textproduct: Chicago

KEY MESSAGES

- An additional round of storms is expected this evening starting at 6 or 7 PM, and lasting through the overnight hours. Confidence on where the next round occurs remains low, but may favor the I-80 or perhaps even I-88 corridor.

- Additional rounds of storms may move through the area tomorrow and Sunday.

DISCUSSION

Issued at 243 PM CDT Fri Jul 3 2026

Rest of Today/Tonight:

Round one of thunderstorms is surely living up to expectations with numerous measured severe wind gusts ranging from 60 to 73 mph. While the main push of storms is now moving toward Lower Michigan, a few more cellular storms continue to percolate along the remnant outflow boundary stretching southwestward from Joliet to Lacon to Macomb, Illinois. North of the outflow boundary, it feels the most comfortable it has in the past week with temperatures largely in the low to mid 70s. Meanwhile, south of the front, it's hot and sticky with heat index values in the 100 to 105F range.

Focus is on the potential for a second round of storms this evening. While southwesterly flow on the southern side of the outflow boundary is not particularly strong (10 to locally 15 kt), have been noting the beginning of the northward retreat of the unstable airmass back toward northwestern Illinois. Just how far northwest the unstable airmass retreats is an item of low confidence, but suspect it will be able to reach I-80 and perhaps even I-88 by this evening ahead of an upper-level shortwave currently moving through the southern Iowa region. As a result, concern remains for another round of storms near the frontal boundary (perhaps extending eastward from ongoing storms near Kansas City, MO) as the wave moves approaches our area. Timing the second round still looks to start sometime in the 6 to 7 PM timeframe with scattered storms continuing through the overnight hours.

While the wave of storms that came through earlier today were anchored by a notable 500mb wind max, flow this evening will be weaker and more on the order of 25-30 kt. As a result, current thinking is that coverage of severe weather this evening will not be as high as it was with the first round of storms. Nevertheless, moisture-laden MUCAPE >3000 J/kg will still support water-loaded microbursts with a threat for torrential downpours and localized wind damage. In addition, with storms more likely to adopt a west-to-east orientation this evening, do have concern for localized flash flooding particularly if the front does make it back toward the I-88 corridor where 3 to locally 5 inches of rain fell last night. If it becomes clear such a threat will materialize, will consider hoisting a short- fused Flash Flood Watch.

Tomorrow / Independence Day:

At least isolated showers and storms may be ongoing at daybreak, though most guidance suggests coverage should wane by mid- morning. Global guidance than advertises another upper-level shortwave or two pinwheeling into the area during the afternoon hours, which will support additional rounds of thunderstorms. At this point, confidence in where the frontal boundary (which will mark the unstable airmass) will be by tomorrow afternoon is quite low and depends on coverage of storms tonight. So, unfortunately, cannot offer more than mid- range chance PoPs areawide tomorrow in favor of further refinement as things hopefully become more clear tonight. The same goes for Sunday.

Borchardt

Beyond the weekend the upper ridge begins to build back north slightly with the embedded waves moving to our north. Meanwhile another wave (or waves) will move within the ridge to our south (and potentially stall out). Overall this favors shower/storm coverage focusing mainly north or south of the area into mid- week. Shower and storm chances return later in the week as the ridge begins to break down again.

Petr

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 101 PM CDT Fri Jul 3 2026

Key messages for the 18Z TAF period:

* Strong the severe thunderstorms targeting the Chicagoland terminals near the start of the TAF period. Strong W wind gusts and IFR vsbys anticipated.

* Additional thunderstorms possible this afternoon through the night with much uncertainty in coverage beyond early afternoon.

A messy line of strong to severe thunderstorms is tracking east to northeast into Chicagoland as of 18Z. These storms will progress across the Chicagoland sites between 18 and 20Z. Strong westerly wind gusts and heavy rain with IFR vsbys are expected to accompany the stronger storms, which should persist over any given site for between 30 and 60 minutes. Wind direction may get squirrelly in the vicinity of thunderstorms.

Additional storms upstream warranted a VCTS through the remainder of the afternoon with likely another round or two moving across the vicinity of the Chicago sites. Uncertainty grows into the evening and overnight with additional scattered storms remaining possible. A relatively better window of opportunity exists from roughly 01 to 08Z, but convection will be possible on either side of that window. More gusty to strong winds and lowered vsbys would be expected with any additional convection. Precip chances dwindle toward daybreak with much of Saturday expected to be dry, although thunderstorm chances increase again late afternoon into the evening.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 7 PM CDT this evening for ILZ023-ILZ032- ILZ033-ILZ039.

IN...Heat Advisory until 7 PM CDT /8 PM EDT/ this evening for INZ010-INZ011-INZ019.

LM...None.