textproduct: Chicago

KEY MESSAGES

- Isolated storm chances exist today through Sunday morning, though many areas could remain dry.

- Expect more widespread showers and at least scattered storms at times Monday-Tuesday, some of which may be severe.

- Summer-like warmth and humidity is in store this weekend through early next workweek. Cooler conditions then return midweek.

DISCUSSION

Issued at 240 AM CDT Sat May 16 2026

Early this morning:

The last of the initial band of showers that moved across southern Wisconsin and far northern Illinois earlier this morning is moving out over Lake Michigan. The trailing outflow boundary and residual cold pool extending southwest of these storms have put a damper on additional shower and storm coverage. The leading edge of this outflow could lead to additional isolated shower development but has been unsuccessful thus far. Meanwhile, a secondary line of showers and storms in central Iowa continues to progress east toward the region which may still give the rest of an area a chance to see a period of showers and isolated (non-severe) storms toward the pre-dawn hours through mid-morning, ending from northwest to southeast.

Today and tonight:

A remnant outflow boundary/gravity wave from the early AM storms is expected to stall out over central IL/IN later this morning where an east to west oriented axis of showers and embedded storms may attempt to redevelop during the day along this feature. There remains some uncertainty in where exactly it stalls, so have maintained 20-30% shower/storm chances south of a Pontiac to Rensselaer line in case it ends up farther north. It could very well remain just outside of the local area though.

As we head into the rest of the day temperatures are expected to warm into the lower-to-mid 80s (locally cooler near/under any showers to the south). In tandem, the airmass will become increasingly unstable with little to no capping in place. However, the majority of the area is mostly removed from any notable forcing mechanisms other than a potential weak lake breeze in the afternoon. As a result, hi-res guidance remains highly variable from run to run in the the placement and coverage of potential for spotty thunderstorms during the day. Accordingly, capped shower/storm chances at 15-20% for the to lower confidence. The more likely scenario is that the majority of the area remains dry the rest of the daytime hours.

A mid-level disturbance lifts into the area late this evening and overnight which could bring renewed shower and storm chances to portions of the area, highest near and north of I-80 (40%). We will also have to keep an eye on upstream convective trends in the event that a more vigorous convective complex happens to hold together across Iowa. Stay tuned.

Petr

Sunday onward:

Guidance continues to indicate breezy (30-35 mph gusts) south winds will develop Sunday afternoon, after the warm front lifts north of the area. With any morning showers/storms (favoring near/north of I-88) departing and decreasing cloud cover in the warm sector, high temperatures in the mid-upper 80s appear likely at this distance. The exception may be near the Lake County IL shore, as a just east of south component to the winds could cap highs in the 70s later in the day. Breezy and mild conditions look to persist Sunday night, with lows generally in the mid to upper 60s.

Thunderstorm chances then return Monday and Tuesday, as a Central Plains surface low pressure slowly lifts northeast through the northern Great Lakes and eventually trails a cold front through the forecast area. Ahead of the cold front, the forecast area will reside in breezy warm southerly flow, characterized by daytime temperatures in the 80s and increasing low-level moisture return (surface dew points potentially upper 60s-to near 70F) from the western Gulf. Resulting instability, combined with mid-level flow near 40 kts would potentially support strong/severe thunderstorms (as well as some threat for flooding given high column moisture), though forcing mechanisms remain unclear within the broad warm sector making convective timing/evolution of low confidence at this distance. Monday's overall messy look also lends uncertainty to how much instability can be realized at peak heating, particularly if morning activity ends up fairly widespread.

On Tuesday, in the wake of probable continued overnight/early day convection, the aforementioned cold front will approach. It's not uncommon in these setups for renewed storms and any associated strong to severe threat ahead of the front to focus primarily near/southeast of I-55, though time will tell in this regard. For the Monday-Tuesday period, SPC continues to extend severe probabilities into our area, which appear reasonable. Quiet and cooler conditions are then expected by mid-week, as surface high pressure builds across the region in the wake of the Tuesday cold frontal passage.

Castro/Ratzer

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 631 AM CDT Sat May 16 2026

Aviation forecast concerns for the 12Z TAFs:

- Brief period of SHRA/TSRA early this morning. 15-20% chance for isolated TS this afternoon, but too low for TAF mention.

- Low chance (<20%) for isolated afternoon/evening SHRA/TSRA.

- Somewhat low confidence in late afternoon wind directions, though a trend from south-southwest to the southeast is likely by early evening.

- Another chance (30-40%) of TSRA after midnight tonight.

Early morning radar imagery depicts another thunderstorm complex moving east across northern IL. Northern portion of the complex has been weakening with decreasing lightning noted in the past 30-60 minutes, though isolated embedded TS remain possible. More solid TSRA to pass mainly south of the terminals through about 14Z, and have included a tempo for TS at GYY due to its farther south location. Expect subsidence in the wake of this morning activity which should make for quiet, VFR weather conditions into this afternoon. Some high-res CAM runs (RAP/HRRR) continue to indicate isolated to scattered TSRA development across the forecast area late this afternoon/early evening, and though can't completely rule this out confidence is rather low in this scenario. TSRA chances do increase late tonight/predawn Sunday however, as a mid-level disturbance (currently over the central Rockies) tracks northeast across the region. Have maintained PROB30 mentions for TSRA after midnight for this.

Lower than usual confidence in wind directions exists today, with some variability possible for a couple of hours after the morning showers/storms move through though a general trend to light south-southwest is expected. While a lake breeze that would affect ORD/MDW can't be ruled out this afternoon, the model consensus is for more of a synoptic shift to the southeast by 00-01Z. A southeast to east component is then expected to persist overnight (barring complications from late night storms) before turning more south-southeast and gradually increasing after daybreak Sunday.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.