788
FXUS63 KLOT 190840
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
340 AM CDT Wed Aug 19 2026

.KEY MESSAGES...

- Areas of shallow fog may become locally dense early this
  morning outside of Chicago.

- A corridor of training thunderstorms may set up near and
  south of a Pontiac to Fowler line this afternoon into the
  evening. Primary hazards include gusty winds and torrential
  rainfall which may lead to flash flooding.

&&

.DISCUSSION...
Issued at 340 AM CDT Wed Aug 19 2026

Residual low-level moisture early this morning has allowed
shallow fog to develop across portions of the area, including
interior northern Illinois and into portions of northwest
Indiana. The lowest visibilities are focused mainly where the
coverage of high clouds extending north into the area from
convection in southwest Illinois is lowest. Generally expect at
least patchy fog to persist through daybreak, some of which
could be locally dense near low-lying areas and open fields.

The surface cold front that initiated convection last evening
has since washed out over the region. Instead of a true frontal
passage, the surface flow may simply flop northerly after
daybreak this morning for most of the area. Surface flow may
remain southerly across central IL/IN, however, which will
likely set up a corridor of surface convergence near and south
of a Pontiac to Rensselaer line. This may lead to at least
isolated shower and thunderstorm development this afternoon
depending on the strength of the low-level convergence amidst a
largely uncapped environment. Additionally, a reinforcing lake
breeze boundary is expected to drift inland today which could
serve as another lifting mechanism for at least a few showers
into portions of the Chicago metro. Also can`t fully rule out
lightning with any of the deeper showers north of the
Kankakee/Illinois Rivers.

Meanwhile a mid-level wave and associated vort currently over
southern South Dakota early this morning is forecast to drift
east southeast over the area today. This feature may bring the
necessary forcing to increase coverage of showers and
thunderstorms along the aforementioned surface convergence axis.
Increasing PWATs ahead of this wave will support rainfall rates
in excess of 1-2 inches per hour with the heaviest
showers/storms. Hi-resolution guidance has remained fairly
consistent in developing this corridor of heavy showers and
thunderstorms over central Illinois and Indiana, including
portions of the local area, but have wobbled north or south by a
county or two. Due to the increased susceptibility to flooding
over much of our area, have elected to hoist a Flood Watch for
Livingston, Ford, Iroquois, and Benton Counties this afternoon
and evening where confidence is higher in heavy rainfall
occurring.

Surface high pressure builds into the area on Thursday bringing
a much needed break from the shower and thunderstorm activity.
Lower dewpoints paired with temperatures in the mid 70s to
around 80 will make for a pleasant day compared the the more
humid airmasses we have been experiencing the past couple of
weeks.

High temperatures through the end of the week will remain near
to slightly below normal, in the mid 70s to lower 80s.
Additional waves moving within the upper flow Friday into the
upcoming weekend may bring additional showers and storms to the
area at times (30% chance). Dry conditions are then favored into
early next week as upper ridging builds into the region.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1252 AM CDT Wed Aug 19 2026

A weak and diffuse cold front currently over central Wisconsin
will drift southward across northern Illinois this afternoon.
Meanwhile, high pressure will drift eastward toward the western
Great Lakes. The combination of these two features will result
in a gradual veering of the winds (at around 5 knots or less)
from W early this morning, NNW by daybreak and NNE by mid-
morning. A lake-enhanced wind shift will then allow winds to
settle NE early this afternoon and through the remainder of the
period, with speeds 10 to 15 knots at times this afternoon.

Light/calm winds, passing mid/upper-level clouds, and plenty of
surface-layer moisture will allow fog and/or LIFR stratus to
develop across interior northern Illinois early this morning
through sunrise. Have included TEMPO LIFR visibility at RFD and
IFR visibility at DPA from 10-14Z, though both sites could see
VLIFR visibility. Have also include VFR BR at ORD/MDW during the
same window, with a low chance (under 30%) that fog well
northwest of the core of the Chicago metro drifts toward ORD/MDW
as IFR stratus around daybreak.

With the abundant low-level moisture in place ahead of the cold
front, diurnal mixing should yield a period of MVFR ceilings at
all sites mid to late morning. Stratocumulus will then lift
into VFR levels and scatter through the afternoon, with drier
and nearly SKC conditions tonight.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch from 1 PM CDT this afternoon through late tonight
     for ILZ032-ILZ033-ILZ039.

IN...Flood Watch from 2 PM EDT this afternoon through late tonight
     for INZ019.

LM...None.

&&

$$

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