textproduct: Chicago

KEY MESSAGES

- A couple rounds of thunderstorms are expected on Sunday, the first round in the morning through early afternoon and the second in the afternoon and evening. Exact timing and coverage of storms with second round remains uncertain.

- Any storms on Sunday could be severe, especially during the afternoon and evening hours, with all severe hazards possible including flash flooding.

- Additional periods of storms are expected Monday through mid- week with the chance that a few storms could be severe especially on Monday.

DISCUSSION

Issued at 227 PM CDT Sat Aug 8 2026

A surface high is settling into the northern Mississippi Valley and western Great Lakes this afternoon in the wake of the this morning's cold front. Despite the northwest winds and the hazy skies, temperatures have still managed to warm into the lower 80s and should be able to warm another 2-3 degrees prior to sunset. Winds will turn more southeasterly overnight as the surface high slides overhead with temperatures cooling into the lower to mid-60s.

While we enjoy a quiet summer night tonight, a shortwave trough will be ejecting across the northern Plains. This shortwave is expected to lead to the development of a cluster of thunderstorms across SD and MN overnight that will gradually west-southwest with time. There is also a chance for some scattered showers and storms to develop on the nose of the low- level jet in IA and MO tonight, but guidance has been inconsistent with coverage of storms there so confidence on that occurring is low. Regardless of what develops in IA/MO tonight, the northern complex is expected to ride the instability gradient and drift towards northern IL. Initially it looked as if the better instability would remain south and west of our area which in turn would keep storms outside of our area through Sunday morning. However, recent trends now show last night's front lifting back north as a warm front overnight into Sunday morning which looks to bring the better instability axis into our area prior to the complex arriving. Therefore, confidence is increasing that a complex of thunderstorms will move across northern IL and northwest IN during the morning hours on Sunday. Since the storms are forecast to arrive prior to the atmosphere fully destabilizing, the expectation is for the storms to move through in a decaying state. As a result, confidence in storm coverage with eastward extent is lower but the thinking is at least some isolated to scattered storms will traverse across most of the area between 9 AM and 12 PM CDT. Given the decaying nature of the storms, the severe threat in the morning looks low at this time but instances of gusty winds and hail could occur with the strongest storms.

Heading into Sunday afternoon, the aforementioned warm front should be stalled across northern IL and northwest IN which will serve as the focus for a second round of thunderstorms Sunday afternoon and evening as another shortwave pivots through. While confidence on the second round of storms occurring is fairly high (70-80% chance of occurrence), confidence is lower in the exact time storms will redevelop and where the greatest coverage will be. The reason is because the morning round of storms could limit how quickly the atmosphere destabilizes and thus delay the redevelopment of storms until later in the afternoon or even the evening. Regardless of when and where storms initialize, the environment in place Sunday afternoon and evening will be very unstable (MLCAPE values around 3000-5000 J/kg) with 35-45 kts of deep layer shear which will support thunderstorms organizing and becoming severe. The main severe threat with the storms will be damaging winds upwards of 60+ mph, but instances of hail and perhaps a tornado or two cannot be ruled out given the steep mid-level lapse rates and veering low-level wind field respectively. Due to this threat the Storm Prediction Center has maintained the slight risk (level 2 of 5) across our entire forecast area.

In addition to the severe weather threat, there is also a conditional threat for flash flooding to materialize given the humid air mass (PWATs around 2 inches) and favorable storm parameters. The condition, though, is whether or not storms train over the same locations Sunday afternoon/evening or if they remain transient enough to limit the heavy rainfall in any one location. With the aforementioned uncertainties in storm coverage Sunday afternoon/evening, confidence is too low to warrant the issuance of a Flood Watch at this time but something we will be monitoring as the event unfolds. Furthermore, there is also the potential for storms to keep festering overnight Sunday night as the low-level jet ramps up in the proximity of the stalled frontal boundary. If coverage of storms Sunday night is more expansive, then the threat for flash flooding could quickly increase especially if coverage of afternoon/evening storms is higher. That said, there is a chance (20-30%) for a swath of 2-3+ inch rain amounts to occur somewhere in our forecast area so remain on guard particularly if you live in an urban or lower-lying area.

Another period(s) of thunderstorms are forecast for Monday as another shortwave pivots across the region. Similar to Sunday, the environment will be warm, humid, and unstable which should support a fair coverage of thunderstorms especially during the afternoon and evening hours. However, confidence on coverage and the exact timing of storms remains low and will depend on how storms evolve on Sunday and Sunday night. Nevertheless, any storms that materialize on Monday will also have a chance to become severe with damaging winds the main threat. Thus, SPC has maintained another slight risk (level 2 of 5) for severe weather on Monday. Thunderstorms on Monday will also have the potential to lead to locally heavy rainfall and flooding given the humid air mass.

The northwest flow pattern aloft is forecast to remain in place through much of the upcoming week which will continue to support periods of showers and thunderstorms through at least Wednesday night. As mentioned in previous discussions, there is the chance that as repeated rounds of storms occur for the effective frontal zone to shift south of northern IL and northwest IN but confidence on when or if this actually happens remains low. Therefore, our forecast will continue to advertise shower and thunderstorm chances through at least the middle of next week. Outside of the rain, expect seasonable temperatures to prevail with daily highs in the 80s and overnight lows in the 60s.

Yack

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 705 PM CDT Sat Aug 8 2026

A complex forecast is in store through Sunday night, with the potential for multiple rounds of TS (some strong to severe) to affect all terminals. Currently, there are two main forecast scenarios for this TAF period:

1) A mid-level wave over eastern Montana and High Plains dry line will allow developing convection to grow/congeal into a larger organized TS complex over eastern South Dakota overnight. The complex would track southeast across southern Wisconsin and northern Illinois in a decaying phase with associated gusty winds mid to late morning. Behind the TS, winds would shift SE or even E as an outflow boundary tracks well south of the terminals. The boundary would then retreat north later in the day and evening, providing a focus for a 50 knot low-level jet to induce robust training convection mid-evening into the overnight hours near or south of the Chicago terminals.

2) Developing convection in the High Plains fails to congeal into a TS complex. The wave over Montana would then provide the main focus for scattered clusters of strong TS to develop over southern Wisconsin and far northern Illinois late morning into early/mid afternoon. Coverage would likely be high enough to push an outflow boundary over the terminals. However, the boundary would be weaker and more prone to quickly retreat to over northern Illinois by the evening. This would lead to several hours of strong TS with torrential rain over the Chicago terminals with the low-level jet focused over the area.

Confidence in both scenarios is about equal at this time, but impacts from TS in or around the Chicago metro later Sunday morning/early afternoon and especially Sunday evening appear likely.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

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