textproduct: Chicago

KEY MESSAGES

- Another round of showers and thunderstorms is expected later this evening into tonight. Some of the storms could be severe. Locally heavy rainfall could result in flooding issues.

- Windy and unseasonably warm weather today will be replaced by much colder weather over the weekend into early next week.

- Frost/freeze conditions are expected many areas Sunday and especially Monday mornings.

MESOSCALE

Issued at 303 PM CDT Fri Apr 17 2026

Latest radar and satellite trends indicate that storms are following a mostly discrete supercell structure in the vicinity of far northwest Illinois. Recent model guidance has also trended to favor this scenario, suggesting the bulk of convection through at least 5pm in our area will remain generally northwest of a Dixon to Boone County line. A less favorable environment highlighted by diminishing theta-e from modest diurnal mixing and limited near-term upstream moisture advection does raise some questions about how far east the more organized convection will survive over the next couple hours. However, modestly favorable low-level hodographs amid slightly less-backed flow do support the potential for supercell modes to survive with any convection that does clip the far northwest CWA. Primary threat will be large hail and possibly a tornado.

Focus then turns to an expected expansion of convection along the cold front (currently from just east of Des Moines to Kansas City) as it quickly moves eastward into a more favorable environment. As this convection marches eastward through late afternoon, increasing coverage of pre-frontal convection associated with deeper low-level moisture over Missouri will surge toward west-central Illinois. This should result in a well-formed QLCS with the potential for damaging winds and embedded brief tornadoes across a large swath of Illinois this evening. Will need to closely monitor any bowing segments, particularly around and after sunset, as low-level flow increases and lengthens hodographs.

Kluber

DISCUSSION

Issued at 136 PM CDT Fri Apr 17 2026

For near term details on the convective threats through this evening, see the mesoscale discussion (and/or additional future discussions) above. Additional concerns for tonight will be centered on heavy rainfall and localized flash flooding. Precipitable water values still lie at the upper echelon of typically observed values for Mid April, and this coupled with some enhanced forcing should lead to areas of 1-1.5" rainfall in a relatively short window of time. We cannot rule out any higher overall totals (pushing 3"), given that we have several windows of forcing both in the open warm sector and along the cold front. Much of this rain will go immediately into runoff. In addition to any urban locations, it is hard to pinpoint the more favored areas in the watch area, however the most susceptible areas overlapping with higher confidence rainfall would be in the Rock River Basin, and then extending eastward into the Fox and Des Plaines River Basins across northern Illinois. A secondary heavy rain axis may setup south, but if we have a forward propagating system, that may limit the duration of heavy precipitation.

A taste of reality will return to the lower Great Lakes region behind tonight's strong cold front. High temperatures will reached at midnight. While some temperature recoveries will occur as clouds break through the morning, rather stout cold air advection through the day will prevent temperatures from plateauing higher than the mid 40s to low 50s area wide. Normal highs for mid April are right at 60. A stiff northwest breeze will make it feel even less pleasant, with "wind chill" readings in the lower 40 in many areas in the afternoon. This will set the stage for a cold night Saturday night.

Cold advection gets reinforced as the upper trough axis will be shifting overhead. The center of the surface high will be off to our southwest Saturday night, thus we will maintain a bit of a wind component, however things look to decouple enough to suggest some areas of frost.

Reinforced northwest flow on the backside of the upper trough will maintain a similar airmass on Sunday and provide a carbon copy day from Saturday. With high pressure nearby Sunday night, a widespread frost appears increasingly likely, with some area in outlying areas heading for a potential hard freeze.

Upper heights recover nicely and warm advection on the back side for the departing surface high will encourage temperatures to rebound back above normal through mid week with generally dry conditions. There will be periods of onshore flow keeping cool readings near the shore.

KMD

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 116 PM CDT Fri Apr 17 2026

Key Messages:

- High confidence in a line of thunderstorms impacting the terminals late this afternoon and evening. Localized wind gusts in excess of 50 kts may occur in some portions of the line of storms.

- Southerly winds this afternoon will gust to around 30 kts at times before a gusty westerly to northwesterly wind shift occurs with both the line of storms and again with a trailing cold front later tonight.

- Post-frontal MVFR ceilings are expected tonight.

Confidence remains high in all terminals being affected by thunderstorms today. Uncertainty remains with the precise timing of the storms, but this evening still appears to the most likely time frame for storms at the Chicago metro terminals, while RFD could see storms as early as the mid-late afternoon. The storms will likely affect the terminals as a solid line, though some potential exists for RFD to be affected by more cellular or supercellular convection before storms grow upscale into a line with time. A gusty westerly to northwesterly wind shift and localized wind gusts in excess of 50 kts may occur in some portions of the thunderstorm line, and some hail and tornado threat exists too -- primarily at RFD. The storms should last for about 2-3 hours at any one location (possibly longer at RFD if multiple rounds of storms materialize there) with rain then continuing for a bit longer until a cold front pushes across the area.

Prior to the arrival of the storms, southerly winds will gust to around 30 kts at times this afternoon. Another gusty northwesterly wind shift is expected behind the aforementioned cold front with blustery post-frontal west-northwesterly winds then continuing through the remainder of the TAF period.

Lastly, MVFR (and potentially IFR) ceilings are likely to occur during the thunderstorms and again behind the cold frontal passage. A return to VFR conditions is then likely to occur sometime tomorrow morning.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch through late tonight for ILZ003-ILZ004-ILZ005- ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105- ILZ106-ILZ107-ILZ108.

IN...Flood Watch through late tonight for INZ001-INZ002-INZ010- INZ011-INZ019.

LM...Small Craft Advisory until 10 PM CDT Saturday for the IL and IN nearshore waters.