textproduct: Chicago

KEY MESSAGES

- Another round or two of severe storms are expected today with the period of greatest threat between 4 and 11 PM. All hazards (tornadoes, destructive winds, destructive hail) are possible across the entire area. - In particular, there is a threat for strong to intense tornadoes this evening (4 to 8 PM) especially along and north of Interstate 88.

- After a dry and cooler day on Friday, there is another threat for severe weather on Saturday.

DISCUSSION

Issued at 304 AM CDT Thu Jun 11 2026

...THREAT FOR STRONG TORNADOES AND DESTRUCTIVE WINDS IN THE REGION THIS AFTERNOON AND EVENING...

Early morning water vapor imagery depicts broad aggregate upper-level troughing centered on the northern Plains. Several embedded short-wavelength troughs are parading around the perimeter of the broader trough, including a notable impulse moving across central Wyoming at press time. Per recent mesoanalysis data across the central Plains, the impulse has already led to the development of a lee cyclone centered in western Kansas. Meanwhile, a broad region of warming cloud tops extends across the Lower Great Lakes associated with a gradually-decaying mesoscale convective system. Per a recent surface analysis, outflow from the convective system has (temporarily) displaced a warm frontal zone southward, with the boundary currently stretching from central Indiana through south-central Illinois and northeastern Missouri. Between both large-scale features, convection is developing across southern Nebraska associated with presumably a mid-level gravity wave originating from unbalanced upper-level flow (a notable 75kt jet streak is departing the base of the aforementioned Wyoming shortwave). Indeed, am noting the hallmarks of developing gravity wave associated convection (GWAC) along the I-80 corridor in Nebraska at press time.

As the Wyoming shortwave ejects east-northeastward into the Great Lakes this morning, the southern Plains low will rapidly shift northeastward from near Kansas City, Missouri toward central Iowa. Ahead of the surface low, the developing convection in southern Nebraska is expected to intensify and consolidate into a compact mesoscale convective system or GWAC supercell complex and track east-northeastward toward the Mississippi River and eventually into northern Illinois. It is unclear if the convection would ultimately become surface-based as the boundary layer warms and synoptic warm front retreats northward (more on this soon), though the presence of steep mid- level lapse rates (contributing to MUCAPE > 2000 J/kg) and increasing shear profiles (effective shear in excess of 50 kt) will support a threat for destructive winds (>70 mph) and hail (>2" in diameter) with the first round of storms. The time window for the early-day storms looks to be generally between 11 AM and 3 PM, mainly north of Interstate 80. (Am somewhat skeptical of CAM guidance depicting early-afternoon convection decaying while approaching Lake Michigan).

Later this afternoon and evening, ensemble model guidance remains in agreement that the surface low will deepen toward the lower to mid 990s mb range while lifting into central Wisconsin. Intensifying south to southwesterly flow on the southeastern side of the surface low will lead to rapid airmass recovery across northern Illinois as well as the advection of an EML plume back into the Lower Great Lakes. CAM-based guidance suggests that dew points in the warm sector will soar into the mid to locally upper 70s this afternoon, apparently a consequence of evaporation of water from the heavy rain yesterday evening and early this morning to our south and west. Resulting mean 1km mixing ratios nearing 19 g/kg and mid-level lapse rates of around 8 K/km will contribute to MLCAPE of locally 4000 J/kg in the warm sector. At the same time, low- to upper-level flow will rapidly intensify across the warm sector during the afternoon and evening leading to strong deep-layer shear profiles.

Thunderstorms appear poised to develop across central Iowa by mid-afternoon and then expand in coverage and increase in intensity while moving toward northern Illinois and southern Wisconsin this evening and eventually northwestern Indiana after sunset. Given the degree of strong forcing, strong instability, and high moisture values, a somewhat messy storm mode is expected with a mix of high-precipitation supercells and line clusters eventually merging into a squall line with a threat for all hazards (tornadoes, destructive winds, and wind-blown damaging hail). The main time window for severe weather this evening appears to be from 4 to 11 PM.

Am quite concerned in a relatively narrow west-to-east zone favorable for strong to intense tornadoes (EF-3+) somewhere across southern Wisconsin or northern Illinois particularly between 4 and 8 PM. Forecast soundings along the warm front this evening depict a rare and volatile combination of exceptional instability augmented by locally pooled moisture (MLCAPE >3500 J/kg) and intense low-level shear (350 J/kg of 0-1km SRH for eastward-moving storms, nearly all of characterized by streamwise horizontal vorticity), leading to effective STP values in the double-digits. Pinpointing the eventual location of the warm front will be paramount to identifying the zone of greatest tornado potential today with reasonable outcomes ranging from the front stalling near Interstate 88 in northern Illinois to the I-94 corridor in southern Wisconsin (consensus is near the Wisconsin state line). Such a threat for strong to intense tornadoes is contingent on a supercell storm mode being maintained, though the degree of low-level shear would still support strong QLCS tornadoes near the warm front.

Just want to conclude by saying that environments supportive of strong to intense tornadoes often have many off-ramps and are accordingly never a guarantee. Will the warm front surge well north of our area taking the primary tornado threat with it? Will storm mode remain far too messy to sustain strong low- level rotation and tornadoes? Or, will there be a discrete supercell that latches onto the warm front draped across our area? These factors are essentially noise within the broader signal to craft a forecast and can only be determine as an event is unfolding (sometimes literally just an hour or two beforehand). Taken altogether, today is a day to stay very connected to the forecast and to ensure you have multiple ways to get warning information for the off chance that the dominoes do end up falling in the wrong direction in our area.

Friday onward:

A brief break in the pattern is expected on Friday with noticeably lower humidity values and temperatures. Broad upper- level cyclonic flow will remain across the northern United States through the weekend and beyond, maintaining periodic opportunities for showers and thunderstorms. Will note another threat for severe weather on Saturday as a more notable shortwave and associated cold front moves into the Great Lakes and interacts with a narrow EML plume and low- level moisture return. At first glance, the main threats on Saturday should be damaging hail and damaging winds.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1211 AM CDT Thu Jun 11 2026

Aviation forecast concerns for the 06Z TAFs:

- Area of SHRA/embedded TSRA ending for CHI terminals 07-08Z.

- A period of TSRA possible late morning into early afternoon across far northern IL, with highest likelihood at KRFD. Did add PROB30s for all CHI metro sites except GYY, where potential decreases with southeastward extent.

- Higher probability (80%) for a line of severe thunderstorms late afternoon into mid-evening ahead of a cold front. All severe threats possible.

- Period of strong southerly winds expected Thursday afternoon/evening, with gusts AOA 30 kts possible. Winds shift west and remain breezy behind cold front Thursday evening.

Second round of thunderstorms has largely pushed south/southeast of the terminals as of midnight, though an area of rain with embedded lightning continues to track east across the area in association with a convectively-induced mid-level circulation. This should end from west to east across the Chicago terminals between 07-08Z. Relatively quiet conditions are then expected overnight into Thursday morning with modest south-southwest winds, though there continues to be support in some model guidance for MVFR ceiling development around sunrise. Confidence in coverage of this remains low at this time, though have maintained a SCT015 mention which can be updated to ceilings if necessary.

An increasing model signal persists for a late morning/early afternoon round of thunderstorms across far northern IL. Highest probabilities (~60%) is at KRFD toward midday, with probs decreasing to ~30% for the Chicago metro sites early in the afternoon. Have included a TEMPO at KRFD, and PROB30 mentions for this at KORD/KDPA/KMDW. Likelihood decreases farther southeast and have left out for KGYY. A more widespread thunderstorm threat then looks to develop late Thursday afternoon and evening, with a line of severe storms expected. Some timing differences remain in guidance, though general agreement suggests a few hour period starting around 22Z for KRFD and 00Z for Chicago sites. All severe modes possible, especially damaging winds.

Ahead of the squall line, synoptic winds are expected to back south or even slightly south-southeast in the afternoon and strengthen. Gusts at or above 30 kts are possible (with some high-res model guidance even 40+ kts late - though not quite confident enough to go that high at this distance). Surface winds will shift westerly with the passage of the cold front Thursday evening, likely remaining breezy around 25 kts for a time into the night.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 4 PM this afternoon to 10 PM CDT this evening for the IL and IN nearshore waters.