textproduct: Chicago

KEY MESSAGES

- Dense fog is expected through mid morning across portions of northern Illinois.

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

- Waves of showers and a few thunderstorms will be possible late today into Thursday (along with drizzle at times).

- 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 343 AM CST Wed Mar 4 2026

Through Thursday:

Fog has expanded in coverage across northern Illinois early this morning where a Dense Fog Advisory is currently in effect through 9 AM CST. There is a mid-level cloud feature slowly lifting north across the dense fog area that has allowed for some improvement in the visibility along the southern periphery, but this has not been consistently for all areas. Thus, suspect at least locally dense fog will persist into mid-morning, especially near the Wisconsin line. Temperatures are near and below freezing across far northeast Illinois which could lead to patchy slick spots developing on untreated surfaces.

Confidence in temperatures trends is low owing to uncertainty in how long the fog persists this morning along with any remaining low clouds. Current forecast favors some erosion of this cloud layer (which may be too optimistic), and allow temperatures to still warm into the mid-upper 40s inland of Lake Michigan. However, if low clouds remain overhead for most of the day, temperatures would struggle to warm out of the mid 30s near the lakeshore and lower 40s farther inland north of I-80. There may be enough warm air advection south of I-80 (ESE winds) to allow temperatures to still warm into the mid to upper 40s amidst overcast skies.

Dry conditions are favored for most of the area through this at least mid-afternoon. There is a low chance of light showers and/or drizzle to lift north across part of the area late afternoon into the evening. Hi-res guidance (RAP/HRRR) is most bullish on this in depicting this though there remain questions as to whether we saturate sufficiently for precipitation to occur. An increase in precipitation chances during this timeframe may be needed with later updates if confidence increases.

Waves of showers are then expected to lift across the region tonight into Thursday though there remains a lot of variability in the coverage and exact timing of showers here locally. Opted to maintain a blended approach to precipitation chances which has shower coverage increasing from south to north through the overnight hours then ending from west to east into the afternoon on Thursday as the center of the surface low moves across the area. While instability looks rather meager during this timeframe (MUCAPE < 500 J/kg), a few rumbles of thunder can't be ruled out with this activity.

This pattern remains favorable periods of fog as low-level moisture increases across a still cold Lake Michigan. Given onshore flow is expected the next few days fog will likely advect inland at times across northern Illinois, some of which could be dense. Have maintained and expanded fog coverage wording in the official forecast from late tonight through the day on Thursday to account for this.

Petr

Thursday Night:

The main concern Thursday night into Friday morning is fog trends, with the synoptic setup likely favorable for at least patchy dense fog. Initial fog Thursday evening should focus over the northeast 1/2 or 1/3 of the CWA and particularly near the lake, where areas of dense fog may already be oozing inland prior to sunset on Thursday. As the strong warm front to our south makes northward progress overnight, fog development looks more favorable areawide. Flow aloft should remain sufficiently light to not be much of an impediment to fog development most of the night. Increasing southerly flow just off the deck towards and after daybreak Friday should tend to lift the existing low stratus and fog bank northward. We'll need to monitor this period for advisory worthy coverage of sharply reduced visibility. For now, with the exception of right along the lakefront (areas of fog), opted for patchy fog mention in the gridded forecast.

Castro

Friday Onward:

On Friday, mid-level southwesterly flow will strengthen as a mid- level jet ejects northeast across the Plains into the Upper Midwest. This will send a low amplitude mid-level shortwave racing northeastward into the Upper Midwest sometime late Friday morning into early Friday afternoon, though there is variance in the guidance regarding the track and strength of this feature. An arc of showers and isolated to widely scattered thunderstorms accompanying this feature may impact parts of the region for a period into Friday afternoon. However, it still appears there will be a good amount of dry time during the day (more than implied by the currently high gridded 12 hour PoPs).

Temperatures on Friday are expected to soar well into the 60s into the low 70s (near record territory for ORD and RFD which is 70 at both sites on Friday) for most through the afternoon as a surface warm front surges northward across Illinois and Indiana. However, as is typical this time of year, onshore flow ahead of this front across far northeastern IL (including parts of Chicago) may keep conditions much cooler through much of the day until the warm front clears this area. Patchy fog may also hang on near the Illinois shore until surface flow turns offshore.

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. 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 severe threat with these storms is low. The currently more likely scenario appears to be the convection remaining elevated/just north of the front, but it could be close. 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 could set up 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) for a period. Assuming these capped/primarily dry conditions come to fruition, it will feel like a warm, slightly muggy, and windy May 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. For the Friday through Friday night period, attempted to provide approximate temporal details for graphical/tabular/matrix type depictions of the gridded PoP forecast, in line with general thinking outlined in this discussion.

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 somewhere in the greater region. If this were to happen in our area, it would be a classic case of March "weather whiplash".

Castro/KJB

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1057 PM CST Tue Mar 3 2026

An area of IFR stratus is expected to gradually expand and build down across the region tonight. Specific trends remain a bit unclear owing to holes in the stratus layer currently, and unscheduled amendments may be necessary. Cigs may build down to LIFR late tonight and into Wednesday morning. While areas of fog are possible, the presence of stratus as well as increasing winds just off the surface through late tonight lowers confidence on FG development. Have continued with generally IFR vsbys within TEMPO groups as a result.

Cigs will slowly lift through midday Wednesday but will build back down during the afternoon and especially during the evening and overnight. An initial period of showers and/or drizzle may develop during the afternoon, with a general expansion of drizzle and eventually widespread LIFR cigs and IFR to LIFR vsbys Wednesday night into Thursday morning. Given the extended time range, have confined a mention of sub-LIFR conditions into a PROB30 during the extended ORD/MDW TAFs, but suspect this may need to be converted to TEMPOs or prevailing groups in subsequent issuances.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 9 AM CST this morning for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ103.

IN...None. LM...Dense Fog Advisory until 9 AM CST this morning for Winthrop Harbor to Wilmette Harbor IL.