textproduct: Chicago

KEY MESSAGES

- Periods of showers and thunderstorms will continue through Sunday. A few storms could be strong to severe with a threat for damaging winds especially today near and south of I-80.

- Locally heavy rainfall will also accompany any thunderstorms and may result in flash flooding especially if storms develop over areas that received heavy rain the past couple days.

- Drier and more seasonable conditions expected to start the week before stormier weather returns late week.

DISCUSSION

Issued at 312 AM CDT Sat Jul 4 2026

A decaying line of showers and thunderstorms continues to drift across north-central IL and northwest IN early this morning. These showers/storms are expected to continue to wane from north to south by daybreak leaving the area with dry conditions to start the day. However, there is a cluster of thunderstorms in central IA that, if it holds together, would move into the western portions of the forecast area by mid-morning. While latest forecast guidance continue to show this cluster weakening and dissipating prior to arriving, the decent organization noted on radar and the fact that instability should be increasing as this cluster moves east supports the idea that the storms may survive into portions of northern IL. Despite the lower confidence on the storm's evolution, have opted to introduce some 20-40% POPs for areas near and north of I-80 through the morning to tease the potential for this cluster to make it into the area. If the storms do survive the main threat with them will be lightning strikes and locally heavy downpours, but some locally gusty winds cannot be ruled out either.

Regardless of how the morning storms pan out, the shortwave trough in NE and its associated cluster of storms is expected to move into northern IL and eventually northwest IN this afternoon and linger through the early evening hours. Given the much better environment forecast for this afternoon this second cluster will have no problems making it into the area. The only question with it is how could the morning storms alter the environment in our northern CWA to perhaps make storm coverage or intensity slightly lower there. At this time the thinking is that a line of scattered thunderstorms will move across the entire area between the hours of 1-8 PM (plus or minus an hour) and last about 2-3 hours at any one location. While wind shear today is not as good as yesterday (only around 20- 25 kts) the decent instability and forcing from the shortwave may support a few strong to severe storms capable of producing damaging winds especially near and south of I-80 where SPC has their level 1 of 5 severe risk. In addition to the gusty wind threat, these storms will be capable of heavy downpours that may result in more flash flood, particularly in areas that have seen heavy rain over the past couple of days.

Once the line of storms moves through this evening there is a decent signal for a period of tranquil weather for the later portion of the evening. Should this hold true it looks as if we may get a break just in time for July 4th fireworks. With the shortwave still expected to be pivoting overhead some isolated to widely scattered showers/storms cannot be ruled out at times overnight, but most areas should be dry. However, the combination of light winds and ample low-level moisture does look to support a period of patchy fog tonight into Sunday morning. Since confidence on fog coverage and location is rather low and will be dependent on storm trends this afternoon (where the best moisture resides from the rain) and inland progress from Lake Michigan, have opted to forego a formal fog mention in the forecast for now.

Heading into Sunday, another shortwave trough is progged to dig out of the northern Plains and phase with the aforementioned shortwave that is expected to still be lingering over the Great Lakes. The phasing of these troughs should allow upper-level heights to deepen and give way to more scattered showers and thunderstorms Sunday afternoon. Recent CAM guidance trends have started to indicate that the deepening trough overhead should allow a weak surface low to develop in central IL and traverse towards central IN Sunday afternoon. If this does occur then it seems that the better instability should stay more into the southern portions of our area (if not fully south of us) and therefore limit shower/storm coverage with northward extent. Though, if the low develops further north then more of the area could see showers and storms once again. For now have opted to keep some 30-50% POPs areawide (highest values south of I-80) for this potential but suspect these will be able to get refined in future forecasts. Outside of the rain chances, temperatures on Sunday will be a tad cooler (highs in the lower to mid-80s) due to east-northeast winds. In fact highs near the lake on Sunday may struggle to get out of the 70s.

By Monday the upper trough will be ejecting eastward and taking with it the chances for showers and thunderstorms as a surface high settles into the western Great Lakes. It should be noted that our official forecast does have some 20-30% POPs in our far southern CWA Monday afternoon where some slower guidance has a stray shower/storms trying to develop, but the overall environment locally should support dry conditions areawide so suspect these POPs will get removed in future updates. Nevertheless, temperatures on Monday will be on the seasonable side with highs once again in the lower to mid-80s inland with 70s expected near the lake due to onshore winds. Tuesday will feature similarly dry conditions, but with slightly warmer conditions (highs in the mid to upper 80s) as winds turn back southerly.

Unfortunately the tranquil weather is not forecast to last long as guidance continues to show the return of west-southwest flow aloft for the middle and latter half of next week. While temperatures look to remain seasonable, humidity will be on the increase which will also bring the return of shower and thunderstorm chances.

Yack

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 655 AM CDT Sat Jul 4 2026

Key aviation forecast messages:

- First round of storms to move through this morning.

- Greater coverage of storms favored this afternoon into early evening, some potentially strong. - Low confidence in wind direction trends today, eventually turning NE behind a lake breeze in the afternoon.

- Potential for fog/low stratus off the lake late tonight into early Sunday AM.

The thunderstorm complex over central Iowa, while weakening, has managed to hold together into northwest Illinois. Have accordingly introduced TEMPO groups for TSRA into the Chicago area terminals for mid to late morning period (14-16Z at ORD/MDW), though some slowing can't be ruled out which may require shifting that later by an hour.

Given the initial round of storms is not handled well in model guidance, how things evolve into this afternoon remains lower confidence overall. An MCV feature over south central Iowa may serve as the focus for additional thunderstorm development this afternoon. Given its slower movement have shifted the TEMPOs for TSRA back an hour at most sites to when greater coverage of showers and storms is favored (19-23Z). Diminishing coverage is then expected heading into the mid to late evening hours. Have maintained dry TAFs through the remainder of the TAF period though a stray shower or storm can't be ruled out.

Wind directions are expected to remain highly variable through midday before a lake breeze moves inland in the afternoon turning winds more solidly northeast, though this could end up being delayed by any convection that occurs this afternoon.

Lastly there is a signal for fog (potentially dense) and/or low stratus (IFR/LIFR) over Lake Michigan that may try to ooze inland late this evening through early Sunday morning. Confidence remains too low to include with this update but will be monitored closely given it may also have some dependence on how convective trends also evolve later today.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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