textproduct: Chicago

KEY MESSAGES

- Expect plenty of dry hours through Memorial Day Weekend. Isolated to widely scattered thunderstorms are possible late Saturday night into Sunday, potentially focusing primarily southeast of I-55 Sunday afternoon.

- Below average temperatures to close out the work week will trend to near to above average into next week, though notably cooler conditions will prevail near Lake Michigan.

DISCUSSION

Issued at 240 PM CDT Thu May 21 2026

Through Saturday:

Seasonably strong surface high pressure centered over eastern Canada will keep fairly status quo weather conditions, with at times gusty northeast/east-northeast winds (strongest near the lake) easing some tonight and then picking back up on Friday. Extensive mid to high level cloud cover through Friday will result in early Friday morning lows not quite as cool as this morning (Thursday morning) but then only mid 60s to low 70s highs inland Friday afternoon, perhaps approaching 60 lakeside. Can't rule out a few sprinkles out of the mid deck across portions of far northern Illinois early to mid Friday morning as a weak impulse passes by.

A fairly complex mid and upper level pattern will unfold Friday afternoon into Saturday morning, with two main short-waves of note. The first is a robust neutral to negatively tilted impulse pivoting from the northern Plains to the central Canadian Prairies. Our main short-wave of interest is a positively tilted southern stream impulse (and weak surface low pressure reflection) moving from the lower MS Valley to the central Great Lakes.

The existing dry sub-cloud layer air due to surface high pressure influence may be tough to erode, especially with north and northwestward extent, as stronger forcing will focus into our southeast CWA and points east. There are also non-negligible differences in the strength of the southern stream impulse, with the ECMWF/EPS camp stronger and thus more aggressive in spreading light rain/showers into a good chunk of the area. Given the weaker forcing and drier posture of much of the non EC guidance, kept PoPs in the 20-40% range (highest southeast of I-55) and indicated sub mentionable (<15-20%) PoPs up in the Rockford area. If and where rain does occur, a bulk of it would likely occur in the overnight hours and thus less likely to compromise outdoor plans.

In the wake of any lingering light rain/showers Saturday morning over the eastern 1/3 to 1/2 or so of the CWA, expect dry conditions the rest of the afternoon and early evening hours. Temperatures will be a bit warmer, reaching primarily the low- mid 70s away from Lake Michigan.

Saturday Night through Thursday:

Another modest short-wave impulse will move across the region Saturday night into Sunday. Isolated to widely scattered showers and maybe a few storms may drift eastward in tandem with the wave into Sunday morning (~20-30% PoPs). Sunday afternoon, the best chance for isolated to widely scattered renewed convective initiation currently looks to focus primarily southeast of I-55, closer to the departing short-wave. Even if/where any thunderstorms develop, strong to severe storms are unlikely. Temperatures will again step up some on Sunday away from the lake, reaching the 75-80F range.

Summer-like warmth is forecast inland (highs in the lower to mid 80s) Memorial Day through midweek next week, while east to southeast synoptic flow will be conducive to daily lake breezes and cooler conditions lakeside. Some rain chances may return midweek next week, though the progged pattern does not appear supportive for organized convection, and there may be a play for a bulk of the rain remaining south of our area.

Castro

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1200 PM CDT Thu May 21 2026

There are no significant aviation weather concerns anticipated through the period.

A deck of MVFR stratus continues to quickly dissipate across the Chicago area terminals. Accordingly, aside from some higher level cloudiness at times, VFR conditions are anticipated through the period. Otherwise, a large area of high pressure centered over southern Ontario into the Upper Great Lakes will favor a persistent easterly wind through Friday.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through Saturday afternoon for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement through late tonight for INZ001-INZ002.

LM...Small Craft Advisory until 10 AM CDT Saturday for the IL nearshore waters.

Small Craft Advisory until 4 AM CDT Friday for the IN nearshore waters.