textproduct: Chicago

KEY MESSAGES

- Additional periods of storms are expected at times through mid-week. Some of these storms could be severe and lead to additional flooding.

DISCUSSION

Issued at 300 AM CDT Mon Aug 10 2026

Forecast remains complex through mid-week, as a nearly stationary frontal boundary lingers across the forecast area beneath an active west-northwest flow pattern aloft. Periodic shower and thunderstorm chances will persist with somewhat low confidence in timing and coverage, ultimately modulated by individual short waves and daily diurnal heating and instability trends. Episodes of severe weather and locally heavy rainfall remain possible.

In the near-term, latest surface analysis depicts a nearly stationary frontal boundary stretching from an area of low pressure over western KS northeast into western IA, then east along roughly the I-80 corridor across northern IL/northwest IN. A very warm/humid airmass is in place along/south of the boundary, characterized by temps in the upper 70/around 80 and dew points in the mid-70s at this late night hour. Aloft, seasonably fast west-northwest flow was in place across the region, supplying a track for a series of mid-level short wave disturbances of varying amplitude. One such wave is currently tracking from IA into northern IL, supporting a southwesterly 35 kt low-level jet atop the low-level frontal zone into northeast IL/northwest IN. Several clusters of strong thunderstorms had developed in the enhanced warm/moist advection regime associated with this jet last evening, with a small MCS remaining across our southern tier of cwa counties as of 2 AM on the nose of the jet in the vicinity of the surface frontal boundary. Given ML/MUCAPE values in the 3000-3500 J/kg range, 35-40 kts of effective shear and precipitable water values near 2 inches, severe weather and locally heavy rainfall remain a concern with these storms for the next hour or so, before they eventually slide southeast of the area toward/just after daybreak as the low-level jet gradually veers more westerly and weakens with the passage of the mid-level wave.

Once these storms exit the area, there should be a bit of a lull in convection in the wake of the departing short wave and during our diurnal instability minimum. Can't completely rule out some isolated elevated shower or thunderstorm development however, due to the presence of the front/baroclinic boundary and weaker but still persistent warm/moist advection. Chances should increase again at least a bit toward midday and into this afternoon however, as another weak wave or MCV (currently approaching the Omaha vicinity) tracks east during a more favorable diurnal time. CAM guidance loosely depicts this, though differences in coverage and timing details are evident. Additional development also appears possible later this evening, as the nocturnal low-level jet modestly increases atop the frontal zone. There does appear to be a somewhat consistent signal however, amongst both CAM and global guidance, for an MCS to develop over NE/SD later tonight which would approach the forecast area around/after daybreak Tuesday. This supports the highest pops (60-70%) during Tuesday, and have limited other mentions to just chance (20-40%) later today and tonight. Given otherwise similar instability/shear and moisture parameters, we remain in SPC/WPC slight risks for severe weather and excessive rainfall during these periods.

No significant change in the pattern is evident into mid-week (Wednesday, possibly Thursday), though confidence gradually decreases based on impacts of any preceding convective/MCS outflow impacts. Global ensembles continue to show a signal for our frontal boundary to shift a little farther south Thursday night and Friday, which may decrease our shower/thunderstorm potential briefly late in the week, though this signal is weaker than earlier runs and there are indications that the front may lift back into the area next weekend. Thus, it appears that our seasonably warm/humid and periodically stormy pattern may persist for much of the forecast period.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1240 PM CDT Mon Aug 10 2026

Forecast concerns include...

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

Mvfr cigs will be possible through mid afternoon, especially just south of the terminals. Most of this cloud cover is expected to lift to low vfr by mid/late afternoon. Mvfr cigs may redevelop overnight and continue into Tuesday morning though confidence is low and maintained scattered mention for now.

There will be a chance of thunderstorms from mid afternoon through mid/late this evening, primarily across northeast IL and the terminals. Overall confidence is slowly decreasing for both coverage and duration of possible thunderstorms though the best timing appears to be 22z to 03z and current tempo mention seems to match up with this potential. There may be a few lingering showers or an isolated thunderstorm later this evening into the overnight hours, but confidence is low and low enough to remove previous prob mention.

Much of the model guidance is in agreement with a complex of thunderstorms developing across northeast IA early Tuesday morning and racing southeast across northern IL and the terminals in the mid/late morning and across northwest IL by early Tuesday afternoon. While there is still some uncertainty for the strength, location and movement of this expected line of storms, confidence is slowly increasing and tempo mention seems reasonable at this time. This line of thunderstorms will most likely be severe and gusts to 50kt in the tempo also seem reasonable based on current trends. Overall duration of thunder may end up only being 1-2 hours at any one location. cms

LOT WATCHES/WARNINGS/ADVISORIES

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