Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210046
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
746 PM CDT SAT AUG 20 2016

.UPDATE...
Issued at 746 PM CDT Sat Aug 20 2016

Went ahead and updated the forecast this evening to remove
measurable pops from the forecast. Secondary front has pushed
through the bulk of the region, and drier air is quickly filtering
in. For those who aren`t a fan of the humidity, 50 degree
dewpoints are on their way overnight into Sunday which will feel
very pleasant for this time of year!

KD

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 343 PM CDT Sat Aug 20 2016

A cold front assoc with an upper trof is pushing thru the region
this aftn and currently stretches from nthrn IL to W cntrl IL to
cntrl MO to SW MO to NW AR. Earlier today...an outflow bndry
emanating from last nights convection pushed through the area. It
made it as far SE as sthrn MO and sthrn IL...which is where the
better concentration of precip has been today. This bndry acted
as the effective front which significantly limited the amount of
convection behind it this aftn. Even so...there has been
isld/scttrd SHRAs that have dvlpd along the front across NE MO and
W cntrl IL earlier this aftn. A thin broken band extends SW from
Morgan County IL to Cole County MO. This activity will slowly
drift SE thru the aftn and should dsspt around 00Z with loss of
daytime heating. Another area of precip closer to the remnants of
the outflow bndry has moved into SE MO from nthrn AR this aftn.
This activity should continue to lift NE along the effective front
into early evng before the approaching synoptic cold front pushes
the whole batch of precip to the SE later this evng. High pressure
builds in behind the cold front overnight. Diurnal cu behind the
cold front will dsspt around sunset but clouds assoc with the cold
front not clearing the SErn zones til closer to midnight.
Otherwise...expect light NW winds...drier air and cooler temps
tonight.

2%

.LONG TERM...  (Sunday through Next Saturday)
Issued at 343 PM CDT Sat Aug 20 2016

Spectacular weather is expected on Sunday due to cooler
temperatures and lower dew points in the wake of the
aforementioned cold front. Any lingering cloudiness within
cyclonic flow aloft should diminish on Sunday when the upper
trough departs and a surface high pressure center builds into the
area.

20/12z model runs continue to trend quicker with the progression
of the sfc high. A few days ago, model solutions depicted the
center of the high located overhead by 12z Mon, leading to ideal
radiational cooling conditions for Sunday night. Now, it looks as
though the center of the high will be well east of the CWA at 12z
Mon, and winds will have already turned southerly over most of
the LSX CWA by 22/06z. Although nearly clear skies and light
winds are still favorable for radiational cooling, the quicker
passage of the surface high (and the resulting multi-hour period
of light southerly flow) does not favor going much lower than the
current forecast lows in the mid to upper 50s (slightly higher in
urban areas).

The next upper trough and its associated sfc reflection and
frontal boundaries will begin to affect the area during the
middle and end of the upcoming week. Models now depict a warm
frontal passage on Tuesday night instead of Wednesday, which
brings SHRA/TSRA chances into MO/IL as early as Tuesday. There
are timing differences for the passage of a trailing cold front
which extends from the Great Lakes into the plains, but there is
general agreement on the Thu night/Fri time frame. Depending on
the speed of the cdfnt, there may be a relatively quiet day
between pds of SHRA/TSRA associated with each fropa.

Kanofsky

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 603 PM CDT Sat Aug 20 2016

VFR conditions are expected through the TAF period. A few
sprinkles/light showers along a secondary cold front will push to
the east of SUS/STL/CPS over the next hour or so, putting an end
to the precip chances at all sites. Winds may stay up a bit into
the evening behind the front, but will slacken overnight as high
pressure builds into the region. Skies will go mainly clear
overnight, with few/sct diurnal cu expected on Sunday with WNW
winds around 10 knots.

SPECIFICS FOR KSTL:
Light sprinkles around the terminal will come to an end over the
next hour or so, with VFR conditions prevailing through the
period. Clouds should dissipate overnight, with few/sct cu
redeveloping Sunday afternoon. WNW winds will slacken overnight,
increasing to around 10 knots during the day on Sunday.

KD

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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