textproduct: Chicago

KEY MESSAGES

- An outbreak of severe thunderstorms with the potential to produce damaging tornadoes, destructive straight line winds, and large hail is possible this afternoon into the early evening.

- A flood watch has been issued for today. Locally heavy rainfall is expected to result in an enhanced threat of localized flash flooding.

- Tranquil weather is expected to close out the work week in the wake of this system.

- Another strong storm system is expected to bring a return of showers and thunderstorms later this weekend, with some potential for heavy rainfall and/or severe weather somewhere in the general region.

UPDATE

Issued at 1100 AM CDT Wed Jun 17 2026

Now pushing through the latter half of the morning, we find a mature bowing MCS diving southeastward across central IL. This line has been producing torrential rainfall and damaging wind gusts since maturing over central Iowa during the predawn hours. The system is riding along the northern flank of the MUCAPE gradient with little to no elevated instability coincident with the core of the complex, at least until just recently. The 12Z RAOB out of DVN sampled no instability through the column, but it certainly picked up on the impressive shear profile working into the region. With the limited instability, this system is being largely driven by strong kinematic forcing, very efficient moist upglide, and a likely gravity wave influence. Latest SPC mesoanalysis depicts a 60kt SW 850mb jet nosing into western IL beneath 70+ kt of diffluent WNW upper level flow downstream of the approaching jet max. These features make for strong vertical lift through the column. This low-mid level flow is resulting in impressive moisture/theta-e advection feeding into this complex with 100mb mean mixing ratios near 17g/kg and PWATs of 1.7" and climbing. The resultant heavy downpours with MRMS picking up on 3 to 4" per hour rates recently prompted a number of flash flood warnings to be issued across parts of SE IA and western IL.

The severe bowing segment will continue to across central IL through the end of the morning along the instability gradient, which includes southern and southwestern portions of our CWA. MLCAPE building north across central IL with time coupled with elongating hodographs could result in a tornado threat along the leading edge of the line in the coming hours. A Tornado Watch was recently issued around our southwestern CWA for this potential. However, most recent guidance suggests the MLCAPE gradient should remain safely south of our CWA through the morning and damaging wind gusts will continue to be our primary concern with this current push. The flash flood threat will also translate into our area with this line and favor areas south of I-80 through the morning.

Additionally, a wake low appears to be moving into northern IL behind the showers, north of the severe bowing segment. Widespread 2- hour surface pressure falls of 3 to 5 mb have been observed across northern and eastern IA with a couple as high as 7 mb. Additionally, a number of sites and personal weather stations have been recording 50+ mph gusts out of the southeast in the wake. Accordingly, we issued a Wind Advisory through 1 PM for areas along and north of I-88 for non- thunderstorm wind gusts up to 50 mph. As things are shaping up, these wake low winds may be our overall biggest concern through the rest of the morning.

Beyond the morning hours, the going forecast remains on track, although many of the same uncertainties still linger. The big question of the day is how well the atmosphere will recover into northern IL following this morning convection. Most of the newest CAM guidance from this morning has the effective warm front, and by extension MLCAPE gradient, locking up well south of I-80, if not just south of our CWA boundary this afternoon. There is still enough support though for the front to surge farther north and open up more of the CWA to the more favorable severe environment. The RAP has consistently been farther north in the position of the front this afternoon parking it near the I-88 corridor. While an outlier among CAM guidance, it's hard to argue this possibility given the kinematic field at play. The front is currently sharpening up over south-central Illinois, but we will unfortunately have to wait until much of this current activity clears away before we can hope to gain a handle on trends into later today with the augmented front, but steady rain and storms will continue to muddy up our insight and make it extraordinarily difficult to anticipate trends. Still anticipate a highly favorable severe environment this afternoon near and south of the warm front. As additional storms develop along and ahead of the impending cold front, those in the warm sector will be exposed to uncapped instability and outlandishly long, looping hodographs capable of resulting in destructive wind gusts and strong, long tracked tornadoes. The flash flood potential remains alive and well through the day across the CWA regardless of the position of the front. See the full discussion below for finer details regarding severe threats through the afternoon. Regular updates will be made to the forecast to reflect latest thinking.

Doom

DISCUSSION

Issued at 212 AM CDT Wed Jun 17 2026

Through this evening:

Complex set-up today with difficult to predict mesoscale details regarding early day convection and its impacts on surrounding environment likely to play a key role in the magnitude and location of severe potential this afternoon and evening. Significant regional severe weather outbreaks are often a result of mesoscale "accidents" happening withing an otherwise favorable synoptic scale environment for severe weather.

Synoptically, today fits that bill well with an unseasonably strong 120kt+ 250mb upper level jet nosing east-southeast into the mid-Mississippi Valley this afternoon. A rather sharp and vigorous 500mb trough will reach the Upper Mississippi Valley later this afternoon, with a remarkably strong (for mid June) 80kt+ H5 jet streak rounding the base of this trough. At the sfc, a near record deep (for June) double barreled low pressure system is progged by 00z to have one low over southern Lake Michigan east of the IL/WI border with a trough extending northwest to a second low over northwest Wisconsin. From a large scale perspective, this set-up looks exceptionally favorable for a regional severe weather outbreak including the potential for intense tornadoes, however the devil may end up being in the mesoscale details that unfortunately just cannot be accurately predicted this far in advance.

A strong low level jet over the eastern Plains is rapidly transporting deep moisture northward into the lower Missouri Valley. This process can be seen well on GOES TPW imagery. The combination of strong large scale ascent associated with the upper level jet and strong isentropic ascent with the aforementioned low level jet has already led to the development of showers and scattered thunderstorm from northern IA west- northwest across South Dakota and southern Minnesota. Convective allowing models have a pretty strong signal depicting more vigorous, potentially severe, convection developing over eastern IA just prior to dawn this morning as this strong forcing encounters a rapidly northeastward expanding axis of elevated instability driven by the strong low level theta-e advection from the low level jet. The severe threat with this convection looks like it will be confined more to areas south and west of our CWA, closer to the expected elevated instability axis, but cannot rule out a severe threat later this morning creeping into our far southwestern CWA.

There is considerable spread in guidance in how this convection evolves, but with low level jet remaining strong through the day as the nose shifts east into Illinois, it is certainly plausible that this convection will increase in coverage and organize into an MCS. Depending on the track, longevity, and intensity of the potential MCS, the synoptic scale environment will be disrupted heading into the afternoon. Again, there's considerable spread in guidance with respect to the placement of what will likely become the effective boundary, a composite of the synoptic scale warm front and outflow boundary from this MCS. A number of models indicate an extremely rapid northward surge of low level moisture and result instability in the wake of this MCS, while other guidance indicates that the effective boundary may struggle to even reach the Highway 24 corridor across our far southern CWA.

Normally it is safe to expect a farther south than progged location of the effective boundary in the wake of an early afternoon MCS, however today's set-up features an unseasonably deep surface low for mid June with a near peak high sun angle. These factor make it difficult to write off the models that depict this boundary and unstable air mass surging rapidly north in the wake of the MCS, possibly making it to nearly the Wisconsin border later this afternoon. The northward extent of the boundary should be demarcation between a more substantial severe weather and tornado threat, and a potentially more muted (but still existent) severe weather threat to the north.

Several of the CAMs are depicting convective initiation along a pre-cold front trough in the warm sector. Given the expected shear, should air mass recovery actually be sufficient, then this would create an exceptionally favorable environment for discrete, fast moving tornadic supercells. Given how quickly these cells will tend to propagate off of the initiating boundary, their ability to survive and thrive will hinge on them traversing a sufficiently unstable/weakly capped environment. Where and whether this type of environment will exist, will hinge on how the difficult to predict mesoscale details play out this afternoon. Also, the magnitude of instability will play a role in how quickly these storm develop along the boundary as low level convergence along the prefrontal trough isn't progged to be terribly strong. So if weak convection quickly moves off the boundary to even a weakly capped air mass, it could struggle to mature. Where and whether this type of environment will exist, will hinge on how the difficult to predict mesoscale details play out this afternoon.

Forecast hodographs within the warm sector feature an amazingly strong low level shear profile with 0-1km SRH well over 300 m^2/s^2, maximized closest to the composite warm front/outflow boundary where some backing of the low level flow is likely. Given that LCLs and LFCs will be quite low, there could be rather strong low level CAPE despite the progged abysmal low level lapse rates (less than 6C/km).

Ultimately, today is going to require close monitoring of the mesoscale environment to determine if and where the greatest tornado threat will exist this afternoon and early evening. The current SWODY1 placement of the MDT risk highlights the most likely area within our CWA to face this potential, however it also wouldn't be hard to envision a scenario where early convection results in the more favorable environment developing farther south/east than the current MDT risk. Given these uncertainties, not planning any significant change to our messaging for the severe potential at this time, however as observational trends begin to shed a light on the ultimate evolution of the mesoscale environment, some changes to today's severe whether forecast may be needed.

Finally, the influx of richer Gulf moisture combined with the strong synoptic forcing continues to look favorable for locally heavy rainfall totals today. While storm motion will be fast, rainfall efficiency will be high, so localized totals in excess of 3" (most of it falling in a very short time) are certainly well within the realm of possibility. Flash flood guidance remains low, so in coordination with neighboring offices will be issuing a Flood Watch for today.

Beyond this evening:

Gusty westerly winds are expected overnight as the deep low pulls away. Dry and pleasant weather is expected Thursday and Friday. Another strong shortwave trough is progged to move across the Midwest later this weekend, likely resulting in another round of potentially heavy rainfall and/or severe weather in the general region. Too soon to say how much, if any of our CWA will be impacted, but our area is certainly well within the proverbial cone of uncertainty and pops are very high Sunday accordingly.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 647 AM CDT Wed Jun 17 2026

Forecast concerns include...

Scattered thunderstorms from mid morning through early evening. Mvfr, possible ifr cigs, late morning through late afternoon. Wind directions mid/late afternoon. Strong/gusty westerly winds overnight into Thursday morning.

A thunderstorms complex across southeast IA is expected to continue moving southeast this morning as rain with embedded thunderstorms moves across northern IL and the terminals later this morning into early this afternoon. Previous forecast remains generally on track with these expected trends and no significant changes with the 12z forecast. However, there is increasing uncertainty for thunderstorm coverage this afternoon into early this evening and changes may be needed with later forecasts as trends emerge. Any thunderstorms today will be capable of producing torrential rain and visibilities possibly under 1sm. Lingering showers are possible this evening and then dry conditions are expected for the rest of the period.

Winds are in the process of becoming southeasterly and may turn easterly for a few hours during the mid/late morning, before turning back to the southeast with gusts into the mid 20kt range. A warm front was expected to lift as far north as ORD later this afternoon, possibly turning winds to the southwest but this warm front may not move as far north, thus southeast winds may continue, until the entire storm system moves east of the area, and winds shift to the west/southwest areawide. Winds will turn more westerly overnight when gusts into the mid/upper 20kt range will be possible, which will continue into Thursday morning.

Mvfr cigs are possible later this morning and are likely this afternoon, as are ifr cigs, but overall confidence is low for prevailing ifr cigs this afternoon. Cigs should lift into vfr early this evening with more mvfr cigs possible overnight into Thursday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 5 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ008-ILZ010-ILZ011.

Wind Advisory until 1 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103- ILZ104.

Flood Watch until 9 PM CDT this evening for ILZ006-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Beach Hazards Statement through late tonight for ILZ006-ILZ103- ILZ104.

IN...Flood Watch until 9 PM CDT /10 PM EDT/ this evening for INZ001- INZ002-INZ010-INZ011-INZ019.

Beach Hazards Statement from late tonight through Thursday afternoon for INZ002.

LM...Small Craft Advisory until 9 AM CDT Thursday for the IL and IN nearshore waters.