textproduct: Chicago

KEY MESSAGES

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

- There may be a few opportunities for showers and thunderstorms at times, but it does not currently appear to be a stormy pattern next week.

DISCUSSION

Issued at 159 PM CDT Fri Aug 28 2026

Tonight through Saturday Night:

Expect primarily quiet conditions through the day on Saturday. Tonight will be the last relatively cool night for a while as we enter the advertised much warmer and more humid pattern into next week. On Saturday, decaying convection will cross the mid and upper MS Valley, which may yield some sprinkles or light showers to parts of northern Illinois during the afternoon, particularly closer to the IL/WI state line where we have some low PoPs. The main sensible weather effect will be fairly thick mid-level overcast, which will likely keep temperatures a bit cooler. The mostly cloudy skies paired with dew points in the lower to mid 60s will make for an all in all comfortable late August day.

On Saturday night, an initial warm front will lift north into Wisconsin. Conceptually, the best chance for nocturnal convection will be closer to the front (ie. somewhere over central/southern Wisconsin). This will be the focus for stronger warm and moist advection and a better chance to erode MUCIN in an overall modest mid-level lapse rate regime. Can't rule out some spotty showers late Saturday night into early Sunday over parts of far northern Illinois, though forecast soundings look prohibitive enough for lightning generation to forego any mention of thunder in the grids. It will be a milder night than the past several amidst steady south-southeast winds.

Sunday Onward:

Strong mid-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. Sunday will feature well above normal highs solidly in the mid to upper 80s along with breezy southwesterly winds in the wake of a secondary, stronger warm frontal passage. Monday then has a decent 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 possible 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 mid and upper level 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.

Castro

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 640 PM CDT Fri Aug 28 2026

Only aviation forecast concerns for the 00Z TAFs are the lake breeze which is currently stalling across the northeast side of O'Hare Field, and the low (15-20%) chance of some showers or sprinkles mainly north/northwest of the Chicago terminals Saturday afternoon/early evening.

Currently, as mentioned above, the lake breeze boundary has stalled across the northeast side of ORD, with the wind sensor at the approach end of 27R (and briefly 27C and 9L) indicating east winds 6-10 kts. Had amended early to include a tempo easterly wind with this in mind (boundary not completely crossing the field) and have maintained that through 01Z before winds settle light SSW overnight.

On Saturday, a mid-level disturbance is forecast to track east across the region, enhancing south-southwest warm/moist advection into the upper Midwest. The strongest forcing and best potential for showers looks to be to our northern into WI, though a few sprinkles/showers can't be ruled out into northern IL during the afternoon. Probability/coverage appears fairly low this far south (15-20%) so have not mentioned in the TAFs. Otherwise, southwest winds will gradually increase and eventually turn more southerly later in the day/evening. Thickening mid-level VFR cloud cover during the day may limit gust potential somewhat, though gusts approaching 20 kts are possible by mid-late afternoon as the gradient tightens a bit. There is dome model support for surface winds to turn south- southeasterly at least briefly Saturday evening.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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