textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms are expected on Tuesday. A storm may become severe east of I-39 after 12 PM.

- Additional thunderstorms are expected in the region Wednesday into Wednesday evening with a level 3 Risk for severe weather. All hazards are possible including large hail, damaging winds and tornadoes.

- Torrential rainfall and localized flash flooding are possible Wednesday.

- Possible high waves and dangerous swimming conditions may occur at Indiana Lake Michigan beaches Thursday.

DISCUSSION

Issued at 251 PM CDT Mon Jun 15 2026

Conditions are comfortably mild on this Monday afternoon with temperatures in the lower and middle 70s and a mix of fair weather cumulus and broken cirrus filtering our largely blue sky. A dry regional airmass in place will keep conditions quiet through the night while the high level cloud cover continues to expand ahead of our next precip chances. Morning lows are forecast in the 50s with slightly warmer temperatures generally expected in and around the city.

Water vapor imagery depicts the early stages of a shortwave disturbance spinning up over the northern Plains on the nose of the upper jet max. This wave will sharpen up through the night and drive a surface low across the upper Midwest tomorrow. The associated cold front is progged to pass through during the afternoon. Before it does, a line of showers and storms is expected to develop along the front over MN tonight and outrun it before moving across the CWA tomorrow morning. Little forcing, dry air, and limited low level instability out ahead of the front look to limit thunder coverage through most of the morning. A number of camps do have the line intensifying to some degree across our eastern CWA late morning and into the afternoon as instability builds. As the actual front moves across in the afternoon, the added forcing for ascent both along the front and on the nose of a modest low level jet, an increase in shear, and a possible tongue of higher dewpoints ahead of the boundary will make it possible for a stronger storm or two to fire up in the afternoon. Guidance is in favor of storms building south along the front later in the day, but a big majority of camps hold out until later in the afternoon when the front is just off to our east. The NAM and NAM Nest are most bullish on this afternoon potential owing to a sliver of lower 60s dewpoints in our east ahead of the front (in typical NAM fashion) while most others insist the dry air will be too limiting. SPC's Level 1 of 5 Marginal Risk east of I-39 seems very appropriate in this conditional setup.

An upper jet max will push onshore the British Colombia coast tonight and evolve into a vigorous shortwave as it ejects southeast out of the High Plains late Tuesday into Wednesday. At the surface, a deepening center of low pressure will cut across the Midwest on Wednesday. Coupled with a speedy southerly warm conveyor featuring 60+ kt at 850 mb and a strong westerly 500mb jet max nosing into the region, the kinematic environment looks to be highly conducive of organized severe convection during the latter part of Monday. The limiting factor in this case may be the amount of available instability and, as per usual with such a dynamic system, there remain considerable model uncertainties.

Guidance is in favor of a push of showers and thunderstorms during the morning as the more impressive kinematic features work into the region. Some camps even resolve a messy MCV spinning up in the high shear environment, but limited elevated instability should hinder much of a severe threat during the morning. However, we will find ourselves in a favorable environment for gravity wave associated convection on the eastern edge of this strengthening jet max which could provide a sneaky severe wind threat with this morning push despite the lack of instability. Today's 06 and 12Z runs of the RRFS even hint at this by resolving evenly spaced thunder cores with strong wind gusts during the morning, a tell tale sign of GWAC.

The better severe potential exists during the afternoon and evening as the storm's cold front passes through. The big question for the PM severe potential is how far north the effective warm front lifts amid the morning convection prior to the cold frontal passage. The greatest severe threat will be found near and south of this front where just about all boundary layer instability will be confined. The anomalously strong low- mid level wind field will result in eye-catchingly long, looping hodographs that will be especially concerning for surface-based storms. All severe hazards are on the table for areas along and south of the front including damaging to destructive wind gusts and tornadoes, some possibly strong. There is uncertainty in how much elevated instability will be found north of the front for an elevated wind or hail threat, but shear still pushing 30+ kt in the 3-6 km layer could certainly make for organized elevated storms farther north. There's quite a lot of spread still in the position of the front during the PM hours with models varying from far southern WI, which would put our entire CWA in the all hazards warm sector, or down into central IL which would pull most of the area out of the stronger, surface-based severe threat, including the tornado threat. More recent model runs have begun trending toward a farther south solution, but it'd be unwise to get too comfortable with that idea right now, especially when dealing with such a rapidly-evolving system. All eyes over the next couple of days will be on model trends regarding the placement of that front.

The strong southerly mass response ahead of the system will advect rich, deep moisture into the region with PWATs exceeding 1.5" into Wednesday setting the stage for more torrential downpours with these storms. Additionally, storms during the morning may attempt to backbuild into the higher instability working in from the west. Given that much of the area is likely in for at least a couple of rounds of showers and storms and with the high soil moisture from recent heavy rains, flooding is another concern for Wednesday, and that goes for areas on either side of the warm front. Most recent runs of every piece of available deterministic guidance that looks through Wednesday evening resolves a widespread swath of over an inch of QPF around the CWA with many suggesting 2-3"+ is possible over a rather large area. WPC has placed us in a Slight Risk (Level 2 of 4) for excessive rain on Wednesday.

Outside of thunderstorms on Wednesday, anticipated gusty south winds, especially across areas south of the front that can mix into some stronger flow aloft. Additionally, afternoon temperatures may vary sharply on either side of the warm front, but there's quite a lot of spread in how models handle temps on Wednesday.

For the latter half of the week, that Hudson Bay upper low will gradually get sheared out across the eastern half of Canada and lock the Midwest into northwest upper flow pattern through the weekend. Surface high pressure will work in from the west behind Wednesday's storm system and dry conditions are forecast for Thursday and Friday. The high looks to move into our southeast late Friday and introduce some mild return flow. Scattered shower and thunderstorm chances then return as early as Saturday with medium-range deterministic and ensemble guidance favoring the late Sunday into Monday window for another organized system of precip. Cooler than normal conditions late in the week look to trend warmer for next week.

Doom

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1225 PM CDT Mon Jun 15 2026

Main Concern:

- Potential for SHRA/TS Tuesday morning through early afternoon

Quiet VFR conditions will prevail through most of tonight. Widely scattered SHRA may develop prior to sunrise over parts of northern Illinois. As a cold front approaches during the morning, SHRA coverage is expected to increase, along with the potential for isolated to widely scattered TS. A few additional TS may develop immediately ahead of the front in the early afternoon, with higher TS coverage favored east of Chicago at this time. Broadened out PROB30 TS mention for the Chicago metro terminals with this issuance to capture the mid to late morning and early afternoon windows. The cold front passage between about 18-20z Tuesday will bring the TS threat to an end.

Breezy westerly winds gusting to near/around 20 kt this afternoon will quickly diminish and become southwest with sunset this evening. Southwest winds will gust to around 25 kt or so ahead of the front through midday Tuesday, followed by a wind shift to westerly with gusts to 25-30 kt.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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