textproduct: Chicago

KEY MESSAGES

- Expect periods of dense fog through Friday morning. The longest duration of very low visibility will be near the lakeshore, where there may not be much if any improvement.

- Scattered showers and perhaps an isolated thunderstorm may move across the area later this morning through midday, mainly south of I-88.

- Friday through Friday night will be windy and unseasonably warm, with a period of showers and embedded thunderstorms during a portion of 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 330 AM CST Thu Mar 5 2026

Through Tonight/Early Friday:

Early this morning, most of the region (including the entire NWS Chicago forecast area) is "socked in" with dense fog. Stagnant flow through most of the column paired with low-level moisture trapped beneath a strong inversion has resulted in extensive fog. Widespread 1/4 mile or less visibility near/north of I-80, including frequently 1/8 mile at both ORD and MDW overnight will probably be steady state past sunrise (and deterioration where visibility is >1/4 mile as of this writing). During the 2 AM hour, there was some temporary improvement in spots farther south, but suspect this will be short-lived as well. As noted in the update yesterday evening, the main axis of widespread showers and embedded storms has primarily remained just south of the CWA, grazing areas south of US-24. This will pave the way for drizzle to likely expand northward with time, causing visibility to deteriorate where it temporarily improved, and reinforcing the very low visibility farther north.

We're watching an area of showers and isolated thunderstorms near the Iowa-Missouri border associated with the cold core of a 500 mb short-wave. This feature tracking east-northeastward through the morning may yield some widely scattered showers and perhaps a few isolated thunderstorms over roughly the southeast 2/3 of the CWA. Any heavier showers able to materialize may theoretically be able to stir up the low levels enough for modest visibility improvement. The most likely contributor to improvement in the fog today, where this occurs, will be the stronger early March sun. Interior sections of the CWA, especially with south and southwestward extent will probably be the best candidates for noticeable improvement. With light reinforcing light onshore flow from the cold marine layer of Lake Michigan, the stagnant surface pattern will likely be conducive to dense fog maintenance across northeast Illinois and perhaps near the Indiana shore as well. Have made no changes to the current dense fog advisory expiration times today, with the day shift using observational and web cam trends to inform where the advisory needs to be extended (possibly into Friday morning).

A continued stagnant surface pattern tonight through pre-dawn Friday will result high boundary layer moisture trapped beneath an inversion. Where the fog is actually able to erode today, which seems unlikely to a meaningful extent near the lake, will once again be conducive for fog expansion and areas of dense fog. Southerly flow off the deck will increase towards daybreak Friday north of an advancing warm front, which may start the improvement process with southward extent.

Castro/Borchardt

Friday Morning through Wednesday:

Forecast attention the rest of Friday into early Saturday 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.

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 highest chances for showers and scattered storms across northern IL during the late morning and early afternoon period (approximately the northwest 1/2 or so of the CWA).

Outside of the morning-early afternoon shower and thunderstorm chances, temperatures Friday afternoon will soar well into the 60s and even 70s (near record territory for ORD and RFD which is 70F 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 lower 60s (locally mid 60s?), will support a weakly to moderately 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 and early evening, 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, possibly yielding a threat for mainly sub-severe hail. 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 (PoPs 15-25%) still appears plausible for a time late Friday afternoon and evening due to capping and/or a general lack of large scale forcing (neutral height tendencies) 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. Unless confidence increases in surface based warm sector storms Friday afternoon and early evening during peak instability, the higher severe threat may focus farther west than the current level 2 of 5 threat area in the SPC day 2 outlook (ie. west of the Chicago metro, east-central IL, and far northwest Indiana).

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 and Monday 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 widespread precipitation mid next week, bringing a return to colder conditions. Tuesday PM may have a threat for thunderstorms ahead of the cold front, followed by widespread rain into Wednesday (non-zero chance of mixing with wet snow in spots before ending).

Castro/KJB

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1146 AM CST Thu Mar 5 2026

Key Messages:

- Fog and low stratus persist through early Friday afternoon.

- Thunderstorm threat, particularly Friday morning.

While some minor diurnal CIG and VSBY improvements are expected this afternoon, dense fog is expected to quickly build back inland off of Lake Michigan after sunset this evening. It thus looks to be another soupy night, especially across the Chicago area terminals. Areas of dense fog will likely persist in this area into early Friday morning, with conditions slatted to quickly improve with the northward passage of a warm front into Friday afternoon.

In addition to the low stratus and fog, there also continues to be a threat for a 2-4 hour period of showers and thunderstorms Friday morning. The best potential for this round of showers and thunderstorms is across northwestern IL into parts of far northeastern IL. With this in mind, have opted to include a tempo group for TS at RFD. However, for the Chicago terminals I have opted to hold onto the going PROB30 for now.

Winds will be light to calm through tonight, primarily from an east to northeast component. On Friday, winds will increase from the southeast (10+ kt) in the morning, then become breezy from the south-southwest with the northward passage of a warm front early to mid Friday afternoon.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until noon CST today for ILZ003-ILZ004- ILZ005-ILZ011-ILZ012-ILZ020-ILZ106-ILZ108.

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

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

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