textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms this afternoon into this evening. Some thunderstorms may be severe with heavy rain and localized flooding.

- Possible line/complex of severe thunderstorms mid/late Tuesday morning into early Tuesday afternoon. Severe wind gusts to 80 mph possible, along with heavy rain and localized flooding.

- Continued thunderstorm chances through the end of the week.

MESOSCALE

Issued at 512 PM CDT Mon Aug 10 2026

We continue to see attempts at deeper/taller thunderstorms stretching from the far northern Chicago suburbs to near the Quad Cities. These storms have remained mostly elevated. However, some of the storms are moving into a more favored environment south of a diffuse bounday extending roughly along the I-88 corridor.

South of the front and north of the Illinois/Kankakee River valleys, modestly strong shear for mid-August will favor the development of a hybrid between supercells and multicell clusters with primarily damaging winds and isolated instances of large hail. North of the boundary where shear is stronger, elevated supercells will favor primarily large hail (especially with left-splitting supercells) and some instances of damaging winds. Overall coverage remains in question, favoring isolated/scattered storms versus widespread storms.

The tornado risk remains non-zero, particularily during storm interactions or with any supercells near the main boundary. Lastly, some instances of flash flooding are possible if multiple storms/clusters train over the same location this evening. We will continue to assess the need for a Flash Flood Watch, but current risk/coverage remains low enough at this time.

Looking ahead, a mid-level wave currently over far southeast South Dakota will track across southern Wisconsin and far northern Illinois toward daybreak Tuesday coincident with increasing shear and elevated instability. This may result in elevated supercells with the primary risk of large hail over northeast Illinois around daybreak. With this activity, the flash flood threat is predicated on an existing low-level focus, likely an outflow boundary from this evening's storms.

Kluber

DISCUSSION

Issued at 217 PM CDT Mon Aug 10 2026

The primary forecast challenge remains thunderstorm timing and strength through midweek with a complex of severe thunderstorms possible mid/late Tuesday morning into early Tuesday afternoon.

A very moist airmass is in place with dewpoints in the mid/upper 70s with a few locations tagging 80. Isolated thunderstorms have persisted across eastern IA and far northwest IL the past few hours but have been slow to strengthen. However, the expectation is for this activity to continue moving east and to slowly increase in coverage and intensity. As it taps into the high instability in place, some of these storms may become severe across northern IL and far northwest IN and persist through mid evening. In addition to the severe threat, torrential rain and localized flooding will also be possible though there remains some uncertainty for how widespread any flooding may become. Consideration was given for a flood watch but have opted to hold off for now. Much of the guidance has trended drier for the late evening and overnight hours tonight and have followed this trend with this forecast though some showers and isolated thunderstorms may linger.

Focus then shifts to a rather consistent feature in most of the models for a complex of severe thunderstorms to develop over northeast IA early Tuesday morning and race southeast across the local area from mid morning Tuesday through early Tuesday afternoon. Some of the cam solutions are quite concerning with a line of severe thunderstorms moving across a large part of the cwa. The updated SPC Day 2 outlook has included a level 3 of 5 severe threat for most of the area and given all of these trends have mentioned wind gusts to 80 mph possible and if these storm mature as shown on some of the cams, pockets of higher gusts may materialize. Based on these trends, have also included severe mention to the grids and zone forecast. Once these storms exit to the east, there should be a lull in the precipitation chances from mid/late Tuesday afternoon through at least the early overnight hours Wednesday morning.

There is another wave that may move across the region Wednesday morning with another potential severe thunderstorm complex near or over the local area. Quite a bit of uncertainty for this time period, but will need to be monitored closely. In addition, all of these periods of heavy/torrential rain will allow each following wave/complex to potentially produce flooding more quickly and over larger areas.

The pattern is now looking quite active into this weekend, though there remains quite a bit of uncertainty for timing and location as a series of high pressure systems and cold fronts move across the upper midwest and northern Great Lakes. How fast these move, how far south they push, etc. will have large impacts on when thunderstorms would be most likely across the local area. cms

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 652 PM CDT Mon Aug 10 2026

Forecast concerns include...

Chance of thunderstorms this evening. Possible line of severe thunderstorms mid/late Tuesday morning. Possible mvfr cigs overnight/Tuesday morning.

Scattered showers and isolated thunderstorms will continue through mid evening, as they move east across the area. There may be additional thunderstorms later this evening into the early overnight hours, but confidence is low. Lowered thunder to prob mention for the Chicago terminals this evening and there may be a gap with perhaps additional prob mention needed later this evening and with later forecasts.

Primary forecast concern is the potential for a complex of thunderstorms to move across the entire area mid/late Tuesday morning through early Tuesday afternoon. Maintained previous tempo timing for now, but latest guidance trends suggest thunder may be just a little bit later and timing adjustments may be needed with later forecasts. If this line of thunderstorms materializes, gusts to 50 knots, possibly higher, will be possible.

Patchy mvfr cigs are possible this evening then mvfr cigs will be possible again Tuesday morning, but confidence is low and maintained scattered mention. There may also be some light fog and mvfr visibilities Tuesday morning given the very moist low levels. cms

LOT WATCHES/WARNINGS/ADVISORIES

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