textproduct: Chicago

KEY MESSAGES

- Waves of showers and storms will move through the area today with torrential downpours and localized flash flooding.

- Tonight will be damp with low clouds and areas of drizzle.

- Tuesday through Thursday will be noticeably warmer and more humid with daily heat indices in the 90s.

- Several rounds of severe thunderstorms may occur in the general region Tuesday night through Thursday.

- Temperatures will trend cooler Friday onward, along with lower humidity.

DISCUSSION

Issued at 320 AM CDT Mon Jun 8 2026

Today and Tonight:

An early-morning surface map revealed a surface low pressure system in the northern Plains with an associated double-barreled warm frontal structure extending eastward across the Lower Great Lakes. Meanwhile, recent regional water vapor and radar imagery depict bands of showers and thunderstorms across the middle Mississippi River Valley associated with a northeastward- moving upper-level low. Gradually increasing low-level isentropic ascent atop the broad frontal boundary ahead of the upper-level low is already supporting scattered thunderstorms across our area at press time, which is foreboding of what looks to be a rather wet and humid day.

After daybreak, the double-barreled warm front will merge and lift northward into the central Great Lakes as the upper-level low approaches. Coverage of showers and storms should increase markedly by late morning across central Illinois and spread into our area during the afternoon hours as the center of the upper- level low moves nearly directly overhead. Peak coverage in our local area will be during the late morning through evening hours (roughly 11 AM to 10 PM), including during the evening commute.

With forecast soundings by early afternoon and evening depicting mid-level lapse rates of 5.9 K/km, freezing levels over 15kft, and PWATs near 2 inches, rain rates today will be tropical-like and over 2 inches per hour. As a result, any persistent or redeveloping thunderstorm over the same area may lead to localized flash flooding, especially if overlapping where rain has fallen in the past few days or with the urbanized Rockford and Chicago metropolitan areas. Did give consideration to issuing a Flood Watch south of I-80 where pockets of 2-4" of rain fell in the past few days and over the normally vulnerable Chicago and Rockford metropolitan areas. However, felt it would be better for the day shift to assess emerging trends in the densest coverage of downpours today to inform watch decisions. Finally, cannot rule out a few funnel clouds today owing to modest veering in the low-level wind profile and ambient vorticity within the upper-level low (which may lead to slow-moving low-topped supercell structure as was the case closer to Davenport, Iowa yesterday).

The upper-level wave will gradually dampen while lifting northeast toward Lower Michigan this evening, leading to a gradual decrease in the coverage and intensity of showers in the area. With that said, the passage of a weak surface low reflection and the ambient moist conditions should support expanding drizzle and low clouds across the area. So, while rainfall intensity should decrease after dark, it'll still be a wet night.

Tuesday and Tuesday night:

Owing to a weak low-level pressure gradient, trapped low-level moisture, and gradually rising mid-level heights, Tuesday may start rather dreary with cloudy skies and residual pockets of drizzle. Expansive upper-level troughing across the western United States will then set the stage for southwesterly low- level warm air advection into the Lower Great Lakes, leading to rising surface temperatures. Mean EPS and GEFS height and 850mb temperature fields support afternoon highs in the mid to upper 80s across the area (assuming clouds do clear as expected). With dew points still in the lower 70s, it will feel more like the low to mid 90s. A lack of robust low-level forcing mechanisms and somewhat muted mid-level lapse rates support daylight hours of Tuesday being dry.

Tuesday night, continued southwesterly low- to mid-level flow will advect an EML plume originating in the southern and central Plains northeastward toward the Great Lakes. Being on the northeastern edge of am EML plume can be quite precarious as any subtle perturbation or shortwave moving into the region can lead to the development of severe thunderstorms rooted to the nose of associated local enhancements of the low-level jet. With a growing signal for one such wave to move through the region during the nighttime hours (ECMWF/EPS in particular are quite bullish), have cautiously increased PoPs to the middle chance (30 to 50%) range through the nighttime hours. Have also touched base with SPC to see if a northeastward expansion and increase in severe probabilities will be needed in our area for the Day 2 outlook pending the 12Z suite of guidance.

Wednesday and Wednesday Night:

Any lingering thunderstorms at daybreak Wednesday should end by mid-morning as low-level capping increases at the base of the EML plume and the low-level jet weakens. A largely dry day will then set the stage for temperatures to warm to the upper 80s to lower 90s (heat indices in the mid to upper 90s).

Wednesday evening, an upper-level shortwave embedded in the aggregate troughing will lift into the Upper Mississippi River Valley leading to the development of severe thunderstorms across Minnesota and Wisconsin. A separate region of thunderstorm development may take place in central and eastern Iowa tied to a secondary, more subtle, 500mb wave. Assuming this occurs, convection may try to continue eastward across the Mississippi River and across northern Illinois/northwestern Indiana during the evening and overnight hours. However, do wonder if thunderstorms would tend to struggle into northern Illinois given (1) the upper-level jet and associated region of deep- layer shear will be displaced northwest of our area and (2) the terminus of the low-level jet will be focused into Wisconsin. With that said, any development of a deep cold pool would sustain a threat for outflow-driven convection well into the nighttime hours given an expansive reservoir of instability across the region. In all, will watch the Wednesday night timeframe for a threat for severe weather in the general region.

Thursday:

The forecast for Thursday carries quite a bit of uncertainty. In the wake of the lead shortwave responsible for thunderstorms Wednesday evening, a few individual models are hinting at a trailing subtle secondary wave moving through the central Plains during the overnight hours into early Thursday morning. In such a scenario, initial elevated convection in the Nebraska/Kansas/Iowa region may evolve into a forward- propagating MCS that would be encouraged to roll toward northern Illinois after daybreak. Confidence in this scenario is rather low.

Assuming there isn't an MCS rolling into the area during the morning, Thursday should be another warm and humid day with highs in the 80s and heat indices remaining in the 90s. A strong upper-level shortwave and associated cold front is then expected to swing into the Midwest during the afternoon and evening hours. This time frame looks to be the most concerning for an episode of severe weather in our local area with an overlap of strong low-and deep-layer shear, steep mid-level lapse rates, and moisture-laden instability profiles. A mix of supercells and bowing segments certainly seems realistic with a threat for all hazards.

Friday Onward:

In the wake of the cold front, Friday and Saturday should be noticeably cooler and less humid. Ensemble mean highs from the EPS and GEFS range from the upper 70s to lower 80s with overnight lows in the mid to upper 60s. Am quite confused as to why NBM guidance putting chances for showers and thunderstorms in the forecast for our area Friday night into Saturday given a large surface high pressure system will be moving through the area in the wake of the cold frontal passage. Ensemble guidance supports temperatures trending below average (highs in the low to mid 70s) and a return of rain chances next week as aggregate troughing develops over the northern United States in some fashion.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 645 AM CDT Mon Jun 8 2026

Main Concerns:

- Periods of SHRA and TS today, with TS most favored from the early to mid afternoon through the early to mid evening.

- Low CIGs and reduced VSBY tonight into Tuesday morning.

Following the exit of overnight activity, additional convection is already streaming northward. Based on proximity of SHRA and TS not far south of RFD as of this writing, opted to start the TAF with SHRA VCTS mention. Didn't change PROB30 timing for direct TS impacts at RFD yet, but expect refinements with the next update. For the Chicago area terminals, aside from VCSH through this morning, better daytime heating and a secondary disturbance shifting northward from central IL should translate to SHRA and at least VCTS and/or convective cloud tops >25kft. There's still enough lingering uncertainty regarding timing of direct TS impacts to maintain a longer PROB30 window in the Chicago metro TAFs. Any heavy showers or storms passing overhead will result in low VSBY, down around 1SM at times.

While TS can't be ruled out later in the evening and overnight, coverage should be limited by then. As a weak surface low pressure tracks across northwest IL overnight, light southerly winds and very moist conditions will probably support 1-5SM BR and/or drizzle amidst on and off showers. RFD has the highest chance to observe IFR VSBY at this time. CIGs will likely build down to IFR and potentially even LIFR with time. Expect steady improvement in flight categories by mid to late Tuesday morning.

East-southeast winds this morning will become south-southeast with occasional gustiness by early this afternoon. The wind direction will shift to west-southwest Tuesday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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