textproduct: Chicago

KEY MESSAGES

- Today and Thursday will be hot and humid with multiple rounds of severe storms.

- Thursday continues to look like the day of greatest concern for severe weather with a threat for all hazards (tornadoes, damaging hail, damaging winds, and flash flooding)

- Friday and Saturday will be dry and noticeably cooler. A generally cool and at times wet pattern will develop next week.

DISCUSSION

Issued at 242 AM CDT Wed Jun 10 2026

Today and Tonight:

Early morning GOES-19 water vapor imagery depicts aggregate upper-level troughing centered over the northern Rocky Mountain region. There are several upper-level shortwaves embedded within the expansive cyclonic flow, including one currently supporting a squall line moving across western Minnesota and another in the central Plains. Meanwhile, an elevated warm frontal zone is lifting northeastward across southern Wisconsin, northern Illinois, and northeastern Indiana this morning as identified by a zone of persistent ACCAS visible on GOES-19 nighttime microphysics imagery RGB imagery. So far, there have only been a few attempts for convection to initiate within the warm frontal zone this morning. At this point, am cautiously optimistic we are out of the woods for an isolated severe storm (call it a remaining 10% chance). Finally, a region of marine fog is stalled across portions of Lake and far northeastern Cook county this morning. The lowest visibility (one quarter of a mile) remains confined roughly near and east of I-94, and will continue to handle the locally reduced visibility with a Special Weather Statement through 5:30 this morning.

The stage is set for a hot and stormy day. After daybreak, the elevated warm frontal zone will shift northeast of our area. Warm overnight lows (current temperatures are largely in the low to mid 70s) will act as a springboard for temperatures to race into the mid to upper 80s by late morning. When combined with dew points in the low to mid 70s, heat index values are expected to reach 95 to 100 degrees by noon. Focus then turns to the potential two windows for severe weather today.

The first window will be in the afternoon (1 to 6 PM) as the aforementioned upper-level shortwave currently in the central Plains races into the Lower Great Lakes. Ahead of the wave, the overlap of steep mid-level lapse rates (sampled near 8 K/km per the 00Z DVN RAOB), moist low-to mid-levels (mean 1km mixing ratios of nearly 17 g/kg and PWATs near 2 inches sampled by the same 00Z DVN RAOB), and warming boundary layer temperatures into the upper 80s will lead to the development of appreciable instability characterized by MLCAPE >3000 J/kg. While deep-layer shear will be lower than typical for severe weather days with the first wave (effective and cloud-layer shear in the neighborhood of 30-35kt), the strong instability and deep moisture profiles will quickly encourage initial cellular development in eastern Iowa to merge into one or more outflow- dominant clusters with a threat for damaging to locally destructive winds (60 to 80 mph). The highest coverage is favored to be along and north of I-80 tied to the core of the wave, though at least scattered coverage may occur as far south as the US-24 corridor.

The second window for severe storms today will be in the evening hours (6 to 10 PM) as a secondary upper-level shortwave races into the Midwest. While the core of the upper-level wave will track decidedly northwest of our area, increasing isentropic ascent atop what would probably be an outflow- reinforced warm front draped across eastern Iowa and northern Illinois would nevertheless support renewed thunderstorm development. Now, the intensity of renewed development including any potential for severe weather is unclear, as the highest coverage of storms may end up north of the outflow-reinforced warm front. With that said, any storms that develop near and south of the front would be capable of producing damaging winds. Suppose there could be a conditional threat for QLCS tornadoes should any bowing segment develop and intersect the outflow- reinforced warm front, as well. Coverage of storms should decrease from northwest to southeast during the evening hours.

At this point, the expectation is for combined cold pool/outflow pushes from the two rounds of storms to temporarily push the warm front south of our area by midnight. Assuming this occurs, any threat for festering convection driven by the low-level jet riding atop the frontal boundary should be south of our area across central Illinois giving our local area a break.

Thursday:

Thursday continues to look like a day of greater concern for severe weather. Ensemble model guidance remains steadfast that the most dominant shortwave embedded in the aggregate cyclonic flow in the northern US will pivot northeastward into the Great Lakes on Thursday. Surface pressure falls west of the Mississippi River ahead of the wave will consolidate into a coherent surface low in central Iowa, leading to rapidly increasing low-level southwesterly flow across the middle Mississippi River Valley. As a result, the expectation is for the warm front to rapidly retreat northward into eastern Iowa, northern Illinois, and possibly southern Wisconsin by midday Thursday. (Accordingly, Thursday will be another hot and humid day for much of our area with peak heat index values in the upper 90s to around 100).

Like today, there may be two rounds of severe weather tomorrow. Curiously, CAM guidance continues to depict the development of thunderstorms across Nebraska and Iowa late tonight potentially tied to mid-level gravity waves emanating from unbalanced upper-level flow. (The HRRR in particular does depict GWAC supercells in central Iowa by daybreak). At this point, current thinking is morning activity would be poised to lift northeastward toward northeastern Iowa, northwestern Illinois, and southern Wisconsin (more specifically, northwest of a line from Dixon to Crystal Lake) in the 11 am to 2 PM timeframe. Even while any early-day convection would probably remain north of the warm front, strengthening upper-level shear ahead of the approaching short wave and steep mid-level lapse rates would still support a threat for damaging hail up to 2 inches in diameter. If the storms do end up tied to gravity waves, would have to watch for corridors of destructive winds (70+ mph) as well. Will note that sometimes these morning rounds of storms in these kinds of patterns can end up somewhat unexpectedly impactful given so much attention on the afternoon period. Speaking of which...

The second window for severe weather will be during the afternoon and evening hours (3 to 9 PM) as the upper-level shortwave ejects into the Great Lakes. An ensemble of CAM guidance is in agreement that the associated surface low will deepen toward the lower 990s mb range while lifting northeastward into central Wisconsin Thursday afternoon. Thunderstorm development is favored both along the northward- moving warm front and along the eastward-moving cold front by late afternoon.

With MLCAPE >2500 J/kg and effective shear >50kt, the environment within the warm sector of the cyclone will be very supportive of severe weather. At this point, the expectation is for initial supercell development along both frontal zones with a threat for all hazards (tornadoes, damaging winds, damaging hail) with eventual upscale growth into a southeastward-racing squall line. Of particular concern is the rapid increase in low- level flow on the southeastern flank of the surface low during the evening hours, which will lead to 30 to 40 kt of 0-1km shear and over 300 J/kg of SRH for eastward-moving storms especially along the warm front in northern Illinois or southern Wisconsin. Accordingly, should supercell storm modes be maintained along the warm front, there will be a threat for strong (EF-2+) tornadoes. Thunderstorm coverage is expected to end from northwest to southeast across the area as the evening progresses.

Not to be forgotten, the potential for up to four rounds of severe thunderstorms within a high-PWAT airmass in the next 48 hours raises concern for flash flooding. The rounds of thunderstorms today will likely "prime the pump" for a more noteworthy flash flood threat tomorrow, especially considering storms today should be more progressive and storms tomorrow may have a better opportunity to train along the warm frontal zone. Indeed, the 00Z 48-hr PMM QPF depicts a wide swath of 3 to locally 7 inches of rain centered on eastern Iowa, southern Wisconsin, and northern Illinois. Meanwhile, combined 24-hour LPMM QPF from the experimental REFS (no 48-hour PMM available) depicts potential for pockets of up to 8 inches of rainfall through Thursday evening. Did consider hoisting a Flood Watch this morning, but felt it would make more sense to do so either on the day shift today or midnight shift tonight (again focusing on the more pronounced threat on Thursday). Also, did touch base with WPC on the potential need for an upgrade to the current Level 2/4 threat level for flash flooding valid for Thursday.

Friday onward:

A break in the hot and stormy pattern is expected Friday and Saturday as a surface high pressure system works through the region. Highs on Friday should top off in the upper 70s (locally 80), and lows Friday night should fall into the lower 60s. Ensemble guidance than depicts the development of expansive troughing across the northern United States next week. While the main reservoir of hot and humid air would get shunted well south of our area, a parade of shortwaves may lead to periodic opportunities for showers and thunderstorms across our region through next week.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 553 AM CDT Wed Jun 10 2026

- +TS with a period of strong to damaging wind gusts likely this afternoon

- Additional TS likely this evening, possibly with variable strong and gusty winds again

An upper disturbance lifting northeastward out of the central Plains and very strong instability should set the stage for a linear cluster (or clusters) of severe storms moving across the region this afternoon. The exact evolution and timing remains in question, but signal is strong enough for continued TEMPO mention with 45-55 kt wind gusts. Localized gusts >55 kts are within the realm of possibility. In the wake of the expected +TS, a period of VRB to E/ESE winds may occur.

Additional SHRA and at least VCTS appear probable this evening, which may include organized TS with gusty VRB winds. The SHRA/TS should end by the late evening, followed by much quieter conditions the rest of tonight and Thursday morning. Additional TS may affect the C90 airspace toward midday Thursday, though direct impacts at ORD and MDW are not expected until beyond 18z Thursday.

Southwest winds will quickly increase by the mid to late morning today, with gusts in the 20-30 kt range until the afternoon TS arrival. Following any lingering post-convection variability in winds late this evening, expect lighter southwest winds through Thursday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Dense Fog Advisory until 7 AM CDT this morning for Winthrop Harbor to Wilmette Harbor IL.

Small Craft Advisory from 2 PM this afternoon to 9 PM CDT this evening for the IL and IN nearshore waters.