharp mid- level impulse, and accompanying mid and upper level speed max, is expected to eject northeastward across the Plains into the Upper Midwest Friday into Friday night. Meanwhile, at the surface, a cold front, along the southwest periphery of a surface low, will shift southeastward from the northern and central Plains into the western Great Lakes by early Saturday morning.

Thursday night into Friday morning, strengthening low-to-mid level warm air advection, atop a northward shifting warm frontal boundary, should result in an uptick in convection to our west- southwest. We will be watching the evolution and extent of this activity, as its focus should generally spread northeastward towards our area later Friday morning. With no real change in forecast thinking with this activity, we continue to focus the high chances for showers and storms across northern IL during the late morning and early afternoon period.

Outside of the morning shower and thunderstorm chances, temperatures Friday afternoon will soar well into the 60s into the low 70s (near record territory for ORD and RFD which is 70 at both sites on Friday) as the surface warm front surges northward across Illinois and Indiana. However, as is typical this time of year, onshore flow ahead of this warm front across far northeastern IL (including parts of Chicago) will keep conditions much cooler through much of the day as the warm front struggles (at least initially) to overcome the colder marine airmass over southern Lake Michigan. Patchy fog would also be likely to hang on near the Illinois shore until surface flow turns offshore sometime later in the day.

As we have mentioned for the past couple of days now, the combination of the very warm temperatures and unseasonably high dew points into the upper 50s to near 60F, will support an unstable and strongly sheared warm sector supportive of a conditional threat of severe thunderstorms Friday afternoon and night. The main question at this point is if any of these early afternoon storms will be able to become surface based near the quickly approaching warm front before the main focus for them shifts to our northeast later in the day. If they do become surface based, then severe storms would be more of threat for a period Friday afternoon, particularly near the surface boundary. However, at this time confidence on the severe threat with these early storms remains low. However, the currently more likely scenario appears to be the convection remaining elevated/just north of the front. Ultimately, it is going to come down to the timing of the northward frontal surge into northern IL Friday afternoon.

A period of nearly storm free weather still appears plausible for a time late Friday afternoon and evening due to prominent capping following this initial impulse, though south- southwesterly winds do look to become gusty (35+ mph) in the warm sector Friday afternoon and evening. Assuming these capped/primarily dry conditions come to fruition, it will feel like a warm, slightly muggy, and windy later spring season night for those out and about. Expect shower and thunderstorm chances and coverage to ramp up again late Friday night into early Saturday morning in association with a cold front sweeping eastward across the area.

The currently unfavorable diurnal timing of the cold front passage late Friday night into early Saturday does raise questions as to the threat for organized severe weather with this activity locally. However, the combined presence of near record warm temperatures and dew points deep into Friday night within a strongly sheared kinematic environment certainly will support at least a conditional risk for some stronger storms capable of damaging winds late Friday night.

Rain chances will come to an end following the cold frontal passage on Saturday, at least for a few days. Temperatures will cool down a bit immediately behind the front Saturday PM, but generally remain well above average through early next workweek. For areas that don't receive much rain in the upcoming stretch, we'll need to monitor for elevated fire danger on Sunday due to mild temps paired with fairly low dew points and gusty southwest winds. Both next Monday and a part of Tuesday look quite warm (highs possibly 70+ for a good chunk of the area). Thereafter, recent medium to long-range ensembles have continued point towards a stronger cold front passage in the wake of a possible formidable mid-latitude cyclone with potentially widespread precipitation mid next week, bringing a return to colder conditions, possibly accompanied by wind driven accumulating snow somewhere in the greater region. If this were to happen in our area, it would be a classic case of March "weather whiplash".

KJB/Castro

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1227 PM CST Wed Mar 4 2026

Key messages for the 18Z TAF period:

* MVFR cigs this afternoon dropping to LIFR this evening. IFR to LIFR cigs expected tonight and Thursday morning

* Periods of showers, drizzle, and fog throughout the period beginning as early as late afternoon with associated IFR to LIFR vsbys expected

MVFR cigs will continue to fill in across northern IL from the south through mid-afternoon. IFR looks to move over early this evening. Confidence has recently grown in a push of light showers or drizzle late this afternoon into early evening, mainly around Chicagoland. Intermittent periods of drizzle and light rain are then anticipated through the night and into Thursday. Areas of fog will also spread into the area this evening and is expected to linger through the rest of the period. Fog may be dense at times with periods of VLIFR possible, especially in drizzle.

Another wave of more appreciable showers will move across northern IL predawn into Thursday morning. Latest guidance suggests coverage may just miss the Chicago sites to the southeast. Timing on this push would be roughly 10 to 15Z. Drizzle may linger through the end of the morning, but dry conditions are expected for the afternoon. The low cigs and at least MVFR vsbys are likely to stick around through the afternoon.

At RFD, while a few showers will be possible, mainly into early evening, precip chances through the period are significantly lesser overall and generally favor more drizzle than true showers. Still anticipating IFR cigs and vsbys out toward RFD, which look to take hold this evening.

Meanwhile, winds through the period will be largely below 10 kt, easterly this afternoon going NE this evening for the rest of the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Dense Fog Advisory until 3 PM CST this afternoon for Winthrop Harbor to Wilmette Harbor IL.