textproduct: Chicago

KEY MESSAGES

- Areas of dense fog early this morning. mainly west and south of the core of the Chicago metro.

- Another cluster of thunderstorms may bring a severe weather threat to our southern cwa this afternoon.

- Stormy pattern persists into the weekend with a threat of repeated rounds of heavy rainfall increasing the risk of flooding in some areas.

DISCUSSION

Issued at 329 AM CDT Thu Aug 13 2026

Primary forecast concerns are dense fog early this morning, another MCS and severe weather threat for our far southern cwa this afternoon, and the potential for a series of heavy rain producing disturbances to increase flooding potential across parts of the area later tonight into the weekend.

Outflow from Wednesdsay's MCS has pushed the effective frontal boundary well south of the area this morning. With modest mid- level height rises/subsidence in the wake of yesterday's disturbance, relatively clear skies atop a residual moist boundary layer has led to fog development however, with calm winds and mostly 0F temp-dew point spreads noted. Combination of current GOES nighttime microphysics imagery and surface observations depict fog quickly filling in south and west of the urban core of the Chicago metro, and expect this to continue with several more hours until sunrise. Thus have issued a Dense Fog Advisory for much of the area through 9 AM (except McHenry/Lake and central/northern Cook counties for now) where current obs and model trends suggest widespread dense fog is likely. Visibilities will improve as fog gradually burns off a few hours after sunrise this morning.

Attention then turns to yet another convective complex currently tracking across eastern NE/SD, associated with the next mid-level (convectively enhanced by the presence of an MCV) short wave which is progged to transit the forecast area later this afternoon. Current suite of CAM guidance is in reasonable agreement in this MCS approaching our western cwa toward midday, with the strongest convective elements expected to track southeast along the northern periphery of the low-level instability gradient into central IL (brushing our southwestern tier of cwa counties) into this afternoon. While some guidance has indicated a gradual weakening of convection this far east later today, diurnal warming should maintain instability along it's path and this may maintain primarily a damaging wind threat mainly along/south of about a Mendota IL to Fowler IN line early this afternoon in addition to the potential for heavy downpours. SPC's new Day 1 outlook has placed a slight (level 2 of 5) severe risk across this far south/southwest portion of our forecast area. Low level instability is expected to decrease markedly with further northward extent, limiting any severe weather threat farther north.

Concern then increases for the potential for several periodic rounds of heavy rainfall and an attendant flood/flash flood threat across portions of the area from tonight into at least Saturday. Guidance is in good agreement in strengthening an upper level high pressure ridge across the Gulf Coast northward across the lower Mississippi and Ohio valleys during this period, setting up a period of persistent west-southwest warm/moist advection flow around the northern edge of the ridge into parts of IA and northern IL/IN. Model precipitable water values reach 2-2.3" across the area as a series of short waves top the ridge to our south. There is a consistent signal for portions of these areas to potentially receive several inches (3+) of rain, which on top of wet soils and elevated river/creek levels in spots from recent heavy rains suggest an increasing concern for flooding. The first round (aside from rainfall with the afternoon MCS) would be tonight into early Friday morning, as a short wave currently emerging from the high plains of western CO/NE transits the region. Model QPF suggest a NW-SE arc of potentially 2-plus inch rainfall generally from a bit south of roughly a Forreston IL to Valparaiso IN line overnight. While exact details remain a bit uncertain, elected to issue a Flood Watch for these areas from 7 PM tonight through 10 AM Friday. Depending on how rainfall lays out, the watch may need to be extended (or another issued) into the weekend for additional periods of heavy rainfall.

Eventually, global models/ensembles shift the core of the Gulf Coast upper ridge westward late Saturday night into Sunday, which may allow the lingering frontal zone to our north to finally push south of the area sometime late Sunday. Rain chances would likely diminish behind the front with mainly dry weather Monday/Tuesday. It does appear however, that another front may push southeast across the area mid-week which may support some additional rain chances at that time.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 128 AM CDT Thu Aug 13 2026

Key Messages:

- Fog and low ceilings are likely to be observed this morning, especially at RFD, DPA, and GYY.

- A period of rain is likely this afternoon with a 20-25% chance for an isolated thunderstorm or two to occur near the terminals.

- A corridor of widespread showers and storms will set up in the region Thursday night into Friday morning, though most of this activity is favored to remain south of the terminals.

Light/calm winds and mostly clear skies have allowed for fog to start developing in the area early this morning. Increasing cirrus coverage overnight introduces uncertainty to how dense this fog might get, but IFR or lower visibilities appear to be a distinct possibility, primarily at RFD, DPA, and GYY. Low stratus coverage may also increase over the next few hours and also result in MVFR or lower ceilings being observed at the terminals for some time this morning.

The next disturbance in this very active weather pattern will arrive in northern Illinois this afternoon. It appears that the stronger thunderstorm activity associated with this disturbance will track safely south of the terminals, but a period of rain is nevertheless still likely to occur as this disturbance passes through. There is about a 20-25% chance for an isolated thunderstorm or two to develop on the back side of this rain shield and affect the terminals. This is still too low of a chance to warrant a formal TSRA or VCTS mention in the TAFs, but it remains possible that such a mention will eventually be needed as forecast and observational trends become more apparent.

In the wake of this disturbance, showers and thunderstorms are likely to fire along the northern periphery of an elevated instability plume and persist into Friday. Shower and storm coverage across this corridor is likely to be widespread and feature training convective elements. While this axis of convective activity is favored to largely remain focused south of the terminals, some of the associated rain may eventually lift far enough northward to affect the terminals. If this axis ends up setting up farther north than presently expected, then it is possible that the terminals will see an extended period of storms and torrential rainfall Thursday night into Friday morning, but this is not the most likely outcome at this time.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ003- ILZ004-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107- ILZ108.

Flood Watch from this evening through Friday morning for ILZ008-ILZ010-ILZ011-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032- ILZ033-ILZ039-ILZ106-ILZ107-ILZ108.

IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

Flood Watch from this evening through Friday morning for INZ010-INZ011-INZ019.

LM...None.