textproduct: Chicago

KEY MESSAGES

- An axis of showers may drift across the area Friday evening through early Saturday AM (30-40% chance) with isolated embedded non-severe storms possible (20% chance).

- A period of heavy rain with embedded thunderstorms is expected Sunday which may lead to flooding.

UPDATE

Issued at 851 PM CDT Thu Jun 18 2026

Quick update this evening to add some low-coverage (20-30%) showers across north-central IL. Expectation is that these will slowly weaken/dissipate over the next couple of hours, with non-zero but decreasing lightning potential.

A small cluster of showers was approaching western Ogle and northwest Lee counties as of 830 pm, in association with a low- amplitude short wave transiting northwest flow and accompanied by a subtle surface trough/weak cold front. While modest surface-based instability will diminish with diurnal cooling and east/southeastward extent, modestly steep (6-6/5 C/km) mid- level lapse rates may allow these showers to track across the I-88/I-39 corridors south of Rockford through late evening. Can't rule out an occasional lightning strike early, though the overall lightning trend has been downward. Have noted a few 20-30 mph wind gusts at some of the northwest IL AWOS sites (Dubuque, Savanna and Freeport), so some brief gusty winds are possible. Have added some 20-30% pops for this activity for the evening hours, and will monitor trends to see if they need to be extended any further east/southeast.

Otherwise, remainder of the forecast appears to be on track with no other significant changes needed.

Ratzer

DISCUSSION

Issued at 302 PM CDT Thu Jun 18 2026

Northwesterly flow across the area in the wake of yesterday's strong weather system paired with decent coverage of diurnal cumulus has helped keep temperatures cooler today with temperatures in the low to mid 70s across the area. A lake breeze hugging the lakeshore into Cook County and well inland across northwest Indiana have held temperatures in the 60s there. Meanwhile, a few light showers have developed into portions of southeast Minnesota associated with a weak wave diving across the Upper Midwest. This may try to sneak in a few light showers or sprinkles into far northern/northwest Illinois this evening but have held off on a formal mention in the gridded forecast with this update given weak forcing amidst an overall dry airmass.

While winds will remain generally out of the west to northwest into Friday (away from the lakeshore), lower cloud coverage should help temperatures warm into the mid-upper 70s across the area. A weak lake breeze may cool temperatures back into the upper 60s/lower 70s near the lakeshore in the afternoon. Heading into the evening hours, a shortwave is forecast to dive southeast within the broader upper level northwest flow. This feature may lead to shower and thunderstorm development across portions of Minnesota and Wisconsin during the day on Friday. This activity may drift into northern Illinois in a decaying mode. Some hi-res guidance maintains showers and possible isolated embedded thunderstorms along an elevated fgen axis that slides south across the area through the night into early Saturday morning. Any lingering showers should come to an end by late morning south of I-80 with temperatures warming into the upper 70s to lower 80s.

A compact system is expected to eject off the Rockies and move east across the Central Plains late Saturday into Sunday bringing a period of inclement weather to the region on Sunday. There remains model variability in the handling of the associated surface low, including its strength and exact track, which will influence which areas are most favored to experience a longer duration of heavy rainfall and any possible severe thunderstorm and flooding threats. More northerly solutions (NAM/ECMWF) would support heavy rainfall across nearly the entire area, whereas the more southerly solutions (GFS/UKMET), focus the heavy rain axis south of I-80. The varying solution tracks may be tied to differences in handling any influence from an expanding Canadian surface high and the position of closed mid-upper lows meandering around the southern Hudson Bay/Ontario region. All in all, Sunday continues to be a day to monitor, particularly for any flooding concerns (currently a level 1 and 2 out of 4 threat for the local area). If a more northern track verifies this could lift a marginal severe weather threat north into portions of the area, mainly south of I-80.

Beyond Sunday we enter a brief dry period as the aforementioned high pressure expands into the area in the wake of the departing surface low. Onshore winds will likely keep temperatures cooler than normal to start the week with highs in the lower to mid 70s (typically lower 80s in mid-late June). Temperatures then gradually warm through midweek into the upper 70s, with our next chance of rain returning during the Tuesday night into Wednesday timeframe as another system dives southeast within the broader northwest flow pattern.

Petr

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1218 AM CDT Fri Jun 19 2026

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

West-northwest winds will prevail at most terminals through the period as surface high pressure shifts eastward across the Lower Missouri Valley today. The only exception to this being at KGYY, where winds look to turn north-northeasterly off the lake for a period this afternoon in association with a lake breeze. It appears unlikely that this lake breeze will shift far enough inland to impact ORD and MDW. Diurnal mixing of the boundary layer may result in some gustiness up to near 20 kt this afternoon, but any gustiness will quickly abate into early this evening. Otherwise, expect VFR conditions to prevail today.

The next weather impulse will track southeastward into the western Great Lakes into this evening. As it does, we should see an increase in VFR cloud cover and a chance (30-50%) for a period of light showers this evening. Accordingly, I have maintained the going PROB30 for this at the main Chicago terminals from 03-06Z this evening, and also added the mention to the surrounding area terminals. The threat of lightning continues to look very low with this activity.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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