textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms expected this afternoon and evening, a few may be severe with large hail and damaging winds.

- Another chance of showers and possibly a few thunderstorms during the day Friday, with another chance of showers and thunderstorms Saturday night perhaps into early Sunday morning.

- First heat wave of the summer expected early next week with the potential for highs in the 90s and peak afternoon heat indices over 100 degrees.

DISCUSSION

Issued at 146 PM CDT Wed Jun 24 2026

Shortwave trough over the western Great Lakes along with associated weak sfc front from northwest Wisconsin south into eastern Iowa should encourage scattered thunderstorm development this afternoon. These storms should move southeastward across our CWA late this afternoon into this evening. Jet streak is evident on water vapor imagery nosing into SE MN/SW WI early this afternoon. Guidance depicts a 40-50kt 500mb speed max spreading across southern WI and northern IL this afternoon and evening. This will result in a increase in deep layer shear into the 40kt range.

Rain associated with lead shortwave should continue to exit the area early this afternoon with at least partial clear already spreading into our far western CWA. This should allow for moderate instability to develop across the area with MLCAPES of 1000-1500 J/kg. The moderate instability coupled with fairly strong deep layer shear should support supercells with a potential for large hail and perhaps damaging winds. Weak mid level lapse rates may limit the magnitude of the hail/wind threat, but the long straight hodographs would support splitting supercells and a hail threat.

This front should clear the CWA late this evening bringing an end to the showers/storms by late evening. In the wake of the front, looks for dry weather Thursday and most of Thursday night. There is a signal in most guidance for another low amplitude shortwave to potentially becoming convectively enhanced before moving toward the mid Mississippi Valley Friday. Instability should be pushed well south of our CWA, so if this MCV takes a more northerly track it would likely be in a weakening state as it encounters more stable and drier low level air mass over our area Friday. Conversely, it is possible that the MCV may tend to propagate more southward toward the instability gradient. In collaboration with neighboring offices, lowered NBM pops for Friday some and kept slight thunder chances confined to areas south of the Kankakee/Illinois Rivers. Cloud cover should keep temps cooler Friday, but if it does rain then highs would need to be lowered even more, especially southern CWA where better rain chances should be.

Over the weekend, an upper level ridge is progged to begin developing over the middle and lower Mississippi Valley region. A warm front is progged to lift northward Saturday night into Sunday. Some threat of showers and thunderstorms will exist ahead of this front and ahead of the EML advecting eastward with the developing upper ridge. Medium range guidance is in good agreement in developing a fairly strong (595-596dm) 500mb ridge over the Mississippi Valley early next week.

Sunday, there's likely a small chance of showers/storms in the morning, then probably dry in the afternoon. With the warm front moving through the area, a weak lake breeze is possible along the IL shore perhaps offering a bit cooler conditions right along the lake. Elsewhere, highs should climb into the upper 80s to around 90 with higher dewpoints likely pushing afternoon heat indices well into the 90s to near 100.

Monday through Wednesday, current forecast has highs in the low to mid 90s with dewpoints in the 70s resulting in heat indices solidly over 100 degrees. Based on the current model guidance, these values are reasonable, but with the heart of the upper ridge progged to be centered south toward the lower Ohio and mid-lower Mississippi Valley it is possible that ring of fire convection currently progged to be mostly to our north could sneak farther south or send outflow south into parts of our area offering some reprieve from the heat. Harder to forecast that this far out, but something to watch as a possible spoiler to what otherwise looks like the first formidable heat wave of the summer.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 104 AM CDT Thu Jun 25 2026

Key Messages:

- Patchy fog and low clouds may be observed at some TAF sites tonight, and possibly again tomorrow night.

- A lake breeze will result in a northeasterly or easterly wind shift this afternoon or evening.

Light winds and residual moisture from earlier rainfall have allowed patches of primarily shallow fog to develop in some areas as of 0600Z. This fog may expand and become more dense through the night, but incoming cirrus and some lingering flow in the boundary layer raise questions over the extent to which that may occur. It is also possible that some of this low-level moisture ends up forming patchy IFR or LIFR clouds instead, but confidence in this also is on the lower side of things. Either way, observational trends will be monitored closely through the night as it's possible that some of our TAFs may need to be amended to be steered in a more pessimistic direction.

Regardless of what happens with the fog and low cloud potential tonight, VFR conditions are likely to prevail by mid-morning and persist through the daytime. A lake breeze will develop and eventually push inland during the afternoon and evening, first resulting in a northeasterly wind shift at GYY, followed by northeasterly or easterly wind shifts at the remaining Chicago metro terminals in the hours that follow. Ample low-level moisture in the marine-influenced air mass behind the lake breeze could result in another period of low clouds and/or fog tomorrow night, but confidence in that is fairly low this far out.

In addition, couldn't rule out a stray shower or two popping along the lake breeze near ORD, MDW, or GYY this afternoon. However, the most likely outcome at this time is that the terminals will stay dry through the current TAF period.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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