textproduct: Chicago

KEY MESSAGES

- A sight chance exists for a few thunderstorms later tonight into early Wednesday. While potential is low, it's possible that a storm becomes severe.

- Wednesday and Thursday will be hot and humid with peak heat indices in the 90s to locally 100. The hottest conditions are favored on Wednesday including up to the Lake Michigan shoreline.

- A few rounds of severe thunderstorms may occur in the general region Wednesday through Thursday. Thursday continues to look like the most concerning day for severe weather locally.

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

DISCUSSION

Issued at 308 PM CDT Tue Jun 9 2026

Through Tonight...

Our Tuesday was largely quiet and warm through the morning aside from a few showers around the Chicago metro. As of about 2 PM, temperatures have climbed into the middle 80s and we're now seeing a broken cluster of early afternoon thunderstorms south of I-80 moving east. These are building off of a vort plume tied to a decaying MCV that's pushing east across central IL. These storms are rather garden variety and don't threaten much outside of brief downpours and lightning. Coverage is expected to remain isolated and south of I-80 before eventually moving away to the east as the vort does.

An upper trough scooting over the High Plains this afternoon will dig east across the northern CONUS tonight into Thursday. As the trough works east, it will shunt the Mid-Atlantic high offshore and open up the region to a southerly mass response pumping a hot low level air mass and attendant EML plume over the Midwest beginning tonight. This will set the stage for several opportunities for severe weather through the end of Thursday. A NW-SE oriented baroclinic zone on the eastern edge of the EML plume will progress across the CWA overnight tonight into early tomorrow morning which some guidance suggests could trigger a line of late night convection. Some lift will be needed to overcome the capping inversion and dry EML base and tap into the 2,000 to 3,000+ Joules of elevated CAPE progged over the CWA. Much better convective chances exist just west of the Mississippi where the nose of the LLJ should do well to trigger thunderstorms, but the kinematic forcing dwindles quickly with eastward extent across northern IL. Efficient isentropic upglide through the low levels along and behind the baroclinic zone may help make up for the lesser forcing. Most recent CAM guidance does resolve at least pockets of convection moving northeastward across the CWA after midnight. If storms develop, the environment will be capable of producing damaging winds and large hail. Torrential downpours will also be a concern with PWATs still hanging out above 1.5". In the forecast, held onto targeted 30-40% PoPs tonight along the boundary. SPC has maintained a Level 1 of 5 Marginal Risk in our area through tonight.

Wednesday and Wednesday Night...

With the arrival of that (even warmer) air mass, nighttime temperatures should only bottom out in the 70s tonight leading to a scorcher tomorrow with high temperatures forecast in the lower 90s area-wide, including up to the lakeshore. Dewpoints into the 70s should push heat indices near or possibly over 100 degrees during the afternoon. Any storms leftover from tonight should work out of the area during the early morning.

There will be a couple of opportunities for strong to severe storms tomorrow. Guidance is in good agreement on an MCS firing up early tomorrow in southern IA/northern MO beneath a shortwave impulse ejecting northeast out of the Plains. This feature looks to hold together, if not invigorate, as it tracks toward the northeast through the morning and across northern IL during the afternoon. Confidence has grown recently in the presence of such a feature, but model discrepancies remain in the timing and strength of this system. The primary window for this potential would be mid-afternoon into early evening, although some CAMs still suggest it could approach I-39 by early afternoon. The afternoon environment will feature little to no capping to over 4,000 Joules of surface-based instability and 25 to 30 kt of effective shear, modest but certainly ample. Damaging thunderstorm winds are the biggest concern here, but severe hail will also be a threat. Can't rule out a spin up tornado either with a veering low level wind profile and an unstable 0-3 km layer.

A much sharper shortwave impulse will track northeast along the lee of the broader upper trough tomorrow into Thursday. This will tighten up a frontal boundary across IA and fire up additional convection during the day west of the Mississippi. Storms are then expected to translate east across IL later in the evening and into the night. There remains a plethora of possible solutions from latest model guidance on how this could all play out. As the wave gets further displaced to the northeast, the front will get laid out across southern WI tomorrow night. With the wonky evolution of the front, storm motions could vary between eastward and south-southeastward. Regardless of the exact nature, any storms that do decide to track across the area will be more than capable of producing damaging winds and hail. The tornado threat looks generally low during the night after the low levels stabilize. Will need to keep a close eye out for any outflow boundaries with storms tomorrow which could act as a trigger for additional convection or a track for training storms. With their late morning update, SPC expanded the Day 2 Level 3 of 5 Enhanced Risk east to include all of northern Illinois, driven by the potential for damaging to destructive winds both during the afternoon and into the night.

Thursday...

Thursday will be another cautiously hot day around the area. The effective front is progged to get hung up in, or just to, our north and highs are again forecast in the lower 90s for most with afternoon heat indices up near 100F.

A potent southern stream wave will develop on the lee of the Colorado Rockies tomorrow night, propagate northeastward, and quickly join forces with the larger upper trough over along the US/Canadian border. The surface response will be a tightly-wound center of low pressure and associated cold front charging east through the region Thursday afternoon and evening. The low center should pass just to our north while the cold front gets dragged across the CWA, and models agree on a strongly forced line of convection with the frontal passage during the evening. This will be a highly dynamic system with 500mb height falls leading up to Thursday night of nearly 10dam/12hr. The environment will be highly unstable featuring 3,000 to 4,000+ J/kg of uncapped, surface-based CAPE. Shear will be more supportive than tomorrow with 35 to 40 kt of effective shear, still not fantastic but certainly more than supportive of organized, severe convection, especially when coupled with this sort of instability. SPC has maintained a level 3 of 5 Enhanced Risk around the area for Thursday, which still seems very appropriate as an all hazards severe event could be on the table.

Friday and Beyond...

Friday will be notably cooler in the wake of the front. Afternoon highs are forecast in the 70s to near 80. High pressure working in to our south will bring sunny skies and dry conditions to Friday. At least the first part of Saturday look similarly quiet with afternoon temperatures forecast to get back into the lower 80s, but another upper trough axis is favored to swing through the Great Lakes with another passing frontal boundary bringing additional chances for rain and storms later Saturday into Sunday.

Doom

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 654 PM CDT Tue Jun 9 2026

Primary forecast concerns include...

Two possible lines of thunderstorms Wednesday afternoon/evening. Chance of thunderstorms predawn hours Wednesday morning. Areas of fog early Wednesday morning near the lake and ORD/MDW.

Current isolated showers and thunderstorms may affect GYY for the next hour but the bulk of the activity this evening is shifting east and southeast of the terminals. Dry weather is expected until the predawn hours when isolated showers and thunderstorms will again be possible. Recent guidance trends have been decreasing this potential and while the chance is low, maintained a two hour prob around/just before daybreak for this potential. If anything does develop, coverage is expected to be low.

Attention then turns to increasing confidence for a line of strong to possibly severe storms racing across northern IL in the early/mid afternoon Wednesday. Further timing tweaks will likely be needed but trends are suggesting an earlier arrival, perhaps by early afternoon across northwest IL. The speed of this line will likely lead to thunder only lasting 1-2 hours at any one location. Guidance is then showing another line of strong to possibly severe thunderstorms developing a few hours behind the first line and again moving across northern IL. Lower confidence for this second line, both for timing and location. A possible outcome would be for a second line to move south of the area the first line moves across. However, prob mention warranted with the new 30 hour ORD/MDW tafs for early/mid Wednesday evening.

Westerly winds generally under 10kts early this evening are expected to become light easterly or light southeasterly by mid/ late evening. Southeasterly winds will continue overnight and slowly turn southerly and then south/southwest by daybreak. Southwest winds will increase through Wednesday morning with gusts into the lower/mid 20kt range through the afternoon, though prevailing winds will likely be affected by the thunderstorms. South/southwest winds will likely continue into Wednesday evening.

While there is currently no fog over southern Lake Michigan, guidance is consistently showing fog developing this evening and then moving it inland and as far west as ORD/MDW during the early overnight hours. Confidence for this is low, but if this fog does develop, visibilities may be less than 1sm, for at least a few hours. Opted to cover this potential with a tempo for now and trends will need to be monitored and changes are likely with later forecasts. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 2 PM to 9 PM CDT Wednesday for the IL and IN nearshore waters.