textproduct: Chicago

KEY MESSAGES

- Warmer and occasionally wet weather is in store from the mid to late week period onward. Confidence is higher in more widespread rain late Wednesday night into Thursday and again late Friday into early Saturday.

DISCUSSION

Issued at 320 AM CST Tue Mar 3 2026

Through Wednesday:

We continue to monitor a weak mid-level impulse drifting east across Iowa and northwest Illinois early this morning which could bring spotty showers to the area early this morning. A stout 800-600 mb dry layer has kept this mainly as virga, aside from an area of deeper saturation in northeast Iowa. This part of the band of precipitation is expected to track east across southern Wisconsin and remain north of the area. As low-level stratus increases in coverage here locally, steepening steep lapse rates within a saturated DGZ layer may be sufficient for seeder-feeder processes to support rain punching through the dry layer on an isolated basis. Accordingly, have maintained the potential for isolated showers in the forecast early this morning.

Temperatures have been very gradually moderating over the past few hours with the majority of the area above freezing, however, portions of interior northern Illinois, roughly between I-39 and the Fox Valley and north of I-88, is still hovering around freezing in low-lying areas. Given the expected limited shower coverage, suspect that the window for patchy slick spots is closing quickly, but still can't be fully ruled out on untreated colder surfaces prior to daybreak. Any sprinkles/spotty showers will shift east of the area shortly after sunrise with any additional shower potential expected to remain well south of I-80 for the remainder of the day.

Farther to the south an axis of warm advective showers and embedded thunderstorms have developed over parts of Missouri early this morning. Hi-res guidance lifts the northern periphery of this activity toward the region through the morning. Confidence in how far north this activity reaches is on the lower side, but it felt prudent to maintain rain chances for areas south of a Pontiac to Rensselaer line mid to late morning. Additional waves of showers and possible embedded storms are expected to continue through the afternoon and evening within the vicinity of an elongated east to west oriented baroclinic zone and associated convergence axis that sets up across central Illinois and Indiana. This activity could continue to graze far southern portions of the forecast area (particularly near and south of US 24) this afternoon and evening, but could also remain just south of the area.

As for temperatures today, onshore flow will likely hold highs in the upper 30s near the lakeshore with lower 40s extending will inland. Western and southern fringes of the forecast area could still manage upper 40s to near 50 west of I-39 and south of US 24.

Looking ahead to tonight into Wednesday morning, hi-res guidance also favors showers remaining well south of I-80, which the ECMWF has remained consistent in depicting the past few days. Have accordingly continued the trend toward lowering precip chances across much of the area, keeping areas north of the Illinois/Kankakee Rivers dry through the morning on Wednesday. Visibility trends will have to also be monitored during this timeframe, first across central Illinois and Indiana where low- level moisture pooling could support fog development. Farther north, if skies manage to clear out there may be sufficient radiational cooling for shallow fog development, particularly north of I-88 into Wisconsin. Still not enough confidence for a formal addition to the forecast yet.

A weak MUCAPE axis may lift north across parts of the area Wednesday afternoon (mainly south of I-88), which could support increasing shower coverage and possible embedded storms. Confidence in the northern extent of showers remains low, with some model camps remaining south of the area through the entire day on Wednesday. Temperatures on Wednesday could end up fairly similar to today, with upper 30s along the Illinois shore, lower to mid 40s across northeast Illinois. and upper 40s to lower 50s south of I-80.

Petr

Wednesday Night through Monday:

There's still some uncertainty regarding rain trends Wednesday night, though highest shower coverage will likely be after midnight. Despite lingering ensemble spread, suspect that the still consistent ECMWF/EPS suite (which seemingly will have led the way prior to Wednesday evening) is on the right track here. This suggests that surface high pressure over the Great Lakes will likely continue to exhibit a drying influence through Wednesday evening and possibly a chunk of the overnight as well, especially with northward extent (ie. north of I-80).

A neutrally tilted mid-level short-wave tracking toward the mid MS Valley by early Thursday should finally help shower coverage expand farther north of weak surface low pressure reflection over central Illinois. Shower activity will continue into Thursday morning and then shift east during the afternoon. Mid-level lapse rates are forecast to be rather steep as the aforementioned short-wave tracks overhead into Thursday morning, so sufficient MUCAPE is plausible for embedded thunderstorms and localized downpours. The modest magnitude of large scale forcing is such that the setup doesn't currently appear favorable for widespread heavy/beneficial rainfall totals, and some locations may end up missing out relatively speaking. The favored track of the weak surface low pressure on Thursday will keep cool onshore flow going into northeast Illinois and far northwest Indiana. We'll also have to keep an eye on the potential for fog oozing inland from the lake as dew points well above current lake surface temps press northward.

Thursday night into the pre-dawn hours of Friday continues to appear primarily dry, with lingering ~20% PoPs probably tied to time-lagged ensemble members. With weak surface high pressure over the Great Lakes Thursday night, this may end up being a favorable setup for fog development for inland areas as well (vs. primarily near the lake during the day). Can envision fog from a build-down of existing stratus, radiational fog if cloud cover is less extensive than forecast, or of course both if there's only pockets of clearing. The fog potential will be assessed for formal inclusion in the official forecast with subsequent updates.

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 or early Friday afternoon, though there is variance in the guidance regarding the track and strength of this feature. Scattered showers and isolated elevated thunderstorms (if sufficient MUCAPE is realized) accompanying this feature may impact parts of the region into early Friday afternoon. Outside of the late morning and early afternoon potential though, it appears there will be a good amount of dry time during the day (more than implied by the currently high gridded PoPs).

Temperatures on Friday are expected to soar well into the 60s to around 70F 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. The combination of these very warm temperatures and unseasonably high dew points, into the upper 50s to near 60, will support an unstable and strongly sheared warm sector in the wake of the earlier day convection. However, it appears there will not be much of a focus for additional convection in our area following the earlier day impulse. Instead, shower and thunderstorm chances and coverage look to be the greatest late Friday night into Saturday morning in association with a cold frontal passage. The currently unfavorable diurnal timing of the cold front passage may also limit the threat for well organized convection locally.

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. Current medium to long-range ensembles 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 by later in the week/mid-March.

Castro/KJB

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 555 AM CST Tue Mar 3 2026

Main Items of Note:

- Lower CIG trends

- Some potential for BR/fog tonight into early Wednesday

Any additional showers will likely remain well south of the terminals the rest of this TAF period. Predominantly MVFR CIGs are expected until scattering early this evening. Light winds and mostly clear skies tonight may be conducive for light fog/BR development, especially away from Chicago. Our current thinking is that any guidance depicting higher coverage of dense fog late tonight is likely unrealistic. MVFR CIGs may then return sometime Wednesday morning.

East to east-southeast winds this morning will become northeasterly this afternoon due to lake influence. Easterly winds around 10 kt are expected after sunrise on Wednesday.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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