textproduct: Chicago

KEY MESSAGES

- Small chance (10-15%) for a gusty thunderstorm along the lake breeze in northeast Illinois through early evening.

- Isolated to widely scattered storms possible Thursday with a gusty downburst threat.

- Hot July 4th and Saturday but not as oppressively humid as the previous rounds of heat recently (~95-100F heat indices).

- More appreciable thunderstorm chances return Saturday night and Sunday with a slowly approaching cold front. There may be a few days of quieter weather to start next work week.

DISCUSSION

Issued at 316 PM CDT Wed Jul 2 2025

Weak high/surface-based capping remains present across the forecast area this afternoon as evident by scattered larger and flatter cumulus on IL/visible satellite. A subtle mid-level impulse across southern Wisconsin is assisting with slightly higher coverage of the cumulus, but is not providing enough ascent to overcome the remaining capping. With that said, as this wave encounters the lake breeze across far northeast Illinois and southeast Wisconsin this evening, a couple widely isolated thunderstorms cannot be ruled out. Modest mid-level shear and a prominent inverted-V profile in the low levels would support gusty winds with any thunderstorm.

A pocket of slightly higher low-level moisture will pivot northeastward around the surface ridge tonight through Thursday. Little to no surface capping during the afternoon and evening may allow for an axis of isolated to widely scatted showers and some storms to develop, particularly if a passing weak upper- level wave provides additional support concurrent with peak diurnal heating. The favored axis where these features align is located from southwest WI toward the Kankakee River Valley. It is possible overall coverage could be quite low, especially with more aggressive mixing of dew points.

Mid-level ridging will then quickly move over the western Great Lakes Thursday night, with the ridge axis crossing the CWA sometime later Friday afternoon into the evening. There will be a notable absence of a subsidence inversion under this ridge, promoting only weak surface-based inhibition Friday afternoon. With weak flow under 20 knots throughout the column and no appreciable forcing aloft, the potential for any very sparse convection will be relegated to a weak lake breeze pinned along the Illinois shore. But with only marginal surface convergence on the boundary and the likelihood that dew points mix out more than currently forecast, will maintain a dry forecast for Independence Day. Temps/heat index values will top out in the low/mid 90s amid partly cloudy skies.

Phasing troughs over the central and northern Great Plains on Friday will become increasingly sheared while encroaching on the departing ridge this weekend. Increasing low-level isentropic ascent may allow for more organized thunderstorms over Iowa Saturday afternoon to reach the far western CWA while in a decaying phase Saturday evening. Otherwise, Saturday will feature another hot day with temps and heat index values reaching the mid 90s.

A cold front associated with the sheared trough will drift SSE across the forecast area late Saturday night through Sunday while encountering increased moisture pooling and resultant higher surface dew points. With weak-shear and generally unidirectional flow parallel to the front, will need to monitor for increasing chances of localized corridors of heavy rain Sunday afternoon and evening.

There is some question as to when the front fully clears the CWA on Monday, with slower solutions suggesting additional convection and heavy rain developing across the southern third of the forecast area Monday afternoon. Behind the front on Monday and across the entire area on Tuesday, seasonable temps and less humid conditions are expected. Shower and thunderstorm chances will increase again late Tuesday into Wednesday with a larger trough axis crossing the region.

Kluber

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 641 PM CDT Wed Jul 2 2025

Generally expecting VFR conditions and winds less than 10 kt through the upcoming TAF cycle.

Lake breeze which had been approaching MDW and ORD late this afternoon has been retreating back toward the lake and appears unlikely to reach either terminal this evening. Westerly winds Thursday are expected to be a bit lighter than today and will probably allow the lake breeze to move across both MDW and ORD late Thursday afternoon or early Thursday evening. Winds behind the lake breeze should shift to easterly, but probably remaining mostly below 10 kt.

An isolated shower or two is possible this evening, particularly at RFD, ORD, and DPA. Isolated SHRA/TSRA are possible Thursday afternoon and evening, but at this time, chances of them impacting the terminals appears to be just below the 30 percent threshold for inclusion in the TAFs. So other than the very low possibility of a shower or storm early this evening or Thursday afternoon and evening, conditions will remain VFR.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

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