Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251942

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
242 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



328 AM CDT

Friday through Wednesday...

Friday through Saturday: Surface ridge will be overhead to start the
day Fri; however, the quasi-stationary boundary will be starting to
drift north across Kansas/Nebraska Fri aftn. Flow aloft appears to
be more fluid, allowing a more progressive pattern to setup. Expect
clouds to be returning late Fri aftn/eve from the southwest, with
some of the operational solutions indicating precip could return to
Northern Illinois as early as Fri eve. Current thinking is that with
the upstream 500mb trough axis remaining over the Northern Plains,
and weak height rises Fri eve then falling overnight into Sat, that
any precip will hold off until the overnight hours. A lobe of
vorticity lifts northeast across Illinois Sat morning, which may
require POPs to trend further up towards likely or possibly
categorical for the first half of Sat. Temps Fri will be more
seasonal, then as clouds thicken late Fri aftn/eve this will hold
warmer conds overnight. Have nudged temps towards the low/mid 60s
Fri ngt, but this may end up needing to be further warmed into the
upper 60s to around 70 given the warm/moist axis returning. Sat high
temps will be tricky due to the abundance of clouds limiting surface
heating, so have held onto temps around 80 to the low 80s.

Sunday through Wednesday: The extended periods continue to be
favored towards a quasi-zonal flow based off of the latest ensembles
forecasts. 500mb flow will initially feature a mid-lvl trough
weakening Sun across the upper midwest, then shifting the jet
further north into Canada. Expect periodic chances for showers and
thunderstorms through the extended periods; however, trying to
highlight a particular period or two remains a challenge given the
lack of a defined shortwave in the extended forecast. Temps will
generally be seasonal in the low 80s to perhaps middle 80s.



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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