textproduct: Chicago

KEY MESSAGES

- Dense fog this morning, mainly near and east of I-55.

- There is a 25-35% chance for thunderstorms at any one location tonight. If storms make it into our area this evening, then they may produce strong winds and small hail in portions of northern Illinois.

- An active pattern looks to bring periods of stormy weather to the region this weekend into early next week.

DISCUSSION

Issued at 328 AM CDT Fri Aug 7 2026

Through Saturday:

In spite of a steady stream of cirrus over our southeastern CWA and patchy mid-level clouds farther to the northwest, light winds and plenty of near-surface moisture (surface dew points ranging from the mid 60s to low 70s) has allowed for fog to develop in some spots within our forecast area. At the time of this writing, most of the noteworthy visibility reductions being reported at area ASOS/AWOS sites have been focused across our southeastern counties near and beneath the cirrus canopy, interestingly enough. In fact, a few observation sites there have already reported 1/4 mile visibility, and with conditions not expected to start improving until after sunset, have went ahead and issued a Dense Fog Advisory for roughly the southeastern half of our forecast area this morning. It's possible that this advisory may need to be expanded northward as radiation fog gets a greater foothold in areas where skies are comparatively clearer, so we'll be monitoring observational trends closely the rest of the night.

One other thing we'll be monitoring through this morning is an arc of convection that sprouted near and east of St. Louis earlier along the nose of a ~35 kt southwesterly low-level jet. This low-level jet is progged to veer more westerly with time, and this may tend to steer the associated convection more towards the east, which would keep most or all of it just south of our area. That said, the 00Z run of the FV3, which appears to be the only convection-allowing model that has a relatively good handle on the ongoing convection (I suppose the saying about a blind squirrel finding a nut every now and then would apply here), doesn't veer this low-level jet as much as other guidance does and brings this convection into our southern counties towards sunrise as a result. Have thus maintained some chance PoPs and lower-end thunderstorm chances for the southern/southeastern third or so of our CWA through this morning to cover this possibility. Otherwise, the majority of the area should remain dry through this afternoon with high temperatures today reaching the low-mid 80s.

Tonight, a cold front will drop into the area from the northwest. There is relatively good agreement across the spectrum of the latest hi-res guidance that convection will be ongoing along the front by midday today in southern Minnesota/northern Iowa, but model solutions then diverge in how this convection evolves thereafter. Would think that the frontal forcing, augmented by lift provided by an upper-level shortwave and upper-level divergence in the left exit region of a northwesterly 70-80 kt jet, should generate enough convection for a consolidated cold pool to eventually materialize somewhere. This cold pool would then drive the associated convective complex towards the east and/or southeast through this afternoon and into this evening. Depending on exactly where along the front this potential MCS develops, this scenario could put northern Illinois in this MCS's crosshairs this evening with some threat for strong to locally damaging winds before the complex weakens with time. On the other hand, if no real organized convective complex takes shape upstream of our area, or if the thunderstorm cluster follows a sharper southeastward or even southward propagation (like a few CAMs are suggesting), then our area could miss out on storms and even precipitation altogether tonight as the aforementioned cold front works its way through the area. Upstream convective trends will thus need to be monitored closely this morning and afternoon, but for now, have nudged our PoP and thunderstorm chances up a bit from what they were previously.

The cold front should stall out somewhere in or near our southern CWA Saturday morning. Humid conditions with low 70s dew points will persist south of wherever the front sets up, while more comfortable air will be found north of the front. High temperatures on both sides of the front are favored to end up in the 80s again, though weak onshore flow into northwest Indiana and part of Cook County will likely keep temperatures in the mid-upper 70s in areas immediately downwind of Lake Michigan. While the air mass along and south of the stalled frontal boundary will be unstable enough to conditionally be supportive of thunderstorms, limited surface convergence along the front, essentially no upper-level forcing support, and weak capping all point towards the daytime hours on Saturday remaining dry across our forecast area.

Ogorek

Saturday Night through Thursday:

The forecast for Saturday night onward generally remains unchanged as a strengthening upper ridge in the southwest CONUS will amplify the northwest flow over the Great Lakes. As a result, periods of thunderstorms are expected to traverse across the Great Lakes as several shortwaves dive through the region. The first period of interest will be late Saturday night into Sunday, when guidance shows the first wave diving through. While guidance does show a complex of thunderstorms developing in the Upper Midwest and diving towards northern IL, there is a lot of variability as to whether or not the complex will track over us or miss us just to the west. Depending on where this complex goes will dictate how Sunday afternoon plays out locally which means we could get more scattered thunderstorms Sunday afternoon or stay dry until the next shortwave arrives on Monday. That said, the environment overhead Sunday afternoon will be one supportive of severe weather with strong instability and decent wind shear forecast to be present. Similarly, Monday afternoon also looks to be another period supportive of severe weather. Though confidence on storm timing and coverage Monday afternoon remains uncertain and will likely be dictated on how things evolve on Sunday. Bottom line, the end of the weekend and beginning of next week looks to start off active so make sure you keep checking back for forecast updates.

Additional periods of showers and thunderstorms will be possible through at least the middle of next week as the northwest flow remains in place. While there will be some dry hours mixed in, confidence on the specifics at this time range are low. Regardless, seasonable temperatures in the 80s (with overnight lows in the 60s) are forecast to prevail through the early part of next week.

Yack

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1231 AM CDT Fri Aug 7 2026

Aviation forecast concerns for the 06Z TAFs:

- Areas of MVFR/IFR stratus and fog early this morning.

- A few isolated SHRA possible at times through this afternoon, though a slightly greater (30%) chance of showers appears to be across the KRFD area late Friday evening.

Terminals remain in the vicinity of a dissipating surface frontal zone early this morning, characterized by a moist low- level air mass and weak forcing for ascent beneath an upper level trough. The combination of moisture and weak ascent may produce isolated SHRA at times through this afternoon, though expected coverage is too low for any TAF mention at this time. The exception appears to be late Friday evening, mainly across the KRFD area, as a weak mid-level disturbance and cold front approaches from the west with a greater (~30%) chance of SHRA entering northwest IL. Current guidance indicates SHRA coverage diminishes with eastward extent Friday night, thus have included a PROB30 mention only at KRFD with this issuance.

Of more immediate concern is the development of low stratus/fog early this morning, again due to the moist low-level environment. Have already noted patchy shallow ground fog and low (800-1500 foot) stratus developing in spots at issuance time, and expectation is that this will likely increase in coverage prior to sunrise. Dense fog appears most likely southeast of the terminals (southeast of the I-55 corridor across eastern IL/IN) where surface winds are lightest and dew point depressions minimal, though thicker high-level cloud cover may mitigate this somewhat for KGYY. Farther north/northwest. low-level winds are slightly higher, which may keep visibilities in the MVFR/low-VFR range but may also allow for additional patchy IFR/low-MVFR stratus to develop. Have indicated TEMPO IFR at GYY through 12Z and MVFR elsewhere, and will continue to monitor cig/vis trends early this morning.

Otherwise, light south winds should become southwest 7-10 kts during the day before easing again with sunset. Winds should turn light westerly late tonight with the aforementioned weak surface cold front approaching from the west.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 8 AM CDT this morning for ILZ021- ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Dense Fog Advisory until 8 AM CDT /9 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.