textproduct: Chicago

KEY MESSAGES

- A storm system is expected to bring widespread soaking rainfall to the area Sunday through Sunday night.

- An axis of heavy rainfall is expected on Sunday which could result in instances of flash flooding and river rises. Favored area for heaviest rainfall is south of I-80.

- Dry and seasonably cool conditions are expected to start next week before another stormy pattern settles in mid-late week.

DISCUSSION

Issued at 200 PM CDT Sat Jun 20 2026

High pressure continues to reside over the Great Lakes this afternoon which will maintain our tranquil weather conditions through tonight. Temperatures this afternoon will top out in the mid to upper 70s despite the breezy northwest winds before cooling into the mid to upper 50s overnight. While the breezy winds (gusting around 20-25 mph) will persist through sunset, winds will ease this evening and become light and variable overnight.

Heading into Sunday, the weather will turn more active as a pair of shortwave troughs pivot through the area. The leading shortwave (currently in central NE) is expected to arrive towards the mid to late morning hours and will bring with it a broad area of moderate rainfall. Given that instability during the morning and early afternoon should be rather meager, little to no lightning is expected. However, as we get later into the afternoon and evening the second shortwave will begin to arrive and be deepening as it does so. The deepening trough will allow lapse rates to steepen (around 6-7 C/km) and may result in some embedded thunderstorms developing especially near and south of I-80.

Associated with the second shortwave will also be a developing surface low that is forecast to traverse across central IL and IN Sunday evening. The expected track of the surface low should keep the northeast IL and northwest IN on the north side of the system and thus limit the overall thunderstorm coverage despite the aforementioned steepening lapse rates. That said, the baroclinic zone associated with the low is expected to be draped across the area and that will serve as the focus for additional forcing and an enhancement in rainfall rates. Couple this forcing with the modest instability and the high moisture content of this system (PWATs around 1.5-1.75+ inches) and conditions remain favorable for an axis of heavy rainfall to materialize. The latest guidance suite continues to favor areas south of I-80 (particularly into central IL/IN) for the greatest heavy rain threat, but given the possible convective nature sudden shifts in this axis may still occur. Regardless, it seems that a swath of rain amounts in excess of 1 inch to locally 2+ inches will materialize somewhere in our southern CWA. Given that this area has already seen decent rainfall from last weeks storms, there is an elevated risk for flash flooding and river rises as these showers/storms move through. For now we will be maintaining our Hydrologic Outlook (ESF) for our southern CWA to highlight this risk because of the lingering uncertainty in exact rainfall placement, but if confidence grows consideration may be given to a Flood Watch.

Finally, there is also a non-zero chance for a couple of strong to possibly severe thunderstorms Sunday afternoon and evening mainly along and south of a Pontiac, IL to Rensselaer, IN line. With effective shear only expected to be around 20-30 kts and the limited surface instability locally, the main severe threat should stay south of our area into central IL. However, if a more organized storm does develop then a gusty to locally damaging wind threat may materialize.

The shortwaves and associated surface low will begin to push east late Sunday night into Monday morning. This will in turn allow rain to end from west to east with most locations expected to be rain-free by mid-morning Monday. Therefore, partly cloudy skies and seasonably cool conditions can be expected for Monday with highs peaking in the mid-70s. Breezy northeast winds will develop behind the departing low and persist through Monday evening resulting in large waves and dangerous rip currents at area beaches Sunday night through Monday night. Winds and waves are expected to subside by Tuesday afternoon as the center of the surface high passes overhead. So expect tranquil weather to persist through Tuesday with temperatures being slightly warmer in the upper 70s. However, onshore winds will keep temperatures notably cooler (upper 60s) near the lake.

The pattern is forecast to turn stormier towards the middle and later half of the week as west-southwest flow develops aloft. This will result in periods of shortwaves traversing across the Midwest and Great Lakes which in turn means more chances for showers and thunderstorms. At this range, details on exact storm coverage, timing, and intensity remains a bit uncertain so if you have outdoor plans Wednesday onward be sure to check back for updates. Otherwise, expect temperatures to gradually warm towards more typical summer values by week's end.

Yack

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1226 PM CDT Sat Jun 20 2026

Key Aviation Messages for the current TAF period:

- Breezy winds this afternoon before a weak lake breeze develops after 00Z

- Soaking rains reducing vis are expected Sunday late morning/afternoon

VFR conditions with west-northwest winds with gusts around 20 knots are expected through the afternoon. A weak lake breeze is expected to push westward after 00Z. There is some uncertainty in the exact time of arrival at Chicago terminals, but should be less than 8 knots and eventually become light and variable through the late evening.

Winds are expected to start out on Sunday out of the east- southeast around 10 knots. The next disturbance will move in from the southwest late morning. Rain rates are expected to increase through the afternoon as the core of the system moves in. Periods of soaking rains are expected in the afternoon reducing conditions down to MVFR levels, with a chance for IFR level cigs. Gustier winds to 20 knots are expected at times during the heavier rain, eventually becoming northeasterly by mid afternoon.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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