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 615 PM CDT Fri Apr 17 2026

The cluster of cyclical supercells over northwest Illinois that ultimately produced a tornado in Winnebago County has shifted northeastward into southern Wisconsin. Frontal convection is not currently surging eastward toward the Mississippi River, with radar trends indicating bowing clusters with embedded supercell structures. While the 21Z KILX RAOB shows a remaining weak cap and diurnal mixing has decreased dew points by a couple degrees over much of the area, the congealing nature of the convection and modest low-level moisture advection from the southwest supports convection remaining quite organized well into the forecast area this evening.

North of the I-80 corridor, a localized area of moisture convection (aligning with a few showers over the west Chicago metro), observed slight backing of the winds, and observed gradual elongation of the low-level hodograph (per KLOT VWP trends) supports a locally higher QLCS tornado threat into at least the western Chicago metro and possibly to the lakeshore with any bowing segments. Otherwise, damaging winds with possibly a corridor or two of 70+ mph gusts are possible through mid-evening.

South of the I-80 corridor, the amalgamation of a large area of convection over over-central Illinois has a potential to organize into a MCV with eastward extent over the next several hours. If this were to occur, another localized damaging wind threat with embedded QLCS tornados would be focused around the IL/Kankakee River valleys.

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 /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 636 PM CDT Fri Apr 17 2026

Forecast concerns include...

A broken line of thunderstorms this evening. Strong/gusty southerly winds early this evening. Strong/gusty northwest winds overnight/Saturday. Mvfr cigs overnight.

A broken line of thunderstorms will move across the terminals this evening. The forward speed of this line may begin to increase in the next few hours and specific arrival times to ORD/MDW remain challenging. Opted for prevailing thunder at 130Z and maintained the 02z-04z tempo, but this may need some adjustment. Once this line of storms moves east, a period of showers with isolated thunder will remain possible during the late evening with showers ending during the early overnight hours. The main severe weather threat is expected to be locally high westerly winds.

Ahead of these thunderstorms, southerly winds gusting into the 30kt range will continue. Prevailing speeds/gusts will diminish behind the storms, likely favoring a south/southwest direction and then shift to the northwest overnight when a cold front moves across the area. Gusts into the 25-30kt range are expected for much of Saturday, diminishing Saturday evening.

Mvfr cigs will be possible both with the thunderstorms and then prevailing mvfr cigs are possible overnight. Only medium confidence and cigs look to lift into low vfr by mid/late Saturday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 5 AM CDT Saturday 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 until 5 AM CDT /6 AM EDT/ Saturday for INZ001- INZ002-INZ010-INZ011-INZ019.

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