817
FXUS63 KLOT 140123
AFDLOT

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

.KEY MESSAGES...

- Band of showers and thunderstorms is expected to result in a
  period of heavy rainfall and flooding overnight. Greatest
  threats for heavy rainfall and flooding is favored near and
  south of a Sterling, IL to Rensselaer, IN line.

- A stormy pattern is expected to remain in place through the
  weekend.

&&

.UPDATE...
Issued at 823 PM CDT Thu Aug 13 2026

A band of showers continues to drift southward across northern
and northeast IL and northwest IN this evening. While the
lightning coverage within the line has been low, some scattered
thunderstorms have developed along the tail end of the line in
eastern IA and as modest instability increases over the next few
hours the lightning may become more frequent with eastward
extent. In addition to lightning, these showers/storms will
also be capable of hefty rain rates (1-2+ in/hr) which will
result in reduced visibility and maybe some ponding in poor
drainage areas.

Heading into the overnight hours, the band of showers and
thunderstorms is expected to stall along an elevated frontal
zone that is draped from the Quad Cities southeastward across
north-central IL into central IN. As this occurs, a modest low-
level jet will develop and help to replenish moisture and
instability which should allow showers and thunderstorms to
fester along this boundary for much of the overnight hours.
While there is still some uncertainty on where exactly this
boundary will stall across our area, latest radar trends and
forecast guidance is favoring the showers/storms to stall near
and south of a Sterling, IL to Rensselaer, IN line. Given the
aforementioned forcing and the high PWATs (around 2.0-2.25
inches) these showers and thunderstorms will be capable of heavy
rainfall which could result in periods of flooding overnight
into early Friday morning. Rainfall totals are currently
forecast to be in the 1-3 inch range within the heart of the
rain band, but some guidance is showing amounts upwards of 4+
inches. With these forecast rain amounts and the already
saturated soils, the Flood Watch has been maintained for our
southern counties through 10 AM CDT Friday morning.

In addition to the heavy rainfall, forecast soundings continue
to show 35-40 kts of deep layer shear overhead tonight which may
support a stronger storm or two. Since most of these storms
should be elevated and the lapse rates upstairs are somewhat
junky (around 5-6 C/km), it seems the threat for widespread
severe weather is low. However, some rain loaded downbursts
cannot be ruled out especially if any of the gravity waves seen
in IA earlier are able to aid in enhancing the storms`
downdrafts. If any gusty winds do occur tonight it seems most of
them would be in the 40-50 mph range but locally higher gusts
will be possible.

The showers and storms tonight should gradually begin to lose
steam Friday morning as the low-level jet weakens. This should
in turn give us mostly dry conditions for the afternoon hours on
Friday. However, another shortwave disturbance is forecast to
pivot through the area Friday night which looks to bring more
showers and thunderstorms. These storms could also be heavy rain
producers and could worsen any ongoing flooding from tonight
and/or cause it to redevelop. The main focus for the evening
updates was on the tonight into Friday morning period so the
prevailing forecast for Friday night was maintained. For more
details on what to expect see the discussion below.

Yack

&&

.DISCUSSION...
Issued at 230 PM CDT Thu Aug 13 2026

At the time this discussion was published, an MCV was tracking
east over US-24. Meanwhile north of the MCV, widespread rain is
falling with isolated to widely scattered lightning. This system
will continue to track east through the afternoon.

There could be a break in the late afternoon for areas south of
Interstate 80, as continued rain is expected north along a
projected low level vorticity axis and 500 mb jet axis along the
Illinois/Wisconsin state line. With the current MCV moving
through the area, it is difficult to pinpoint the exact location
of the stationary front that was previously analyzed just south
of the forecast area. That becomes important because the right
entrance region of the previously mentioned 500 mb jet (and
therefore area of focused upper level divergence) will be
situated just north of the stationary front. High-res CAMs are
projecting weak perturbations within the 500 mb flow this
evening and through the overnight. With precipitable water
amounts between 1.5 to 2 inches, this set-up is increasing
concerns for flooding threat in the area. The one fly in the
ointment for this set-up is the absence of a strong low level
jet. Had there been a stronger low level jet projected, perhaps
there would need to be consideration to expand the previously
issued Flood Watch northward. Showers are expected to develop
over northern Illinois with the risk of training storms over the
same area. It is no surprise that ensembles are showing a broad
swath of 1 to 2 inches of rain that could fall overnight, with
an axis of locally higher amounts over the southwest portions of
the forecast area.

There is lower confidence in the exact timing of when the rain
ends on Friday morning. As the upper level jet weakens and moves
east, rain should slowly taper off by mid-morning with the
potential for a drier period expected in the afternoon. However,
as the broad upper level ridge over the southeastern portion of
the CONUS remains in place, another short wave will ride over
the top of the ridge tomorrow afternoon and evening providing
enough forcing for the next system to develop in Iowa and move
east. Rain could begin in the early evening, but the better
chances for the heavier downpours are expected closer to
midnight when the core of the wave moves over. Given what the
forecast is for rainfall overnight tonight on already saturated
soils, there is growing concerns for flooding threats
tomorrow night into Saturday morning and collaborated with the
Weather Prediction Center to increase the risk to a level 3 out
of 4 for excessive rainfall. The potential for water loaded
downbursts keeps much of the forecast area in a marginal (level
1 out 5) risk for severe storms tomorrow night, but weaker shear
profiles keep the confidence in severe development low.

Models have shown fairly good agreement that yet another wave is
expected to pass over the ridge on Saturday providing another
threat for heavy rainfall and flooding threat across portions of
the forecast area. Global models are not suggesting a break in
the rain until a front drops down out of Wisconsin late on
Sunday. Surface high pressure is expected to grow on Monday for
the potential for a dry day in the region. However, global
models are still suggesting another system potentially bringing
a a renewed threat for precipitation mid-week next week.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 632 PM CDT Thu Aug 13 2026

An area of showers will continue to shift south across the
terminals early this evening. Later this evening and overnight,
expectations are for an additional area of convection to develop
in the region. Some uncertainty still exists on the northern
terminus of this convection, but latest satellite and model
trends suggest the bulk of this activity may tend to focus just
south of the terminals, although it may get close, particularly
at MDW and GYY. Given this, have continued a long-duration VCSH
and PROB30 groups for TSRA to account for trailing stratiform
precipitation. Activity may persist through much of the night
and into Friday morning before diminishing.

Outside of showers, a moist airmass will support the potential
for BR development tonight. MVFR cigs may develop Friday morning
and afternoon with mixing, but confidence on this remains too
low for a mention in the TAFs. East to northeasterly winds will
prevail through the forecast period. An additional round of
convection appears possible Friday evening/overnight.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through Friday morning for ILZ008-ILZ010-ILZ011-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-
     ILZ107-ILZ108.

IN...Flood Watch through Friday morning for INZ010-INZ011-INZ019.

LM...None.

&&

$$

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