textproduct: Chicago

KEY MESSAGES

- Heat and humidity will return on Sunday and persist through much of the workweek.

- There may be a opportunities for showers and thunderstorms at times amidst plenty of dry hours.

DISCUSSION

Issued at 231 PM CDT Sat Aug 29 2026

Rest of Today:

Despite the ominous upstream radar earlier today, drier air at the lower levels has substantially eroded the rainfall footprint this afternoon (as expected). Thus expect occasional sprinkles and perhaps light showers sufficient to wet the ground in a few spots, favoring near and north of I-80 until sunset. Mostly cloudy skies will keep temperatures in the mid 70s to around 80F, coolest northwest.

Tonight through Sunday Night:

Following a mostly quiet evening, isolated to scattered showers will likely blossom overnight (favoring near/north of the IL and Kankakee Rivers) into Sunday morning. Forecast soundings are unfavorable for anything more than isolated embedded thunderstorms (slight chance/20% mention). Lows will be in the mid 60s to around 70F.

Sunday will mark the start of the advertised very warm to hot and humid pattern in store for much of the week. However, relatively extensive cloud cover and a sharp lower level inversion will keep temperatures in check somewhat (mid to locally upper 80s). Breezy southwesterly flow below the inversion will yield gusts up to 30 mph. A mid-level short-wave will cross the mid-upper MS Valley in the afternoon, which will bring another chance (20-30%) for elevated convection (widely scattered showers and a few storms) to areas primarily north of I-80. Fairly meager, skinny CAPE due to marginal mid-level lapse rates rooted above the strong inversion down around 850 mb, and overall modest forcing will likely again serve to keep thunderstorms isolated at most.

Additional isolated to scattered showers (20-30% chance) and a few storms may then move across portions of far northern Illinois late Sunday night/early Sunday.

Monday Onward:

Strong mid and upper level ridging will amplify northeastward from the southern Plains and center over the Ohio Valley next week. While we're past peak crop evapotranspiration, it will be a very warm to hot and generally uncomfortably humid week. Monday still looks to have a good chance to be the first 90F reading of August at Chicago and Rockford, on the last day of the month! September and thus meteorological fall (already?!) will get off to a summer-like start with additional 90+ temps and daytime heat indices flirting with 100F in spots probable on Tuesday and Wednesday. If the ridge flattens out sufficiently later in the week, weak surface high pressure may enable a backdoor cold front to sag south and bring some cooling, particularly closer to the lake.

The ridging currently looks amplified enough to result in more pronounced capping and a better chance for "ring of fire" type convective activity to remain primarily north of the area until perhaps later in the week, when the ridge may flatten out some, as alluded to above. Northern Illinois (especially with northward extent) is close enough to the precarious northern periphery of the ridge to warrant occasional lower end (20-30%) thunderstorm chances (focused at night) through midweek. Ensemble guidance variance, even later in the week, is such that periodic PoPs are still only in the 20-30% range.

Looking ahead to Labor Day weekend, central CONUS ridging will likely remain in control, resulting in continued well above normal temps. The exact position and orientation of the ridging will modulate how warm temperatures will get, along with convective chances.

Castro

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 646 PM CDT Sat Aug 29 2026

Aviation forecast concerns for this TAF period include timing periods of showers, a relatively low potential for minimal LLWS tonight mainly for KRFD, and breezy southwest winds on Sunday.

One band of light showers is over KMDW/KGYY at issuance time, but will move east of the terminals by 00Z. The forecast area will remain within a region of broad warm/moist advection through the period, thus isolated sprinkles or light showers can't be completely ruled out through Sunday. However, the period from late tonight into Sunday morning appears to have the greatest potential for more organized SHRA potential as a 35-40 kt southwest low level jet develops. HREF ensemble guidance supports a roughly 09Z-16Z window (KRFD earliest and KGYY latest) for a few hour period of SHRA. Isolated TS are not out of the question, though potential appears too low for TAF mention. May need to go with a TEMPO for these SHRA with later updates if guidance trends converge on timing/coverage. May also need another PROB30 mention Sunday afternoon, though there is more guidance spread and lower confidence during that time. Otherwise, VFR conditions are expected throughout the period, with showers mainly from mid-level (8-10 KFT) cloud bases.

Wind-wise, surface winds are currently backing SSE in the 10-15 kt range early this evening, though should eventually settle back SSW after midnight. With the low-level jet increasing, expect there will be some sporadic gusts for KORD/KMDW which typically remain warmer/more mixed. LLWS appears marginal from a TAF standpoint even farther west at KRFD where surface gusts will likely be less frequent, and therefore have not included in TAFs. It does appear that surface winds will become gusty to around 25 kt later Sunday morning as we mix into the weakening jet after sunrise.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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