textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms Monday with all hazards possible, large hail, damaging winds and tornadoes. Heavy rain and localized flooding also possible.

- Seasonable and dry conditions this weekend, but cooler temps near Lake Michigan.

DISCUSSION

Issued at 144 AM CDT Sat Apr 25 2026

A cold front is currently moving across the area with brief patchy fog possible along the front. Low clouds are widespread across far eastern WI and much of Lake Michigan and are spreading southwest into northeast IL currently. Expect this trend to continue with a cloudy start to the day for at least northeast IL but these clouds are expected to lift and scatter by midday. Persistent northeast winds will allow for a large temp gradient across the area today, with highs in the lower 70s well inland to only the 50s near Lake Michigan. Low temps tonight will be in the mid 40s for most locations and then temps will rebound into the lower/mid 70s for much of the area Sunday, with another large gradient closer to Lake Michigan Sunday afternoon, as temps only reach the upper 50s near Lake Michigan.

Primary forecast focus however remains the severe potential for Monday. Overall, no significant changes. Low pressure will move from the Plains Sunday to the western Great Lakes region Monday night, as it slowly deepens. Thunderstorms are expected to develop across eastern IA/northern MO Monday afternoon and move east across the local from mid afternoon through late Monday evening. As previously noted, appears the best chance of any discrete supercells will be across the southwest cwa (and areas west and southwest of our area), with an expected evolution into a squall line and potential QLCS tornado threat. Despite the fair/good agreement among the models, were still 60+ hours from the event and additional details/specifics are still to come.

Heavy rain will possible with these thunderstorms and even with a possible fast moving line, with precipitable water values into the 1.5 inch range, there may still be rainfall totals of 1-2 inches. While the usual low-lying/urban flooding would be a concern with these amounts, if they were to materialize over the Rock, Fox or Des Plaines River basins, renewed rises would likely result. While it is far too early for a flood watch, did coordinate with WPC to include a slight risk for excessive rainfall, mainly for areas north of I-80, for Monday or day 3.

There may be some wrap around showers Tuesday morning and then another cold front will move across the area Tuesday night, shifting winds northeast. Another weak system may move across the area Thursday bringing a few showers and additional cooler air. Low temps by the end of next week could be in the mid/upper 30s, thus frost potential will need to be monitored as this time period approaches. cms

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1251 PM CDT Sat Apr 25 2026

Key Messages:

- Very low (IFR) CIGs near the lakeshore and at GYY through the afternoon.

- The IFR CIGs shift inland through this evening, likely impacting the main Chicago terminals by mid to late this evening.

- Potential for the low CIGs to build down low enough to lead to some fog late tonight into early Sunday morning.

Low cloud cover persists over Lake Michigan and areas as far inland as 5 to 10 miles. While this will continue to be the case through the afternoon, we have concerns that this low cloud cover will spread inland after sunset this evening, potentially making it into the main Chicago area terminals by 02 or 03Z this evening. Confidence on the exact timing remains unclear, but once it does work inland, we are likely to experience very low CIGS (IFR to LIFR) through the remainder of the night, before conditions improve mid to late Sunday morning. We may even have a period late tonight into early Sunday morning in which the clouds build down to the surface as fog. However, it remains a bit unclear as to how extensive the fog threat may become. With all this in mind, we have opted to hit conditions harder in the TAFs for tonight. Accordingly, we have prevailing IFR CIGS moving into the main Chicago terminals after 02Z this evening, and a few hour tempo group early Sunday morning for lower VSBYs/CIGs. Conditions will diurnally improve by mid to late Sunday morning, with VFR conditions expected thereafter. Otherwise, winds will remain easterly through Sunday.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.