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239
FXUS63 KLOT 131206
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
706 AM CDT Thu Aug 13 2026

.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 /12Z TAFS THROUGH 18Z FRIDAY/...
-- Changed Discussion --
Issued at 705 AM CDT Thu Aug 13 2026

Key Messages:

- Fog and low ceilings to gradually improve this morning.

- A period of rain is likely this afternoon with about a 25%
  chance or so for lightning to occur near the terminals.

- Additional showers and thunderstorms are possible tonight
  through Friday morning.


LIFR and IFR conditions from fog and/or low stratus are being
observed across most of the area early this morning. While these
conditions should gradually improve over the course of the
morning, couldn`t rule out MVFR ceilings hanging around into
this afternoon in spots.

The next disturbance in this very active weather pattern will
arrive in northern Illinois this afternoon. The associated
ongoing organized thunderstorm complex in Iowa will weaken as it
approaches the area, but the associated rain showers will
persist and result in a period of rain at the terminals this
afternoon. There is about a 25% chance or so for thunderstorms
to redevelop on the back side of this rain shield and affect the
terminals. This is still a bit too low of a chance to warrant a
formal TSRA or VCTS mention in the TAFs this afternoon, 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 afternoon 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. The general
thought is that the bulk of this activity should remain just
south of the terminals, but it may get pretty close, and the
terminals are likely to at least see another period of rain
sometime tonight into tomorrow morning. Opted to add PROB30
groups for TSRA to the TAFs for tonight as it could at least be
a close call with these storms, and in the worst case, a
prolonged period of storms and torrential rainfall may be
observed tonight if this axis of convection ends up positioning
itself a bit further north than presently expected.

Ogorek
-- End Changed Discussion --

&&

.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.

&&

$$

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