textproduct: Chicago

KEY MESSAGES

- Late spring to summer-like temperatures continue through Friday with highs in the 70s and 80s and lows in the 60s.

- Significant all hazards severe weather threat this evening, particularly north of I-80.

- Heavy rain and flash flooding threat from repeated rounds of thunderstorms tonight into Wednesday. - A brief break in the pattern is expected late Thursday into or through the day on Friday before another storm system arrives in the Friday evening to Saturday timeframe.

- In the wake of the Saturday cold front, a brief, potentially sharp, shot of much cooler air could bring frost/freeze concerns into next Sunday and Monday.

MESOSCALE

Issued at 505 PM CDT Tue Apr 14 2026

Convection has taken off over IN in advance of that subtle shortwave that brought some scattered showers and storms to our southern CWA. In the wake of this shortwave, there has been evidence of subsidence, both in satellite imagery and in the ILX 12z to 18z soundings. The 18z soundings from DVN and ILX both have MLCAPE over 2000 J/kg, but with a pronounced capping inversion on both soundings. That strong cap suggests that we it will likely be another couple-few hours before the threat of convection makes it into our area.

Convection has been increasing in coverage across Wisconsin closer to the warm front as well as across central/eastern IA near the sfc low and south along the dryline. Strong deep layer shear and strong instability will continue to support supercells with potential for large to very large hail with this activity in the near term. Given the intense nature of the convection already, plus a glancing shot of synoptic ascent associated with a shortwave pass well to our northwest, this convection should survive and break through the capping inversion as it progresses east toward and across the MS River early this evening.

As sunset approaches, developing nocturnal low level jet should result in a rapid increase in low level shear in advance of this convection across northern IL and southern WI. This will result in storms moving into a much more favorable environment for tornadoes, potentially strong as they get into southwest WI and northwest IL. By this point, there are uncertainties regarding storm mode, but the strong cap in advance of these storms could slow the upscale growth into a QLCS. If any well developed supercell(s) move into this environment the potential would exist for strong tornado(s) for at least a couple of hours.

Eventually, there are indications that this convection should congeal into more of a QLCS with severe threat transitioning to more of a damaging wind with an attendant QLCS tornado threat. Given the very strong low level shear, the threat for a strong tornado or two could even persist even after the convective mode transitions to QLCS.

It still appears the areas most at risk in our CWA is near/north of I-88/I-290 corridors. Given its current motion, the storms over IA may not reach our CWA until after 0100 or 0130z.

- Izzi

DISCUSSION

Issued at 312 PM CDT Tue Apr 14 2026

Through Wednesday night...

The primary weather focus in the near term continues to revolve around the continued threat for a significant severe weather event this evening. This includes the threat of tornadoes, instances of very large hail (2"+ diameter) and damaging winds in excess of 70 mph. The threat of flash flooding is also of increasing concern tonight into Wednesday, due to repeated rounds of very heavy rain producing storms.

A recent surface analysis indicates that the stalled west-to- east oriented frontal boundary continues to reside north of the area this afternoon, generally from northeastern IA eastward across southern WI. Meanwhile, the outflow boundary from this mornings convection has largely washed out, with a notable airmass recovery across northeastern IL, noted by temperatures rebounding into the low 80s amidst surface dewpoints in the mid 60s. Interestingly, while this afternoon's airmass recovery beneath a corridor of very steep mid-level lapse rates has resulted in moderate to strong MLCAPE values in excess of 3,000 j/kg, a notable capping EML inversion remains in place. This is apparent on the 18Z DVN RAOB, with a sharp EML inversion noted around 800 mb. While this cap is currently curtailing convective development, the sampled thermodynamic and kinematic environment is quite volatile and will be supportive of significant severe weather as we head through the evening.

While some isolated to widely scattered supercells capable of producing large hail will be possible this afternoon as a subtle impulse continues to slide eastward across central and eastern IL into northwestern IN, the primary severe thunderstorm threat continues to be this evening, particularly across northern IL and southern Wisconsin. Initial severe storm development is expected near the stalled frontal boundary in northeastern IA within the next hour or two. In fact, current satellite and radar imagery indicate several convective attempts already underway in this area.

As development occurs, storm modes will initially favor supercells, though a eventual upscale growth is anticipated through the evening. Very steep mid-level lapse rates and long straight hodographs will favor instances of very large destructive hail with these initial supercells. However, strengthening low- level flow and the associated enlarging low- level hodographs into early this evening will will support a notable tornado threat for a few hours this evening (6-9 pm). This tornado threat is expected to be the highest across far northern IL into southern WI (generally along and north of I-88). Upscale growth into a severe wind producing MCS, with potential embedded QLCS tornadoes is then likely to occur through the evening as this activity progresses eastward through southern WI and far northern IL. Accordingly, the main severe threats look to transition to damaging wind gusts in excess of 70 mph and isolated tornadoes later this evening. Increasingly confidence has prompted the recent issuance of a tornado watch for most of northern IL through this evening.

While the severe threat should largely wane by midnight tonight, the threat for training showers and thunderstorms may continue across parts of northern IL into the overnight hours. A veering low-level jet overnight may set up favorable conditions for training and backbuilding storms as the flow offsets the easterly movement. This adds concern for flash flooding, particularly given that this activity could train over the urban areas of Chicago. At this time we have opted to hold off on a flash flood watch, though as trends become more clear this evening one may be needed. We did highlight the threat for possible flooding concerns in a Hydro outlook (ESF), but again, messaging may need to be ramped up to a Watch this evening.

Additional waves of showers and thunderstorms are expected on Wednesday. The amount of cloud cover and periods of rain does make it unclear as to the extent and coverage of severe storms due to questions of diurnal destabilization. Nevertheless, a level 1 to 2 out of 5 severe weather threat continues on Wednesday afternoon and evening. Regardless of the overall severe threat, flooding and flash flooding will continue to be of concern Wednesday into Wednesday night, owing to the repeated instances of very heavy rainfall. Accordingly, a future flood watch may be needed for the tonight through Wednesday time period.

Thursday onward...

A short period of drier weather is expected late Thursday into Friday morning following the eastward departure of the mid- level impulse ejecting out of the Desert Southwest. However, yet another storm system and associated cold front is favored to shift across the central CONUS Friday night into Saturday morning. Locally, another threat of strong to severe thunderstorms will exist in advance of cold front, which should move across our area sometime Saturday morning.

In the wake of this cold front, a period of much cooler weather will move in across the area Sunday into Monday. The magnitude of this colder airmass may result in freezing overnight low temperatures and afternoon highs only in the low 50s for Sunday.

KJB

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 636 PM CDT Tue Apr 14 2026

Aviation forecast concerns for 00Z TAFs:

- Period of TSRA likely later this evening into the overnight hours. Some TS may be strong-severe. Brief IFR vis/MVFR cigs in TS.

- SHRA/embedded TS may linger for some time overnight, if linear orientation becomes more west-east. Low confidence in end timing of TS overnight.

- Additional TSRA possible especially late Wednesday into Wednesday night, however timing/location will likely be affected by lingering cold pool/boundaries from tonight's storms.

- Breezy S-SW winds through period, though potentially disturbed at times by thunderstorms clusters especially tonight.

Latest surface analysis places low pressure over central IA, with a nearly stationary frontal boundary extending northeast and east across southern WI. Clusters of severe thunderstorms have developed in the vicinity of the low and frontal boundary from central/eastern IA into southern/central WI. Farther south, our terminals are in a warm, humid and breezy warm sector, generally capped by an inversion associated warm EML layer aloft. While can't completely rule out isolated TS developing ahead of current activity this evening, current expectation is that the cluster of TSRA over northeast IA will track east across far northern IL and move into the RFD area after 02Z and Chicago area after 03Z with attendant severe weather threat (wind/hail and isolated tornado) through late evening. Thereafter, additional TS may develop as the low level jet turns more west-southwest and increases and potentially leads to some training of storms across part of the area. Exact location and footprint of this is of somewhat low confidence at this time, though have maintained at least a VCTS mention through 09-10Z for this. At some point TS coverage should wane with a somewhat stable cold pool likely resulting in a lull in activity Wednesday morning.

Redevelopment of SHRA/TSRA appears likely later in the day Wednesday into Wednesday evening, though effects of tonight's cold pool and residual stabilizing effects (and location of any aggregate outflow boundaries) will likely have impacts on destabilization and location of stronger low-level forcing. Have indicated VCTS/prob30 after 20-21Z Wednesday afternoon and into the evening with lower confidence in details, though coverage may ultimately require a tempo/prevailing mention.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 9 PM CDT this evening for Winthrop Harbor IL to Burns Harbor IN.