textproduct: Chicago

KEY MESSAGES

- Period or two of scattered showers and isolated thunderstorms expected today, with a level 1 of 5 severe risk east of I-55 including NW Indiana.

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

- Locally heavy rainfall Wednesday could result areas flash flooding and potentially some river flooding.

DISCUSSION

Issued at 234 AM CDT Tue Jun 16 2026

Today:

Water vapor imagery early this morning depicts a vigorous shortwave trough digging southeast into the mid-upper Mississippi Valley with an embedded compact mid-upper level low. At the sfc, low pressure near Duluth early this morning will deepen as it moves southeast to near Green Bay by early this afternoon. A trailing cold front will sweep across the area later this morning through mid afternoon. While southerly winds ahead of this front will draw some moisture northward, an elongated east-west oriented ridge of high pressure to our south will block the return of any meaningful Gulf moisture today.

A lead band of showers and isolated thunderstorms is expected to blossom over northern IL toward sunrise as the increased ascent associated with a lead impulse encounters the better (though still somewhat meager by mid-June standards) moisture over the area. This lead band of showers and isolated t-storms should sweep quickly across the CWA between 11-16z. Despite the limited moisture, steep lapse rate and cold mid-upper level temps will likely allow for a narrow axis of up to around 1000 J/kg of MLCAPE to develop in the wake of the lead band of showers and in advance of approaching cold front.

Scattered thunderstorms are expected to begin to develop along and ahead of this front early-mid afternoon, with the big question for our area being how far east the front gets before storms develop. Strong shear and moderate instability would likely support a risk of supercells capable of large hail and perhaps locally damaging winds. At this point, it appears the best chance of storms along the cold front in our CWA will be roughly near/east of I-55, especially across northwest Indiana where frontal passage will be latest to occur.

Behind the front, look for gusty westerly winds, probably peaking in the 30-35 mph range at times this afternoon before quickly subsiding toward sunset.

Wednesday:

An unusually powerful shortwave trough is progged to race east-southeastward into the upper Midwest and western Great Lakes Wednesday into Wednesday evening. Model guidance is in good agreement on the synoptic scale with 500 mb heights progged to be 3-4 standard deviations below normal for mid June. At the sfc, an associated low pressure that could threat all- time monthly record for June is expected to develop across Wisconsin and eventually move into lower Michigan. Unsurprisingly, the kinematic fields with such an anomalously strong system are also nearing the upper echelons of what we see this time of year in the Midwest. All of this to say that synoptically, the ingredients are coming together for a potentially dangerous outbreak of severe thunderstorms and potentially long-tracked, intense tornadoes.

On the mesoscale, there are naturally greater uncertainties regarding precisely where (and to a lesser extent, if) the overlap of exceptionally strong deep and low level shear and moderately strong instability will take place. SPC's latest SWODY2 highlights areas south of I-80 in our CWA with a rare day 2 moderate risk (level 4 of 5) for Wednesday. The potential ceiling for bad this severe weather event could get is quite high, but it is also important to note there are obviously still failure modes that could materialize on the mesoscale and prevent the reasonable worst case scenario from unfolding.

The general expectation as it looks now is that a strong low level jet (increasing to 60kt+ at 850 mb by 12z Wed) will result in very strong low level theta-e advection and eventually the development of strong to severe thunderstorms late tonight. Initially development is expected over eastern IA, but as the low level jet translates eastward into IL, it should support this convection developing/moving into northern IL near/after sunrise Wednesday morning. While this convection will be elevated, strong effective shear and a reservoir of 1000-2000 J/kg of MUCAPE should allow for elevated supercells. The greatest severe risk Wednesday morning looks to be over our western and eventually southern CWA. In addition to the threat large hail, strong shearing instability near a sharpening frontal inversion would point to at least some threat for the development of gravity wave associated convection. Should this occur, a damaging wind threat could also develop, despite little or no sfc based instability. In fact, there could even be pockets of locally significant severe wind gusts (>75 mph) near or just west/south of our CWA Wednesday morning. This convection will probably evolve into an MCS as it tracks east across northern IL and into northwest Indiana Wednesday morning.

The morning convection will likely augment the warm frontal position and at least initially slow the northward progression of the composite warm front/outflow boundary, delaying destabilization north into our CWA. This seems to be the most obvious potential failure mode: morning convection retarding the northward surge of the warm front and subsequently the stronger instability, keeping the extreme wind shear profiles and strong synoptic forcing somewhat divorced from the more favorable instability. While that is one potential obvious failure mode, at this point, it seems unlikely to fully succeed in completely disrupting the otherwise exceptionally favorable synoptic set-up from resulting in a high end severe threat. Though the precise location of the most likely area(s) to see a high end event, could change some on the mesoscale as the event nears.

The reason that this morning convection is unlikely to completely stunt the northward surging warm front is the extremely strong mass response expected as a result of the near record deep low pressure system. With a far less intense low last Thursday, an impressively large footprint of a cold pool left behind from an MCS that lingered well into the afternoon was able to be completely overcome in just a matter of a couple of hours late in the day. Something similar seems plausible again tomorrow afternoon where a dissipating outflow boundary from early convection could separate an extremely volatile air mass from a still sufficiently unstable air mass north of the boundary could support severe thunderstorms with damaging winds and isolated tornadoes.

There is variance in guidance with just how far north the effective boundary will get, but somewhere generally in the vicinity of I-80 is where a majority of the models show it reaching its peak latitude in our CWA. To the south of this boundary, the environment looks similar to what's often seen in the cool season major tornado outbreak in the southeastern United States. Forecast hodographs are literally off the chart, with 0-1 environmental SRH >500 m^2/s^2. This sort of extreme low level shear, coupled with low LCLs and resultant strong 0-3km CAPE, fast storm motion, and favorable downstream environment is the classic type of set-up long tracked strong- violent tornadoes.

It's important stress that while the synoptic set-up is classic for a tornado outbreak, we are dealing with convection that often alters the mesoscale environment in ways that cannot be anticipated 12-24+ hours in advance. As noted above, there are certainly scenarios where an alteration of the mesoscale environment could dramatically reduce or shift the area of the greatest tornado risk. For this reason, it is important to monitor later forecasts closely. Finally, in addition to the supercell tornado threat, a fast moving line of severe thunderstorms/possible derecho may also accompany the front producing widespread, potentially significant, wind damage as well as line embedded tornadoes, some possibly strong.

Not to be overlooked, precipitable water values are progged to get to near or just above 2", meaning that convection will likely be extremely efficient rain producers. The stronger convection could easily produce 2"+ of rain in just an hour or so. Given the antecedent very moist ground and generally above average streamflow, there is a threat of flash flooding Wednesday. The greatest threat may be with the first round of convection, since it will likely be a bit slower moving and offer a better chance of some training cells than the second round. When the area most at risk can be better refined, we will need a flood watch for portions of the area for Wednesday as well.

After Wednesday:

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.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1230 AM CDT Tue Jun 16 2026

Forecast concerns include...

Chance of thunderstorms this morning and again this afternoon. Gusty southwest winds this morning, then west/northwest this afternoon.

Overall, current forecast is in good shape with only some timing thunder tweaks this afternoon. A broken line of showers and scattered thunderstorms is expected to move across the terminals this morning. Current tempo mention has this potential handled well, though duration at any one location may end up being 1-2 hours. A second line of scattered thunderstorms is expected to develop either right over the Chicago terminals or just east of the Chicago terminals during the early/mid afternoon hours. Adjusted current prob thunder an hour later but duration during this time may be 1-2 hours as well. A few lingering showers may be possible across far northeast IL/southwest WI and over Lake Michigan through early this evening.

Southwest winds around 10kt overnight will slowly increase toward daybreak when gusts to 20kt will be possible. Winds/gusts will continue to increase through the late morning and then turn west/northwest this afternoon behind the cold front. Gusts into the upper 20kt range are expected. Speeds and gusts will diminish quickly with sunset this evening.

Some mvfr level cigs are possible this morning and again this afternoon, but will most likely be associated with precipitation and not expecting prevailing mvfr cigs. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM this morning to 9 PM CDT this evening for the IL and IN nearshore waters.