textproduct: Chicago

KEY MESSAGES

- Periodic shower and storms persist this week, though there will ample dry periods.

- Hotter and more humid conditions build midweek.

DISCUSSION

Issued at 200 AM CDT Mon Aug 31 2026

Thunderstorms have fizzled out, but isolated showers fester over the forecast area early this morning. Similar to last night, a southwest to northeast low level jet (LLJ) is expected to amplify prior to daybreak and help induce additional shower and storm coverage. There is lower confidence on the exact coverage; but considering how impressive last night/yesterday morning's coverage ended up being, widely scattered showers and storms can be expected through daybreak before gradually diminishing (as the LLJ weakens) and moves east. Forcing becomes fairly nebulous behind the LLJ. Most areas should receive a period of dry conditions around midday, but maintained slight chance PoPs for a stray diurnal shower that might pop-up this afternoon, mainly north of Interstate 88.

Stepping back and zooming out for a second, GOES IR and water vapor satellite imagery currently shows broad cloud cover sweeping across the CONUS from the Four Corners, to the Minnesota arrowhead, down to our nation's capital. Their movement is riding over an upper level ridge that has dominated the forecast for the last several days. IR is showing showers and storms over eastern Colorado/northwestern Kansas that is collocated with an upper level short wave embedded in the ridge. This wave is expected to quickly move across the Great Plains and head toward southwestern Wisconsin today. Model soundings are showing a fairly strong cap and a robust dry air over much of the area this afternoon and early evening with DCAPE values around 1000 J/kg in northwest Illinois. The cap might be enough to inhibit shower and storm development, but lapse rates are expected to steepen along with modest deep layer shear with the arrival of the short wave. Any elevated storm that develops upstream strengthen and possibly produce some hail up to the size of a quarter or potentially localized damaging wind gusts, mainly for areas north of Interstate 88. The main bit of uncertainty is the exact track of the wave. If it follows the ridge, it could move right over northern Illinois, with favored areas closer to the Wisconsin stateline. But some models have a slightly more northern track into Wisconsin which would limit shower/storm coverage.

Even as the wave passes to the northeast, broad warm air advection behind the wave could allow for festering showers once again overnight into early Tuesday morning. For now, kept "silent" PoPs (10 to 14 percent chance) with no formal forecast mention, but trends will be monitored. At the risk of sounding like a broken record, but models are suggesting yet another wave embedded within the ridge is expected to pass nearby on Tuesday morning/early afternoon. However, models are showing a lot of disagreement with this wave. The NAM currently has the northernmost solution that may keep most of the forecast area dry, whereas the latest 00Z guidance is starting to bring that wave closer down into northern Illinois. With increasing instability ahead of the wave, it provides another chance where storms could become strong to severe. This also just highlights the pattern that the region currently finds itself in with persistent waves within the ridge pattern that keeps periodic shower/storm chances in the forecast. However, shower/storm chances on Tuesday could impact the temperature forecast.

As the upper level ridge slowly ambles east through midweek, southwest winds are expected to advect in a warmer air mass and higher dewpoints into the region. High temperatures are expected to climb into the low 90s with dew point temperatures into the low to possibly mid 70s. Models have been fairly consistent in suggesting Wednesday could be the hottest day of the week where heat index values could reach the mid to upper 90s for much of the forecast area, and better chances that they could meet or exceed 100 south of Interstate 80. Confidence lowers in exact heat values from south to north across the forecast area with greatest uncertainty closer to the top of the ridge (north of Interstate 80) given the better chances for showers and storms there.

Models are suggesting that the ridge could break down by the end of the work week. That will help the hottest of temperatures cool off a bit, but it will return the upper level flow pattern to the northwest and thus bring with it additional chances for showers/storms toward and over the weekend.

DK

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1204 PM CDT Mon Aug 31 2026

Per recent regional VAD wind profiles, a southwesterly low- level has redeveloped tonight across the middle Mississippi River Valley and is pointed toward the Lower Great Lakes. While the difference between the surface and 2000 foot wind speeds is around 30 kt, the change in wind direction is 40 to 70 degrees, so opted to toss in a mention LLWS at RFD (the wind at 2000 feet is comparatively weaker toward Lake Michigan). Broken clouds based between 7000 and 15000 feet will prevail through the nighttime hours.

Similar to yesterday, am anticipating the development of scattered showers and thunderstorms toward daybreak as the low- level jet veers leading to increasing isentropic ascent across the Lower Great Lakes. Will maintain the inherited PROB30 groups for thunder at all terminals generally between 11 and 15Z and consider an upgrade to a TEMPO with the 12Z TAF set as activity is presumably developing (by then, should have a better handle on coverage). Southwest winds will increase with gusts up to about 20kt from late morning through early afternoon.

Also similar to yesterday, an upper-level wave currently moving across Nebraska will arrive in the Iowa/Illinois/Wisconsin border vicinity this afternoon leading to an uptick in showers and thunderstorms. Am suspicious of guidance depicting minimal coverage in an otherwise largely uncapped and unstable atmosphere, though do think coverage may be highest near the Wisconsin state line closer to the track of the wave. Accordingly, will maintain the inherited PROB30 groups for thunder generally in the 21-24Z window this afternoon. With a dry sub-cloud layer, cannot rule out locally gusty winds up to around 30kt where thunderstorms occur.

Showers may linger beyond sunset across the area, especially as broad but generally unfocused isentropic ascent festers across the lower Great Lakes region. Will leave the TAFs dry overnight for now, but would not be surprised in the least to have to introduce -SHRA or -TSRA in a later TAF package. Finally, am noting a signal for a weak frontal boundary to wash out over the terminals after sunset. Should that occur, winds may become light and variable through the last 6 to 12 hours of the TAF.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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