textproduct: Chicago

KEY MESSAGES

- A period of showers is expected this evening into the overnight hours. A stray thunderstorm or two and locally gusty winds will be possible in the evening (20% chance).

- There is the potential for heavy rain and thunderstorms in the region on Sunday, though if and where a flooding threat will materialize here locally remains uncertain.

DISCUSSION

Issued at 315 PM CDT Fri Jun 19 2026

Through Saturday night:

Mostly sunny skies amidst broad surface high pressure has allowed temperatures to warm into the low to mid 70s this afternoon. Expect temperatures to warm another 2-3 degrees with highs generally in the mid to upper 70s this afternoon. A lake breeze hugs the shoreline in Illinois which should hold temperatures in the upper 60s to around 70.

A cluster of thunderstorms have developed upstream across northern Minnesota in the vicinity of a weak disturbance diving across the Upper Midwest. The expectation is that any shower and storm activity associated with this feature will remain largely north of the area. However, a separate area of showers and isolated thunderstorms is ongoing across northern Iowa that is tied to a separate sheared out vort and along a low to mid level frontal axis. This activity will continue to slide east southeast toward the area this afternoon. 18Z DVN RAOB shows a layer of marginal instability (MUCAPE of ~550 J/kg) in place to our west. While instability is expected to decrease with eastward extent and with time, lapse rates may be just steep enough here locally to support lightning strikes persisting into at least portions of the area this evening. Have accordingly maintained a 15-20% chance for thunderstorms as the showers move across the area but again this should be very isolated. Given dewpoints have dropped into the upper 40s to lower 60s, the larger dewpoint depressions may support locally stronger gusts being brought down to the surface, potentially as high as 35-45 mph with the tallest showers/storms.

Showers end from north to south through the night with perhaps a few light showers or sprinkles lingering well south of I-80 after daybreak. Saturday will be similar to today, perhaps with just slightly warmer temperatures (upper 70s to around 80) inland with an afternoon lake breeze again keeping temperatures cooler near the lakeshore.

Petr

Sunday:

The most notable upper-level disturbance progged to track through our region over the next seven days will eject eastward out of the central Rockies on Saturday and reach our neck of the woods on Sunday, accompanied by an attendant surface low. Ensemble guidance over the past few days has struggled to get a good handle on how this ejecting shortwave disturbance and associated surface low will evolve, and this theme has continued with the latest ensemble suite. The existing and persisting ensemble variance can partly be explained by slight differences in the configuration of various influencing synoptic-scale features this weekend, but it is also a likely byproduct of the extensive coverage of convection that is expected across the central Plains on Saturday and what effects that may have in modulating the main synoptic wave and surface low. Because of the role that this central Plains convection may play in dictating the strength and track of the shortwave and surface low, their ultimate evolution will likely not be truly known until Saturday night at the earliest, once observational trends with the Plains convection have become apparent (and even then, there's no guarantee that the system's track and strength will become obvious). Despite the existing uncertainties, there is fair agreement across the ensemble suite that this system will track over or close enough to our area for at least part, and quite possibly all, of our CWA to see rain from it on Sunday, so made no changes to the 70-90% PoPs delivered by the NBM.

Model guidance generally favors precipitable water values peaking anywhere between 1.25" and 2" here, which raises concern for notable rainfall rates that could bring about some potential for flooding. Whether such a threat for flooding will ultimately materialize here, though, appears to be contingent upon either 1.) a relatively strong surface low developing and tracking across or just south of our forecast area and inducing a robust deformation band on its northern flank that steadily pumps out rainfall at a good clip as it persists over the same areas for a few hours, and/or 2.) the surface low tracking far enough to the north for the associated warm front, richer boundary layer moisture (70+ degree dew points), and greater instability to enter our forecast area, allowing for convectively-driven torrential rainfall rates to occur with any thunderstorms that develop near and south of the front. Will also note that the latter scenario would also introduce a potential for severe weather for at least our southern counties on Sunday. The envelope of the latest ensemble and deterministic forecast guidance shows mixed support for both of these scenarios, but there are also several advertised outcomes with relatively weak surface lows and more southerly surface low tracks that would make this a relative non-event for our area with fairly insignificant rainfall totals and little to no thunderstorm activity north of I-74. The bottom line right now though is that, while a great deal of uncertainty remains, Sunday continues to be a day to monitor for potential weather impacts in our area, so stay tuned for forecast updates.

Monday through Thursday:

After the Sunday system clears the region, quieter and drier conditions should settle into the area for the beginning of the upcoming work week. However, there remains a signal for another upper-level disturbance embedded within northwesterly flow aloft to bring another round of precipitation to the area sometime in the late Tuesday through Wednesday time frame. Global ensemble meteograms are otherwise in relatively good alignment on temperatures remaining near to below normal for this time of year through at least the middle of the upcoming week.

Ogorek

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1225 AM CDT Sat Jun 20 2026

Key Messages:

- VCTS through 0730Z, mainly at MDW and GYY.

- Breezy westerly winds (gusts 20-25 kt) develop later this morning and continue into the afternoon.

- Lake breeze wind shift likely by early (late) this evening (afternoon) at MDW/ORD (GYY).

Some scattered showers and thunderstorms recently developed along the I-80 corridor in northern IL. This activity is shifting to the east-southeast and will be in the vicinities of MDW and very near GYY through 0730Z, before this activity shifts out of the area.

While there are otherwise no significant aviation weather concerns anticipated through the period, breezy westerly winds (gusts 20-25 kt) will develop later this morning into this afternoon. Then, as the winds begin to abate into early this evening, a lake breeze is likely to shift the winds onshore at ORD, MDW and GYY around, or just before sunset. Wind speeds should ease quickly following the easterly wind shift, with winds likely going light (under 5 kt) and variable by mid to late evening as a surface ridge axis shifts overhead.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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