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 /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 632 AM CDT Fri Aug 7 2026

Key aviation weather messages:

- Period of LIFR/VLIFR at GYY this morning, possibly impacting MDW for a brief time prior to 14Z before improving.

- At least patchy MVFR ceilings likely this morning, and again toward daybreak Saturday.

- Chance (~30%) for scattered SHRA/TSRA to spread into NW IL this evening, possibly affecting RFD.

An extensive area of LIFR/VLIFR fog/stratus has developed south of Chicago early this morning, and has gradually expanded northward over the last few hours. GYY currently in the VLIFR, and GOES satellite imagery indicates the northern edge of the fog bank is approaching MDW (only 3-4 miles south of the field at 1125). Thinking is that the northern periphery should begin to slow as mixing strengthens after sunrise, though it's close enough to MDW that I've tempo'd them down to airfield minimums for a brief time this morning. Otherwise, with continued mixing this morning would expect patchy stratocu/stratus to develop and/or rise into MVFR range before eventually becoming VFR around midday. Surface winds which are light south/variable at this time should become southwest 7-10 kts by late morning.

Otherwise, an area of TSRA developing along the SD/MN border this morning is expected to eventually develop east-southeast into eastern IA and possibly southwest WI/northwest IL by early this evening. Some CAM guidance brings this into the RFD albeit in weakening form, and have moved up the previous PROB30 mention to ~01Z. It's possible the TSRA cluster ends up tracking closer to the Mississippi and remains largely west of RFD.

Another weak cold front is expected to push east across the terminals predawn Saturday morning, shifting winds to the west- northwest. A period of MVFR ceilings may accompany the front before low-level dry air moves in after sunrise.

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.