textproduct: Chicago

KEY MESSAGES

- Periodic shower and storms through the early part of the week, though there will ample dry periods.

- Hotter and more humid conditions build through midweek.

DISCUSSION

Issued at 231 PM CDT Sun Aug 30 2026

A shortwave disturbance over northeastern Iowa will continue to drive showers and embedded thunderstorms eastward through early this evening, with the greatest coverage expected north of I-80. While a 20% chance for lightning remains, poor mid-level lapse rates across northern Illinois should limit thunderstorms to isolated coverage.

Following the eastward departure of this short-wave impulse this evening, a southwesterly low-level jet will ramp up again overnight, with the core expected to reside over IA northeastward into southern WI. Associated isentropic upglide (warm air advection) may thus foster more isolated to scattered showers and thunderstorms late tonight into early Monday morning. Thereafter, capping is likely to curtail the threat of convection for much of the area into Monday afternoon. However, another short-wave impulse is progged to translate eastward towards northwestern IL late in the day, and thus may act as the impetus for isolated to widely scattered afternoon development across northwestern IL after 2 pm. Should this activity occur, ~40 kt of mid-level westerly flow could support a few stronger storms across northwestern IL late in the afternoon. Confidence remains low with the extent of this threat.

Convective trends continue to be of low confidence Monday night into Tuesday morning. Nevertheless, low (20-30%)chances for storms continue due to the persistent warm-air advection pattern in combination with additional smaller scale perturbations likely to transition eastward across southern WI through the mid-level ridge axis. The better focus for storms may remain in southern WI, but at this time, cannot rule out storms as far south as the I-80 corridor. The SPC day 3 severe outlook (for Tuesday) places most areas north of I-80 in a level 1 of 5 threat. So, while confidence in the coverage of storms remains low into Tuesday morning, the meteorological environment will be supportive of a conditional threat for strong storms.

Outside of the threat of storms through Tuesday morning, temperatures and humidity levels will be on the rise through mid-week as the airmass under the expansive mid/upper-level ridge continues to warm. We anticipate high temperatures in the upper 80s Monday, the low 90s Tuesday and the low to mid 90s Wednesday. In spite of us being past the peak timing for evapotranspiration, limited mixing under an inversion centered near 850 mb should keep dew points in the low to mid 70s during peak heating, particularly on Tuesday and Wednesday, resulting in peak heat indices in the 100 to 105 range.

Relief from the heat looks plausible later in the week as a backdoor cold front sags southward into the area. However, this looks to come at the expense of a period of more active weather (a period or two of thunderstorms, some possibly strong) at the end of the week. The specifics such as the timing of the front and best potential for thunderstorms remain a bit unclear at this point, but storm chances begin to ramp up across our area late Wednesday night into Thursday.

KJB

AVIATION /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 639 PM CDT Sun Aug 30 2026

Main aviation forecast concerns for this TAF period are timing potential periods of SHRA/TSRA.

At issuance time, a convectively-enhanced mid-level disturbance was tracking east across northeast IL/southern LM. While some subsidence was occurring immediately behind this feature and resulting in a break in the light rain/showers, additional light rain/embedded TS was occurring within persistent southwest flow across parts of IA/WI and NW IL. Mid-level lapse rates weaken with eastward extent, so not expecting TS to make it into the terminals with any significant coverage, though it does appear that a period of scattered SHRA/-RA will linger across the area through mid-evening.

Though confidence in forecast details decreases later tonight into Monday, current GOES water vapor imagery and CAM forecast trends do support a couple additional periods of better SHRA/TSRA potential. The first is after 3-4 AM into mid-morning morning, and the other is mid-late afternoon Monday as individual mid-level waves propagate through the region. Have maintained PROB30 mentions for TSRA for both, with steeper mid-level lapse rates forecast. Otherwise, VFR conditionsare expected through the period ceilings generally above 7-8 kft. The exception may be brief MVFR vis reductions in heavier convective SHRA/TS.

Winds will remain light south overnight, becoming southwest after daybreak Monday. Gusts around 20 kt appear likely from mid/late morning into the afternoon, easing with the approach of sunset.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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