textproduct: Chicago

KEY MESSAGES

- Isolated strong storms possible early this morning.

- Another fast-moving line of strong/severe thunderstorms is expected later this morning with a damaging wind and heavy rainfall threat.

- Our active, stormy weather pattern is expected to persist into the weekend with periodic chances of severe weather and heavy rainfall which could lead to flooding.

- A period of dry weather may finally arrive early next week.

DISCUSSION

Issued at 329 AM CDT Wed Aug 12 2026

Forecast remains complex within our persistent pattern of active weather. Main concern in the near term is the potential for another fast-moving line of wind-producing storms across at least a portion of the area later this morning/midday.

A nearly stationary frontal boundary remains in place across the Midwest early this morning, stretching roughly from near Omaha to north of Des Moines to Milwaukee. Complicating the surface pattern however, is the remnant outflow boundary from Tuesday's derecho, which intersects the front near Waterloo, IA and extends southeast and bisects IL along a SQI-BMI-MTO-EVV line. The west/northwest periphery of the cold pool appears to be fairly shallow however, per 00Z DVN and ILX soundings (as compared to ILM farther to the east/southeast). Aloft, a subtle mid-level short wave has been evident in GOES vapor imagery transiting northern IA and entering northwest IL, inducing a modest west-southwest 25-30 kt low level jet from northern MO into northern/central IL/IN. With an axis of 3500-4000 J/kg MUCAPE along/north of the front/outflow boundary across the region, there's been concern since last evening for the potential to develop elevated convection within a northwest- southeast arc across the area (along/northeast of the remnant outflow boundary). So far however, only isolated updraft attempts have been noted and successive CAM runs have backed off considerably in convective coverage across the forecast area through daybreak. That said, we'll have to continue to monitor for any deeper updraft development, as the environment would support the potential for strong elevated storms with a hail threat if they develop.

Of greater concern this morning, is an MCS which has been developing southeast from western South Dakota into northern Nebraska overnight in association with an attendant mid-level short wave and MCV. There is notable CAM support indicating this MCS becoming more organized across eastern NE/western IA toward daybreak, then propagating quickly eastward along the aforementioned surface front/outflow boundaries into our IL cwa after 14/15Z (9-10 AM) this morning. While not quite as robust as yesterday's derecho, high-res guidance does explicitly indicate 60-70 mph winds across parts of our forecast area with this MCS. CAMs indicate an easterly propagation of this system across the cwa, though with the remnant outflow likely mixing back north across the southern half of our cwa that may support more of an east-southeast or southeast track as we saw yesterday. SPC has pulled their new Day 1 slight (level 2 of 5) severe risk up to about a Dixon-Renssalear line. It's possible that could be adjusted further if the evolution of the MCS supports it later this morning. For now, will be advertising the severe threat (primarily wind) even a little north of that for this morning/midday. Thunderstorm threat should then diminish markedly from the west into this afternoon in the wake of the MCS and parent short wave.

There may be somewhat of a lull in convection coverage/potential tonight into early Thursday morning, depending on the footprint of the anticipated MCS cold pool, and with some weak mid-level height rises indicated in model guidance. Have trimmed pops during this period especially up north along the IL/WI border and Lake Michigan. Another mid- level wave (currently over NV/UT) is then progged move into the region on Thursday which look to increase shower and thunderstorm chances again during the day and with additional rounds through at least Thursday night. These continued rounds of storms will maintain at least a marginal severe threat and a threat of flash flooding if storms train over the same areas.

Unfortunately, medium-range guidance continues to depict our active pattern remaining in place across the region into the weekend. There does appear to be decent agreement in the global models/ensembles that high pressure eventually settles across the western Lakes region by Monday-Tuesday of next week, pushing our stalled frontal zone to the south and providing a period of dry weather at that time.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 719 AM CDT Wed Aug 12 2026

Key Messages:

- A complex of storms is likely to affect the terminals between mid-morning and mid-afternoon today. Strong to severe wind gusts will be possible with these storms.

- IFR/LIFR ceilings and visibilities possible at RFD this morning, while patchy MVFR ceilings remain possible elsewhere. Additional low ceilings and reduced visibilities possible overnight tonight into Thursday morning.

An organized thunderstorm complex is entering northern Illinois early this morning and will approach the terminals during the mid-late morning. Some uncertainty remains with whether these storm complex will begin to dive southeastward along an instability gradient or whether it will continue to progress more towards the east and into the heart of the Chicago metro. Current thinking is that the strongest winds with this storm complex (50+ kts) may remain south of the terminals, but the threat for lightning at the Chicago metro terminals remains high enough to maintain the going TEMPO groups. Also maintained a VCTS mention at RFD, but the latest radar trends that the storms should remain south of RFD. Conditions are likely to dry out by mid-afternoon.

IFR/LIFR ceilings and some visibility reductions are being observed at/near RFD this morning, while patchy MVFR ceilings are being observed elsewhere at press time. These conditions may stick around for a bit this morning, but a return to prevailing VFR conditions is favored by this afternoon. Persisting rich low-level moisture and light winds may allow for another episode of sub-VFR ceilings and visibilities late tonight into tomorrow morning. The potential for this appears to be greatest at RFD, DPA, and GYY, but based on how some forecast guidance fared poorly with the visibility and low ceiling forecast this morning, confidence in this occurring is only low-medium.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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