textproduct: Chicago

KEY MESSAGES

- A period of showers and embedded thunderstorms is expected after midnight tonight through early Tuesday morning, some of which could be locally strong, mainly west of I-55.

- Additional thunderstorms may redevelop Tuesday afternoon, mainly east of I-55, some of which could be strong to severe. - Summer-like warmth and breezy winds will prevail through Tuesday, then cooler temperatures and dry conditions will return for the middle of the week.

- Milder weather returns heading into the weekend with intermittent shower/thunderstorm potential.

UPDATE

Issued at 846 PM CDT Mon May 18 2026

A robust line of convection from Wichita, Kansas to Des Moines, Iowa, which created quite the severe event in the Central Plaines this evening, is slowly moving east. A strong instability gradient from northwest Missouri toward St. Louis should focus most of the strongest parts of the line's movement through the overnight south of the forecast area toward central Illinois. However, a plume of better lapse rates into central Iowa will slowly meander east ahead of the line of storms. Outflow from the storms should provide sufficient forcing, but being in a diurnally unfavorable time of day, the lapse rates should start to weaken by the time it approaches the Rockford Metro after midnight, but confidence lowers in coverage toward the city of Chicago as it starts to fall apart away from the better forcing.

Most of the forecast area will be dry tomorrow morning, but lingering showers may crop up. The cold front will finally make it to the forecast area and turn winds from the southwest to the west and eventually northwest behind the front. Additional showers and storms are possible along the front, with the best time being in the afternoon. But by that time, the front could already be around and east of Interstate 55. Storms along the front in the afternoon could be strong to even severe with damaging wind and hail the primary severe threats.

Forecast updates this evening were focused on the near-term timing of shower and thunder as well as for tomorrow afternoon.

DK

DISCUSSION

Issued at 244 PM CDT Mon May 18 2026

Through Tonight...

Convection and associated MCV responsible for this morning's severe weather has moved into Lower Michigan. Air mass in the wake of this wave is substantially more stable and combined with modest height rises in its wake should prevent any renewed convective development this afternoon or evening.

Severe convection currently developing over the central Plains is likely to congeal into a large QLCS that will move toward the mid-Mississippi Valley tonight. Most intense convection is likely to focus farther southwest across Missouri, likely intercepting the low level jet and orphaning the convection farther north across Iowa. This should result in convection over Iowa diminishing as it moves eastward into northern IL with some guidance suggesting very little shower activity would actually make it into our CWA. Have reflected this trend in pop grids, but opted to maintain at least chance pops given the inherent uncertainties in forecasting convective evolution.

Pressure field is recovering in the wake of the morning convection which has allowed winds to pick and become rather gusty from the southwest. Gustiness of the winds will generally tend to abate by sunset, except perhaps remaining somewhat gusty tonight in the urban heat island of Chicago.

-Izzi

Tuesday...

Convectively induced vorticity maximum will slide through northeast Illinois into early Tuesday, well outpacing the cold front, and therefore we may have some showers hanging on into the area, but confidence is medium-high that thunderstorm coverage will be on the lower side. Any lingering MCS cold pool footprint and residual cloudiness will likely delay destabilization into midday Tuesday, though additional storm development appears somewhat probable across the southeast half of the forecast area Tuesday afternoon/evening as the cold front finally pushes through the forecast area. Southwesterly flow ahead of the front will maintain mid to possibly upper 60s dewpoints and support MLCAPE approaching 1500 J/Kg. Stronger mid-level flow on the southeastern periphery of the Model guidance would support some additional vorticity maximum riding along the front, and flow to the southeast of the mid level trough across the northern Great Lakes will likely support enough storm development and some organization for a hail/wind threat during this time. This would be favored along and southeast of I-55, with short term ensemble guidance suggesting the window for much of NE IL and NW IN being fairly limited in the mid to late afternoon. For the Chicago metro area the picture is also a bit muddled for precipitation chances. High res guidance is mostly dry for the area, whereas global guidance and NAM/RAP does bring some shower activity across Chicagoland along the front in the early afternoon.

KMD

Wednesday onward...

Following the cold front, surface high pressure is progged to build east into the western Great Lakes region Wednesday and Thursday. Dry, but breezy and cooler weather is expected both days, with temps mainly in the 60s, but only in the 50s near Lake Michigan with northeast-east flow off the lake. Nighttime lows in the 40s are expected. The high then drifts off to the east Thursday night, with the upper level pattern favoring a series of mid-level disturbances tracking through the area Friday into the weekend. This will support milder temperatures, but also some occasional shower/thunderstorm chances into the holiday weekend.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 103 AM CDT Tue May 19 2026

Forecast concerns for the terminals...

- A line of showers and embedded thunderstorms will move across the terminals late tonight into early Tuesday morning.

- Breezy southwest winds to develop Tuesday morning behind the rain and persist through the afternoon.

- Period of MVFR ceilings is expected Tuesday afternoon behind a cold front, but skies should scatter back to VFR around sunset.

A line of showers and embedded thunderstorms is progressing eastward across the Mississippi River and will be encroaching upon the terminals over the next 1-3 hours (earliest at RFD, latest Chicago area sites). While the line is expected to be gradually weakening as it moves through, recent AMDAR soundings out of MDW do show some developing mid-level instability which may allow for a couple of lightning strikes to occur. Despite confidence in thunder actually reaching the Chicago terminals being somewhat low (~20% chance) have opted to introduce a PROB30 for TSRA at DPA, ORD, and MDW as a precaution give the aforementioned instability and lack of broad weakening in the line so far. That said, still think that the most likely scenario at the Chicago sites (especially GYY) is for some isolated to widely scattered showers to occur.

The line of showers/storms should diminish by daybreak and result in a mostly dry morning. Winds will be increasing with southwesterly gusts peaking around 25-30 kts through early Tuesday afternoon, but locally higher gusts could materialize if deeper mixing is achieved. As the main cold front approaches Tuesday afternoon it will bring with it a period of MVFR ceilings and perhaps an isolated shower or two though the greatest shower/storm coverage should establish more into northern and central IN. With the increasing cloud cover, gusts should ease a bit into the lower 20 kt range through the rest of the afternoon. Directions during this time will also transition from southwest to west-northwest before turning northwesterly behind the cold front early Tuesday evening. While a few upper teen to lower 20 kt gusts cannot be ruled out Tuesday night, it seems they frequency of gusts should be more sporadic so have opted to remove gusts from the TAFs from this point onward.

In the wake of the cold front, ceilings should rise back to VFR Tuesday evening and remain as such for the rest of the forecast period. Lastly winds will finally settle into a northeast direction early Wednesday morning.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 6 AM this morning to 4 PM CDT this afternoon for the IL and IN nearshore waters.