textproduct: Chicago

KEY MESSAGES

- Waves of showers and storms are expected starting today and lasting through Sunday. Many hours will be dry.

- A few storms this afternoon and tonight may be severe and produce torrential downpours.

- Temperatures will be fairly seasonable to slightly above average this week before trending warmer next week.

DISCUSSION

Issued at 240 AM CDT Fri Jun 5 2026

A recent hand surface analysis augmented by regional 00Z RAOBs places an expansive reservoir of higher-quality low-level moisture characterized by dew points in upper 60s to around 70 and mean low-level mixing ratios at or above 12 g/kg west of the Mississippi River. Several bands of showers and thunderstorms extend from northern Missouri to central Wisconsin along the eastern fringe of the low-level moisture axis tied to low-level isentropic ascent ahead of an upper-level shortwave lifting into central Iowa. Meanwhile, a secondary upper-level shortwave is readily evident in overnight GOES-19 water vapor imagery moving across the Dakotas, and is supporting a convective system moving into western Minnesota. Finally a larger and much slower-moving upper-level low is currently meandering along the US/Mexico border. The overarching weather pattern is certainly foreboding of a more energetic period heading our way.

Today:

As the morning progresses, the showers and storms near the Mississippi River are expected to slide northeastward generally along and northwest of a line from Mendota to Evanston, IL. Coverage of thunder may be isolated through the morning hours as the main instability axis lags to the west. With that said, the expectation is for episodic downpours amidst a broader rain shield across northwestern Illinois this morning. Meanwhile, southeast of the aforementioned line, residual dry low-level air should limit coverage of rain showers this morning.

An ensemble of CAM-based guidance suggests that an organized MCV will emerge across southern Iowa later this morning, which seems reasonable given the ongoing broad zone of active convection stretching across northern Missouri and the northeasterly trajectory of the parent shortwave. Southwesterly flow will be poised to guide the MCV toward the southern Wisconsin/northern Illinois region by late morning to early afternoon along the eastern edge of the instability axis. As a result, do have growing confidence in a period of thunderstorms this afternoon, favoring areas generally north of Interstate 80. Locally augmented and veering low-level flow along the southern side of the circulation (as strong as 40kt from 925 to 700mb per latest RAP guidance) does raise concern for supercell structures in the most persistent convective cells should the MCV remain on the eastern fringe of the instability axis. For this reason, cannot rule out a few funnel clouds or even a brief tornado early this afternoon tied to the MCV, which again will favor areas north of I-80 and perhaps even east of I-39. With that said, do suspect that convection may struggle to become rooted to the instability axis given the predominant convective- scale inflow will be from the relatively more stable air to the east (though such a limiting factor will not be as much of an issue by late afternoon, should convection still be ongoing).

For areas south of Interstate 80, today is shaping up to be a warm and breezy day. At least filtered sunshine should allow for highs to rise into the low to mid 80s. Mixing into the southern side of the wind field tied to the MCV will allow for periodic wind gusts of 25 to 30 mph. Humidity levels will increase throughout the day, with dew points poised to rise toward 70 degrees by this evening. The air will feel quite a bit heavier than the past week! Lingering isentropic ascent in the wake of the MCV may allow for isolated to scattered storms to develop toward and eventually south of I-80 this evening, though coverage should not be nearly as high as points to the north.

Tonight:

Forecast confidence lowers after sunset owing to a somewhat complex regime. Later today, an episode of convection is expected across Minnesota and central Wisconsin tied to the aforementioned Dakotas shortwave. Increasing isentropic ascent ahead of and beneath the left entrance region of a jet max associated with the wave into the Midwest region, as well as an approaching subtle 700mb speed max itself advecting a remnant EML plume eastward from the Plains, should excite a separate region or two of thunderstorms after dark from central Iowa toward northern Illinois. While pin-pointing exactly where thunderstorms will erupt in this kind of nocturnal regime can be difficult, the latest CAM guidance seems to favor an axis near and south of Interstate 80 between 9 PM and midnight, especially across Iowa.

In spite of somewhat marginal deep-layer shear (as the upper- level jet max will be displaced across central Wisconsin and Michigan), moisture-laden instability profiles aided by mid- level lapse rates near 7 to 7.5 K/km will support MUCAPE of 2000 to locally 3000 J/kg. Taken together, the most vigorous convection may be able to support scattered damaging hail and winds (a higher coverage being limited by a lack of more appreciable deep-layer shear). Perhaps more pervasive will be torrential downpours moving over the same general area, given modest backward corfidi vector magnitudes (a consequence of a somewhat weak low-level jet magnitude). Accordingly, do believe there is a threat for localized flash flooding in the general region tonight in spite of the recent dry stretch. Note that some areas, and perhaps even a large portion of our area, will remain completely dry tonight.

Saturday:

Coverage of overnight showers and storms should wane toward daybreak owing to the diurnal weakening of the low-level jet and subsidence in the wake of the overnight shortwave. Currently do anticipate a fair amount of sunshine by early afternoon, which should allow for temperatures to warm to the mid to locally upper 80s. The generally weak low-level pressure gradient and continued cool Lake Michigan temperatures (in the upper 50s to lower 60s per the recent OMRLOT product) should allow for a lake breeze to develop and push inland during the afternoon hours. The continued humid conditions (dew points in the upper 60s to lower 70s) will support convective temperatures somewhere in the upper 80s, setting the stage for isolated to perhaps scattered "pop-up" diurnal thunderstorms by mid afternoon. Coverage may be locally higher near the lake breeze given locally stronger low- level convergence. Most areas should be dry.

Sunday into Monday:

Toward the start of next week, the upper-level low currently traversing the US/Mexican border will finally begin lifting northeastward toward the Mississippi River Valley. The (by this point somewhat stagnant) humid airmass will hence be poised to support episodes of showers and storms as the upper-level low approaches and eventually moves overhead. Periods of cloud cover, as well as onshore flow forced by a surface high pressure system moving across eastern Canada, will lead to seasonable temperatures generally close to 80 degrees both days.

Tuesday Onward:

In the wake of the upper-level low, ensemble model guidance appears steadfast in depicting the development of persistent troughing across the Pacific Coast and upper-level ridging across the Midwest and Great Lakes region. As a result, temperatures should warm throughout next week with highs climbing into upper 80s to perhaps lower 90s by the weekend. When combined with the continued humidity, it'll start to feel more like summer next week. Will note that there are differences in the placement of the ridge next week, suggesting that there may be opportunities for storms (and associated localized natural air conditioning) in the general region as well.

(Note that the high temperatures in the mid to upper 90s and associated much higher heat indices being advertised by the NBM toward the end of next week are not currently supported by input ensemble data).

Borchardt

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1232 PM CDT Fri Jun 5 2026

Key Aviation Messages for the current TAF period:

- Occasional SHRA/TS with associated vis reductions through the early afternoon

- Another round of showers and storms possible overnight

- Winds switch to the east-northeast tomorrow afternoon with a lake breeze

Winds are expected to remain out of the southwest through most of the TAF window. Gusts around 20 knots are possible through the afternoon, but will diminish toward sunset this evening.

More impactfully are the shower and storm chances today. Scattered showers are ongoing, but the short term thunder trends are down. While the instability is low, it is not zero, so TAFs were maintained with a PROB30 mention for thunder through the afternoon. MVFR or locally IFR conditions are possible during downpours due to a combination of lower cigs and/or vis.

Lingering showers cannot be completely ruled out this evening, but models are trending in a break in the rain which results in not having a mention of it in the TAF. The next wave of showers and storms is set to develop overnight along and south of a KSQI to KVPZ line. While the better coverage for thunder is better south of this line, there remains a PROB30 mention for the Chicago terminals, with updated timing.

Also overnight, attention is going to be paid to what transpires in Wisconsin. Models are continuing to develop a cluster of storms over their area that is expected to move southeastward toward 12Z. But there is a lot of disagreement in coverage and track (many have the storms fizzling out by the state line, others push it out over the lake). While the chances for showers and storms around KRFD and the Chicago terminals kept the PROB30 in the TAF, these storms could send outflow southward and flip the winds over to the northeast for a brief period. For now, due to model uncertainty, any northeast wind mention with the rain was removed, but will be monitored closely.

Rain chances diminish after 12Z and a return to VFR is expected. Winds will start out in the morning out of the west, but a lake breeze is expected to eventually develop in the afternoon switching the wind directions to the east-northeast.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 7 PM CDT this evening for Wilmette Harbor IL to Michigan City IN.