textproduct: Chicago

KEY MESSAGES

- Today through 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.

- Outside isolated showers this afternoon, several rounds of severe thunderstorms may occur in the general region Tuesday night 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 312 AM CDT Tue Jun 9 2026

Near Term / Today:

The center of the upper-level wave responsible for waves of showers and thunderstorms the past few days is now drifting northeastward away from our area. Subtle vorticity maxima rotating around the perimeter of the upper-level low interacting with the very moist airmass (PWATs near 2" and freezing levels above 15kft, per the 00Z DVN RAOB) continue to support regions of isolated to scattered showers in the general region. Mostly cloudy skies envelop much of the local area early this morning, which is leading to temperatures holding steady in the lower 70s.

After daybreak, the strong sun angle will facilitate efficient mixing through the cloud layer leading to a transition from mostly to partly cloudy skies. Given the warm start to the day, temperatures will rocket into the mid to upper 80s with perhaps a few local 90 degree readings not out of question. While mid- level height tendencies will be neutral (indicative of little to no large-scale vertical motion) through the day, the warm and moist boundary layer will nevertheless support potential isolated shower development (perhaps an afternoon storm) throughout the day.

Tonight:

Over the next 24 hours, aggregate troughing across the western United States will shift eastward establishing expansive southwesterly low- to upper-level flow from the central Plains toward the Upper Mississippi River Valley. Such broad southwesterly flow will advect an expansive EML plume eastward into the Middle Mississippi River Valley atop a similarly- expansive region of low-level moisture across the central United States. The net result will be the development of a large reservoir of instability ahead of the aggregate upper- level troughing. Over the next few days, embedded shortwaves moving around the perimeter of the broader long-wave troughing interacting with the reservoir of instability will support periodic episodes of severe thunderstorms across the Midwest. Meanwhile, outside the convective episodes, the combination of seasonably warm temperatures and the resident humid airmass will lead to hot conditions.

Tonight, the eastern edge of the EML plume will approach the Lower Great Lakes. Being on the northeastern edge of the EML plume is often fraught with danger as any subtle wave riding atop the elevated frontal boundary or unexpected moistening at the base of the EML can support nocturnal severe thunderstorms. Guidance remains mixed as to whether any such wave or enough mid-level moistening will be able to activate the northeastward- moving EML plume, with perhaps a better signal northwest of our area. Nevertheless, MUCAPE in excess of 3000 J/kg, convective- layer shear in excess of 30kt, and PWATs in excess of 2 inches would support threats for damaging winds, damaging hail, and flash flooding should convection develop in our area tonight. All things considered, will maintain low-end chance (30 to 40%) PoPs across the area tonight. The SPC Level 1 threat area remains appropriate.

Outside the threat for thunderstorms tonight, the high- moisture airmass will maintain seasonably warm temperatures with overnight lows generally in the 70s.

Wednesday:

Any thunderstorms that develop overnight should shift eastward by or just after daybreak as capping strengthens beneath the northeastward-moving EML plume. The warm start to the day will act as a launch pad for temperatures to spring into the upper 80s to lower 90s across the region by early afternoon. Heat index values may approach or locally exceed 100 degrees where temperatures and dew points are highest. While local Heat Advisory criteria is 105F, conditions on Wednesday will be the warmest we have experienced so far this year so heat safety tips should be practiced.

Attention then turns to the next threat window for severe weather. At this point, am seeing two potential windows for severe storms during the day: The first in the afternoon (2 to 6 PM) and another in the evening (6 PM to 12 AM).

For the first time window, a subtle signal exists in both CAM and global guidance for a convectively-invigorated shortwave or MCV to develop in the central Plains tonight and then lift northeastward toward the eastern Iowa/southern Wisconsin/northern Illinois region during the afternoon hours. Naturally, confidence in where the track of a wave originating from prior thunderstorms in the Plains (let alone whether it will actually develop) is naturally quite low. With that said, the environment ahead of any early-day wave will be more than supportive of severe thunderstorms with MUCAPE > 5000 J/kg and effective shear (itself driven by the convective wave) of 25 to 30 kt. Initial supercellular development would probably be prone to growing upscale into a foward-propegating MCS with a threat for damaging winds. At this point, what subtle signal there is for a lead wave favors areas generally along and north of Interstate 88 as being in the threat zone during the first time window.

The second time window will be driven by a synoptic-scale shortwave embedded in the aggregate troughing along the US/Canadian border. Appreciable synoptic-scale ascent ahead of the wave in the Midwest characterized by 12-hour 500mb height falls on the order of 10m/hr will induce the development of thunderstorms along a warm front stretching across northern Wisconsin into central Minnesota and a separate region of thunderstorms along a cold front stretching into central Iowa during the late afternoon hours. Current thinking is that oblique orientations of the deep-layer shear vector to the cold front will support initial supercellular development in central Iowa in the late afternoon before conglomeration into a squall line toward the Mississippi River during the evening hours (moving into northern Illinois after 5 to 6 PM).

With the upper-level jet and associated access to deep-layer shear displaced northwest of our area, the strongest portion of any squall line may tend to lift northeastward into Wisconsin. With that said, the reservoir of instability across our area as well as intensifying low-level flow (925 to 850mb flow near 35-40kt) may maintain the southeastward spill of an outflow- dominant portion of the squall line into northern Illinois through the evening hours. While locally damaging winds would be a threat, the oblique orientation of the low-level jet to the west-to-east positioning of any outflow-driven portion of the squall line raises red flags for potential training and flash flooding. Also, should convection occur in the first time window and leave an outflow boundary somewhere in the region to intersect with the squall line during the second time window, could easily envision a corridor of heightened tornado potential somewhere in the region. Overall, confidence in the evolution of convection on Wednesday is low to medium. The SPC level 2 to 3 threat level across our area seems appropriate, albeit with higher confidence in impacts typical of level 3 areas occurring perhaps in Iowa or Wisconsin.

Thursday:

At this point, the expectation is for the cold front associated with the Wednesday convective episode(s) to stall somewhere in the southern Iowa/northern Illinois vicinity by mid-Thursday morning. As a result, do expect another hot and humid day across the area (especially south of I-80) with heat indices in the upper 90s to locally 100. We continue to see a signal in various model guidance that an MCS may develop in the Plains Wednesday evening and race toward northern Illinois our southern Wisconsin during the morning (or early afternoon?) hours with a threat for damaging winds, though confidence in this scenario is naturally very low. Should that MCS occur, it would all but certainly modulate the positioning of the front leading to a bust in temperatures across portions of the area.

During the afternoon hours, the dominant upper-level shortwave in the broader upper-level trough in the northern US will eject eastward into the Great Lakes and support the development of a surface low tracking near the Mississippi River during the evening. Conceptually, the pattern matches analogs for significant severe weather events in our area as the strongest upper-level forcing, low- to deep-layer shear, and instability will overlap directly across the Lower Great Lakes.

At this point, it is tough to have confidence in the exactly evolution of thunderstorms given we are still more than 48 hours out, especially considering there could be complicating thunderstorms in the morning. With all of that said, do want to highlight that there is a scenario where morning thunderstorms reinforce a west-to-east orientation of the warm front by late afternoon somewhere in our area. Such a frontal orientation would provide prolonged access to an overlap of strong instability (MUCAPE >3000 J/kg) and low-level shear (0-3 km shear of 30-35 kt with >300 J/kg of 0-3km SRH) for eastward- moving storms. The net result would likely be the development of focused zone of intense thunderstorms with a threat for all hazards (tornadoes, destructive winds, damaging hail, and flash flooding) our area. Even if the front is not oriented in such a fashion on Thursday, an all-hazard severe threat could still emerge as the upper-level waves moves overhead.

The SPC level 3 of 5 threat level for Thursday is very appropriate across our area given the synoptic-scale set-up. Readers are advised to keep close tabs on the forecast.

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 /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1259 AM CDT Tue Jun 9 2026

Main Concerns:

- Occasional +RA at MDW and GYY early this morning

- Low CIGs until mid to late morning

- Early evening lake breeze wind shift at near lake terminals

- Low (20%) chance for a few TS in the area this afternoon-early evening and again late tonight/Tuesday night-early Wednesday

Areas of heavy rain primarily confined to locations south and east of downtown Chicago as of this writing will result in occasional VSBY down to 1SM or possibly lower. CIGs have finally trended lower across the area, with a period of IFR (temporary LIFR in a few spots) and 2-4SM VSBY expected until steady improvement this morning.

A mostly quieter day is in store, though still can't rule out a few isolated PM SHRA/TS. A late arriving lake breeze boundary could serve as one possible focus area. Timing of the northeasterly wind shift (from westerly) currently looks to be in the ~00-01z timeframe, though confidence is only medium. TS may develop over the area in the pre-dawn hours of Wednesday, with low confidence and thus no mention in the 06z TAFs.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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