textproduct: Chicago

KEY MESSAGES

- Threat for isolated to widely scattered thunderstorms near and east of I-55 this afternoon, with a level 1 to 2 of 5 severe risk, including northwest Indiana.

- A dangerous severe weather outbreak, including the potential for long tracked, strong-violent tornadoes, is possible Wednesday, especially south of I-80.

- Periods of torrential rainfall Wednesday could result in areas of flash flooding (some locally significant) and potentially some rapid onset river flooding.

DISCUSSION

Issued at 237 PM CDT Tue Jun 16 2026

Tonight:

This is the calm before the proverbial storm as winds quickly diminish with sunset. Expect temporary mostly clear skies before mid and high level cloud cover overspreads the region in advance of Wednesday's powerful storm system. While the instability axis will remain off to our west through daybreak, some isolated to scattered showers may move across portions of northern and central Illinois between about 4 AM and 7 AM.

Wednesday:

Synoptically, the ingredients appear likely to come together for a potentially dangerous severe weather outbreak somewhere over the broader region (favoring I-80 and south). This includes potentially long tracked intense tornadoes and destructive straight line wind gusts (and line embedded tornadoes).

Meteorological Setup:

An early-spring like (vs. mid June in actuality) mid and upper level pattern in place across northern North America features exceptionally strong west-northwesterly flow aloft around an unseasonably deep closed low southwest of Hudson Bay. A potent neutrally tilted short-wave trough and associated intense upper level jet streak will eject southeastward and result in near record strong surface cyclogenesis (upper 980s to low 990s mb surface low) over Minnesota into Wisconsin by early Wednesday afternoon.

Impressively strong warm and moist advection atop a lingering stable boundary layer and a rapid increase in large scale forcing will quickly join forces Wednesday morning. The likely result will be an area of convection propagating east-southeast across the area through early afternoon (more on the potential severe threat with this initial round to follow).

The effective warm front will likely be suppressed south to an extent in the wake of the morning/early afternoon convection. However, intense mass response and development of a secondary surface low will likely result in the sharpening warm front shifting north during the afternoon. Exactly how far north the front gets is the million dollar question, so to speak, for the looming more concerning mid afternoon through early evening severe threat (discussed below). Regardless of the exact position of the front, much of if not the entire area is fair game for waves of intense convection. The system cold front will sweep across the area between about 7pm and 9pm, quickly bringing the severe weather threat to an end.

Severe Threat Details:

Round 1 (~8 AM to 1 PM CDT): Initial development of strong to severe storms over eastern Iowa tomorrow morning should translate into portions of northern and central Illinois in tandem with a 60+ kt southwesterly low-level jet by late morning into early afternoon. Strong effective shear and up to 1-2k J/kg of MUCAPE could in turn support elevated supercells and a corresponding threat for damaging to locally destructive winds and hail. The pattern with this initial round of convection will be one to watch for gravity wave associated convection, which would potentially up the ante for the damaging wind threat, despite little/no surface-based instability.

Our thinking hasn't changed much with this initial round, with respect to western and southern/southwestern CWA locales being most favored for severe weather from "round 1". This certainly doesn't preclude the threat extending farther north. There still could be isolated strong storms with heavy downpours and hails extending into the Rockford and Chicago metros even if the more intense convection passes over our southwestern 1/2 (or just south of that). Finally, we would be remiss to not mention the possibility of strong wake low winds on the back side of the initial convection.

Round 2 (2 PM to 9 PM CDT): Presently unknown and unknowable mesoscale details can be expected to play a notable role in modulating the locations/corridors of the highest severe threat, and possibly the magnitude as well (at least within our CWA). From our assessment, SPC's depicted level 4 of 5 threat (moderate risk) area on the updated day 2 outlook continues to best highlight the most likely areas to have a dangerous severe weather outbreak. This is tied to where an overlap of incredibly strong low level (40-50+ kt) and deep layer (70+ kt) shear and moderately strong instability is most favored to occur.

As already alluded to earlier, the million dollar question is how far north the sharp warm front is able to push in the wake of the initial MCS. Lingering cold pool influences are typically not handled all that well by high-res guidance. In addition, and on the other hand, we'll quite literally be in uncharted territory regarding the expected mass response (recovery back northward of a volatile parameter space) given the extreme kinematics at play. The range of depictions in the 12z guidance suite suggests the reasonable error bar to be between roughly the US-24 corridor to as far north as near or even just north of I-80. Thus, a key takeaway here is to absolutely not let one's guard down, lest the warm sector push farther north than expected. Anywhere from near/south of I-88 down to the aforementioned US-24 corridor is fair game, albeit generally favoring south of I-80 within the level 4 of 5 threat area.

To the south of the sharp warm frontal boundary (near/south of the path of the secondary surface low), the progged environment is akin to those found in winter and early spring severe weather outbreaks in the southeast US. Discrete or semi-discrete supercells in such an environment, characterized by 500+ m2/s2 of effective inflow layer SRH and extremely long and looped hodographs, present a ceiling of long tracked strong to violent tornadoes. However, with that said, it's unclear if the storm mode within our area will lend itself to long tracked and/or cyclic mesocyclones and tornadoes, or be on the messier/mixed mode side due to the extremely strong forcing.

In addition, depending on how the boundary layer evolves on the mesoscale in the wake of the initial MCS, it's not uncommon to have a south to north gradient from exceptionally favorable low-level lapse rates and instability for tornadoes to less favorable but still sufficient ingredients right near the northern warm front terminus. Should such a scenario unfold, this may be a path to modulate/limit the event ceiling tornado intensity wise, but this is absolutely not something to bank on in the highest threat areas.

Along with the supercell tornado threat in the system warm sector, a fast moving line of severe thunderstorms (potential derecho) may materialize just ahead of the cold front. As we saw last Thursday, widespread potentially significant wind damage as well as (potentially strong) line embedded tornadoes are common in the most intense QLCSs. Considering that effective shear will be more extreme than Thursday, the above scenario is certainly on the table for parts of the area. Finally, north of the sharpening warm front, intense warm and moist advection will quickly replenish MUCAPE reservoir and could yield scattered elevated severe storms (hail and wind).

Flash Flooding Threat:

Owing to the cool season-like synoptic setup, rapid poleward transport of 2"+ PWATs from the 60+ kt low-level jet will "prime the pump" for torrential to locally excessive rainfall (2"+/hour rainfall rates). Given the wet antecedent conditions and elevated river and stream levels across the area, it will take less rain, even in rural areas, to become problematic. For urban and highly developed suburban areas, the wet antecedent conditions would only serve to exacerbate the issues even coming out of dry patterns. In collaboration with WPC, a rare flash flood threat level 3 of 4 (moderate risk ERO) has been issued for Wednesday.

Repeated rounds of storms, if not true backbuilding/training, particularly near and north of the front, meet aspects of a Maddox archetype. With 24-hour HREF LPMM QPF pushing 3-4" in swaths, the level 3 of 4 flood threat appears more than reasonable. Concern would be urban flooding potentially on the level of mid May 2020 if conditions come together just wrong in/near Rockford, Chicago, and flashy river/stream basins in the nearby Chicago suburbs. Expect a Flood Watch to be issued by the overnight shift, informed by further guidance and observational assessment.

Final Takeaway Message:

Given the severe weather parameters in play tomorrow, as well as the threat for potentially significant flash flooding (even outside of the higher severe threat areas), it is important to continue to monitor later forecast updates closely. Review (and practice) severe weather safety plans. Have multiple redundant means to receive warning information, and keep mobile devices charged in the event of power outages.

Castro/Izzi

Wednesday Night Onward:

We should see a break in the active pattern for the rest of the work week. There are indications that we could get into a more active northwest flow type pattern heading into the weekend and beyond. This may include a threat for another strong moisture laden system and an associated severe weather and flooding threat on Sunday PM/Father's Day. In other words, our break may be short-lived, unfortunately.

Izzi/Castro

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 700 PM CDT Tue Jun 16 2026

Key Messages:

- Abating westerly winds with sunset this evening.

- Active weather day expected on Wednesday, with several waves of showers and thunderstorms. The most substantial storms (with a severe and very heavy rain threat) are expected in the afternoon.

- Wind forecast could become "messy" for a period Wednesday afternoon due to effects for the storms and the proximately of a surface warm front to the Chicago area terminals.

- Low CIGs and VSBYs (IFR to LIFR) likely to accompany the heavy rain producing showers and thunderstorms Wednesday afternoon. A very impressive and dynamic weather system will track into the western Great Lakes Wednesday. This promises to produce a significant severe weather outbreak Wednesday afternoon, particularly near and south of a surface warm front. This surface warm front is likely to remain south of the terminals for much of the day as waves of showers and thunderstorms move across northern IL. Accordingly, winds across the Chicago area terminals should largely favor an east-southeasterly direction, which continues to be highlighted in the TAFs. However, wind directions could become tricky, and possibly even differ significantly between say MDW and ORD for a period mid to late afternoon as the surface warm front attempts to surge northward just ahead of an incoming surface cold front. There is thus a play in which the winds at some of the Chicago area terminals turn south-southwesterly for a short (1-2 hour) period late in the day before turning westerly with the cold frontal passage by early evening.

VFR showers should onset across the area terminals by mid morning on Wednesday (14-15Z). This initial precipitation looks to be void of lightning and primarily accompanied by VFR conditions, particularly across northeastern IL, where the low- level airmass will still be rather dry and stable. This will change quickly later in the morning, however, as Gulf moisture quickly surges northward in advance of the approaching weather system. Therefore, a quick uptick in thunderstorms is expected across the area terminals after 16Z. Thereafter, periods of very heavy rain are expected at times through the afternoon, with IFR to LIFR CIGs and VSBYs likely accompanying this activity. Some storms will also become severe, with the main threat being damaging wind gusts in excess of 70 mph. A notable tornado threat will also exist, especially south of the Chicago terminals in the afternoon.

Conditions will begin to improve into early Wednesday evening (00-01Z), as westerly winds set up in the wake of the cold frontal passage.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from Wednesday afternoon through Thursday morning for ILZ006-ILZ103-ILZ104.

IN...None. LM...Small Craft Advisory until 9 PM CDT this evening for the IL and IN nearshore waters.