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.

UPDATE

Issued at 741 PM CST Wed Mar 4 2026

A recent hand surface analysis revealed a weak area of low pressure in southern Illinois along a stalled frontal zone stretching from eastern Oklahoma through central Illinois and into southern Ohio. The lead low pressure system is supporting a region of showers and thunderstorms moving just north of the Ohio River toward southwestern Ohio, as well as showers and drizzle across northern Illinois and northwestern Indiana within a weak region of deformation and upper-level diffluence. Meanwhile, a secondary area of low pressure is located near the Kansas, Nebraska, Missouri, and Iowa border regions beneath an upper-lvel shortwave, which is supporting another round of showers and thunderstorms within the baroclinic zone stretching from far northern Texas through southern Missouri. In all, it's a rather wet evening across the Mississippi and Ohio River Valleys.

Over the next few hours, the lead low pressure system will shift eastward into central Indiana leading to a gradual decrease in local shower and drizzle coverage. As the low-level pressure gradient relaxes within the baroclinic zone ahead of the next shortwave, the low-level wind field beneath 10,000 feet will effectively stagnate. When paired with trapped low-level moisture beneath the inversion, conditions will become ideal for the formation and maintenance of dense fog. Indeed, visibilities across central Illinois (on the backside of the lead low pressure system) have been rapidly falling in the past hour, with many stations logging observations of one quarter of a mile or lower. With this in mind, confidence is fairly high that much of our area will get socked in dense fog overnight, especially southeast of a line from Dixon to Waukegan where rain is falling/has fallen this evening. Will be watching observations closely over the coming hours for the issuance and ultimate northward expansion(s) of Dense Fog Advisories. While not the most interesting aspect of the forecast tonight, temperatures will stay pretty much where they're at from now through at least daybreak Thursday.

Toward daybreak, the secondary low pressure system will lift along the low-level baroclinic zone ushering in another round of showers and thunderstorms across southern Illinois and central Indiana. While some of this activity may graze portions of the US-24 corridor (currently eyeing the activity that is developing just south of Kansas City, Missouri), the brunt may end up missing our area to the southeast. With little change in the position of the warm frontal zone tomorrow and a continuation of fairly light winds in the column due to a baggy pressure gradient, am growing concerned that dense fog will struggle to erode after daybreak and potentially remain through much of the day. Such a threat appears highest across northeastern Illinois where what flow will exist will be off Lake Michigan thereby reinforcing low-level moisture beneath the inversion. Have gone ahead and maintained areas of fog in the gridded database across northeastern Illinois through Thursday afternoon for this reason.

After making a few adjustments to the gridded database (mainly cosmetic), will go ahead and ship out updated text products.

Borchardt

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 /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1150 PM CST Wed Mar 4 2026

Key Messages:

- Low stratus and fog (including dense fog through the morning hours) will persist through the TAF period, resulting in predominantly LIFR/VLIFR conditions at the Chicago metro terminals and IFR/LIFR conditions at RFD.

- Periods of drizzle/light rain may occur overnight through Thursday afternoon.

Abundant low-level moisture and relatively stagnant low-level flow will maintain gloomy conditions with low stratus and fog throughout the current TAF period.

The VLIFR dense fog and LIFR ceilings observed during the late evening at the Chicago metro terminals will likely continue to be observed there through the overnight hours and much of Thursday morning before conditions gradually improve going into the afternoon. Conditions will then likely deteriorate once again after sunset on Thursday. RFD may remain northwest of the densest fog and lowest ceilings, but will still experience IFR and likely LIFR conditions through most or all of the current TAF period. Drizzle/light rain may also be observed at times through Thursday afternoon.

Winds throughout the TAF period will remain below 10 kts and predominantly be from a northeasterly direction when not calm/variable.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until noon CST Thursday for ILZ005-ILZ011- ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ106-ILZ107-ILZ108.

Dense Fog Advisory until 3 PM CST Thursday for ILZ006-ILZ013- ILZ103-ILZ104-ILZ105.

Dense Fog Advisory until 9 AM CST Thursday for ILZ032-ILZ033- ILZ039.

IN...Dense Fog Advisory until 3 PM CST Thursday for INZ001-INZ002.

Dense Fog Advisory until 9 AM CST /10 AM EST/ Thursday for INZ010-INZ011-INZ019.

LM...Dense Fog Advisory until 6 AM CST Friday for the IL and IN nearshore waters.