Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252028

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
328 PM CDT THU AUG 25 2016

241 PM CDT

Through Tonight...

A short wave disturbance and remnant, somewhat ill-defined, MCV
are translating east-northeast near the IA/MO/IL border region.
This is riding along an instability gradient that in the far
southern CWA is characterized by MLCAPE as high as around 1,000
J/kg. The surface cool front which inched its way southward today
is weakly defined in convergence fields across the southern CWA.

Cumulus clouds on visible satellite imagery have shown little
growth thus far but are seeing some uptick over far western
Illinois just ahead of the short wave. Given the forcing and
instability and the time of day, along with consistent support
from the HRRR, would expect scattered convective development
further eastward into central Illinois and as far northward as the
instability gradient. Thus the part of the CWA that would be
likely to see scattered storms remains south of the U.S. Highway
24 corridor, with isolated/scattered showers possible north of
there. The current CAPE/shear/PWAT spectrum would support a few
stronger to possibly severe organized storms capable of gusty
winds, mainly through 8 pm. The better potential for longer-lived
organized storms looks to remain south of the CWA where greater
destabilization has occurred.

A sheath of high clouds is expected to remain over the area
tonight as the moist plume aloft eases eastward. This should help
to limit temperature drop as well as fog threat. If the clouds
do end up being less prevalent though, cannot rule out some
patchy shallow fog, namely over the south where higher dew points



327 PM CDT

Friday through Thursday...

With the surface front south of the CWA on Friday and while high
pressure builds across the region, any precip development during
the day will stay just to the south. Some guidance is hinting at
the possibility for some precip to drift along the southern CWA in
central Illinois and northwest Indiana during the day Friday and
although can`t completely rule out some stray precip, think the
higher probability for any development will be near the front that
will be to the south. Only made minor adjustments to the high
temps on Friday, a slight increase, with only mid/high level cloud
cover expected to be in place.

Active weather will likely return to the region at the start of
the weekend as an upper level trough pushes through the central
conus, and while more energetic mid level flow begins to draw
near. This will aid in a return to southerly flow across the
region, with low pressure developing over the upper midwest and
with the surface front lifting back north. Focus for precip Friday
night looks to initially be over Iowa and far northwest Illinois
but as forcing increases across the region and while the LLJ
continues to veer, should begin to see better precip development
steer over northern Illinois after midnight into early Saturday
morning. Most guidance pretty consistent with this signal and with
several forcing mechanisms in place, did increase pops to high
likely pops across north central Illinois during this time.
Instability which will initially be south of the area will spread
back north into the weekend, with thunderstorm development
becoming more likely. At this time, think the more robust
development will be just to the west of the CWA Friday night into
Saturday but with higher moisture air returning, do think the
threat of heavy downpours will also return to the area.
Forcing/Focus will continue to the north and northeast during the
day Saturday with precip likely in the morning, but with a drying
trend possible into the afternoon. Guidance is showing some
development in the afternoon and although this is possible, I
wonder about boundary layer recovery as well as any good focus to
produce this. Nonetheless, with high dewpoint air in place and
with large scale ascent still overhead, scattered development in
the afternoon will be possible. Warmer airmass in place Saturday
would be supportive of temps well into the 80s but with expected
precip and cloud cover maintained highs around 80.

Confidence lowers with regards to precip chances Saturday night
with guidance showing QPF during this time but question the
potential forcing mechanisms. Did leave forecast of chance thunder
alone Saturday night as it is possible for any additional
development during the day Saturday to possibly linger into a
portion of the evening. Precip chances should lower on Sunday and
did reflect this in the forecast, but do think isolated to widely
scattered precip will be possible in the afternoon.



For the 18Z TAFs...

The challenges with the TAFs are present MVFR cloud cover as of
18Z that should shortly lift, a probable wind shift with a lake
breeze this afternoon, and finally a small chance for showers
this afternoon.

A surface cold front has slowly moved just south of the Chicago
area TAF sites with a light northwest wind in its wake. The light
wind flow has allowed the low clouds to persist, although these
should lift and scatter shortly. The light wind flow also has
allowed for a lake breeze to develop. This has been hugging the
Illinois shore between 11 am and 1 pm, but should make some
progress inland. Confidence is highest in this reaching MDW and a
little less for ORD. The later the wind shift arrives at
ORD...assuming it does arrive...the higher confidence that speeds
will be under 9 kt behind it.

The area of showers that has been moving northeast along the
IA/MO border has been gradually weakening. While we do expect an
uptick in scattered to numerous storms across central Illinois
this afternoon ahead of this feature...possibly as far north as
just outside the southern TRACON area...the chance for thunder at
the TAF sites is very small. There could be isolated showers
around during the mid to late afternoon but expect these tops to
be shallow if they were to develop.

Light or even variable winds will be seen early this evening
before a light northwest wind. Any IFR fog is expected to be
south of the TAF sites. Winds should flop northeast during Friday



227 PM CDT

A cold front associated with low pressure over Ontario and Quebec
has pushed through the lake this afternoon. Winds had generally
become a west northwest direction behind this boundary, but a
weaker surface pattern overhead has allowed for the winds to
become more variable across the lake this afternoon. The trough of
low pressure to the north still moving through the region will
allow the winds over the northern half of the lake to be slightly
more organized and stronger tonight, until a return to a northwest
wind occurs late tonight into Friday for all of the lake. This
will also be due to high pressure moving through the western lakes
Friday, and will help for diminishing winds into the afternoon.
Lighter winds will be in place for most of Friday night until
winds increase out of the south southeast Saturday into Saturday
night. However, at this time, speeds should remain in the 10 to 20
KT range.






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