textproduct: Chicago

KEY MESSAGES

- Two windows for storms exist today: one from around midday through this afternoon, and the other from the late afternoon through tonight. The exact coverage of storms with both rounds remains somewhat uncertain.

- Any storms in our area today will have the potential to be severe. There is also a potential for flash flooding, particularly with the second round of storms.

- Additional periods of storms are expected Monday through mid- week. Some of these storms could be severe, especially late Monday afternoon or Monday night.

DISCUSSION

Issued at 423 AM CDT Sun Aug 9 2026

A prolonged, convectively-active period is expected across the Midwest through much of this week as a muggy and unstable air mass stubbornly hovers over the region over the next several days. That said, confidence in forecast specifics regarding the thunderstorm potential for any given day during this period is inherently low given the nature of this summertime air mass, the involvement of subtle forcing mechanisms, and the influence of one convective cycle on the next and the subsequent domino effects that follow.

First thing's first, though. Clear skies and calm/light winds have allow for some fog develop in our area early this morning, mainly focused outside of the Chicago and Rockford metro area. Thus far, it appears that any denser fog has remained patchy based on area webcams, but we'll continue to monitor the potential need for a targeted Dense Fog Advisory as the morning goes along.

Anyways, circling back to today's thunderstorm potential... There still appear to be two potential windows for thunderstorms in our area today. The first window will be from around midday through this afternoon and will be tied to the convection that is ongoing to our west at this early hour in Iowa and eastern South Dakota. There was concern earlier that the storms eastern South Dakota could grow upscale into a mature, well-organized MCS that would plow through our area later today. While such a scenario isn't quite out of the equation just yet, the extensive, messy nature of the convection ongoing across Iowa does lend doubt to that scenario being realized. However, some loosely-organized conglomerate of the ongoing Iowa and South Dakota storms and/or any new convection that sprouts between or just north of them will still likely drift towards our area, with the associated outflow likely to press eastward into forecast area later today. By the time this outflow arrives here around the early afternoon, convective inhibition to 2000-3000 J/kg of MLCAPE here will have largely been eroded, and new updrafts will be able to go up along it uninhibited. However, questions still remain regarding the strength of the attendant cold pool and the degree of convergence along this boundary, as the relative lack of synoptic-scale forcing support this afternoon will necessitate stronger lift at the surface for storms to develop and harness the full potential of the thermodynamic environment in place. The storms that do end up pulsing high enough this afternoon will be capable of producing localized damaging winds and possibly a few instances of 1+" diameter hail.

The second window for storms in our area is expected to come late this afternoon into tonight as a low-level jet ramps up and a shortwave trough passes by to our north. Note that it is possible that there may not be much of a break between these two potential storm windows. The outflow boundary from the earlier convection will likely be laid out somewhere in or very near our forecast area by this evening, and increasing isentropic ascent atop this boundary will likely generate new convection along it during this second thunderstorm window as long as the storms from earlier in the day don't end up being overly robust and extensive in coverage. How much instability will be available for these later storms to work with will depend on how strong and widespread the earlier storms were, but deep-layer shear looks to be sufficiently strong to support rotating updrafts with any mature convection during this second thunderstorm window. Damaging winds and hail will thus be possible with these storms, and a threat for tornadoes also couldn't be ruled out with any supercells that develop near the outflow boundary if the boundary layer on the cool side of the outflow boundary manages to re-destabilize in time before nocturnal stabilization starts to set in. If the outflow boundary were to get laid out in a northwest-southeast orientation like some CAMs have been showing, then that would increase the likelihood of storms training over the same area. With the muggy air mass being conducive to torrential rainfall rates, this would raise concern for the threat for flash flooding tonight.

Regardless of how the storms in our area play out today and tonight, today's moist and unstable air mass will not budge much going into tomorrow, so conditions on Monday will remain favorable for additional thunderstorms to develop in the region once again. Confidence in forecast specifics for Monday remains relatively low owing to there being some dependence on tonight's convective footprint, but broadly speaking, most of the latest CAM guidance keeps our area devoid of thunderstorm activity during the daytime hours tomorrow due to a combination of capping and a lack of large-scale forcing mechanisms outside of a west-east-oriented corridor of fairly weak mid-level warm air advection/isentropic ascent. This ascent may actually hinder the potential for thunderstorms during the daytime more than it will help as it will likely lead to extensive mid- to high-level cloud cover (and possibly even some high-based showers) that will filter solar insolation to some degree where it is present and thus delay destabilization efforts.

While a storms couldn't be ruled out to develop earlier in the day, the most likely scenario at this time for tomorrow is that convective initiation holds off until the late afternoon or evening, when the low-level jet to our southwest gets going again and a subtle mid-level disturbance is progged to arrive from the west. Exactly where storms will initiate is not yet clear at this time (and it's not yet a lock that our CWA will see storms tomorrow), but it's likely that their point of origin will be somewhere along the aforementioned boundary. Wherever storms do develop, strong to locally extreme instability will be supportive of explosive updrafts, while largely unidirectional deep-layer shear will be robust enough to promote some level of thunderstorm organization, including updraft rotation and/or bowing segments.

Ogorek

Tuesday through Friday:

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 /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1251 AM CDT Sun Aug 9 2026

Aviation forecast concerns for the 06Z TAFs:

- Patchy fog/low stratus possible through daybreak.

- At least two rounds of TSRA possible, some strong to severe, from late this morning through tonight. Confidence in details below do decrease beyond this afternoon.

Late evening surface analysis depicts low pressure in the vicinity of northeast CO/NE panhandle region, with a warm front extending east to near the IA/MO border and into downstate IL/IN. This warm front is forecast to lift north across IA and into northern IL/IN later today, as low-level southerly flow strengthens in association with an approaching mid-level disturbance. This will bring a warm, moist and unstable air mass across the terminals, setting the stage for at least a couple of rounds of strong thunderstorms from later this morning through tonight.

Currently, a small cluster of thunderstorms has developed along the ND/SD border ahead of the aforementioned mid-level wave. This cluster is expected to track southeast into southern MN by daybreak, eventually expanding on the nose of the southwesterly low-level jet into southern WI/northern IL late this morning into this afternoon (basically scenario 2 from our previous 00Z discussion). Coverage is anticipated to be high enough to push an outflow boundary through the terminals this afternoon (potentially with gusty winds on the leading edge of the storms), but this should be able to return north (perhaps as a composite with the synoptic warm frontal boundary) later in the day/evening. This boundary would then likely serve as the focus for additional thunderstorm redevelopment as the low-level jet impinges on the outflow/warm frontal boundary over the area this evening, especially across the Chicago terminals. These storms could potentially linger over/near the terminals in some fashion past midnight before eventually sliding off to the south/southeast as the cold pool expands and the low-level jet veers more westerly. It's important to note that confidence in the details in this scenario decrease considerably beyond the afternoon hours.

Provided storms do shift south/southeast of the terminals late tonight, a period of light/variable winds appears likely along with some potential for MVFR conditions to linger through daybreak Monday.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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