Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161731
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1131 AM CST Thu Nov 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 230 AM CST Thu Nov 16 2017

Surface high pressure will dominate the weather today bringing
tranquil conditions and seasonably cool temperatures. The center of
the high will be centered within the MS Valley Region at daybreak
and retreat eastward into the OH Valley by early evening. Waves of
high clouds will periodically spill thru the area within the
northwest flow aloft.

The high pressure center will continue to slide eastward tonight and
into the Appalachians by 12Z Friday, with surface winds veering to
southeasterly across the CWA. The winds aloft will continue to veer
with height, and above the boundary layer the increasing
southwesterly flow will result in not only prominent WAA but also
increasing moisture transport. The greatest moisture increase will
be after midnight from western MO into central MO, especially after
08Z or so. Soundings at KCOU show the moisture depth to ~ 7 kft.
This moisture profile along with weak lift via WAA should be
sufficient for spotty light showers.

Glass

.LONG TERM...  (Friday through Wednesday)
Issued at 230 AM CST Thu Nov 16 2017

Weak shortwave to slide through region on Friday, so will see
isolated/scattered showers persisting. Otherwise, sfc gradient to
tighten with south winds picking up to between 10 and 20 mph with
gusts near 25 mph. This will help usher in decent WAA which will
persist into the overnight hours. So highs will be in the mid 50s to
mid 60s during the day, then temps to continue rising through the
evening hours as main cold front approaches.

Friday night main shortwave and deepening surface low to begin
lifting northeast towards Great Lakes region. This will drag strong
cold front through forecast area. Ahead of cold front will see
increasing coverage of showers and isolated thunderstorms. Severe
chances continue to look minimal with this system despite the rapid
deepening of the system, as surface instability is limited. The
front will move through UIN by 09z Saturday, then STL metro area
by 14z Saturday, eventually exiting CWA by 18z.

Will see some wrap around showers on backside of system Saturday
afternoon, then taper off by 00z Sunday. Surface gradient tightens
once again, so will see northwest winds pickup to 15 to 25 mph
gusting to 35 mph at times. As for temps on Saturday, highs expected
during the morning hours, then either remain steady or slowly fall
through the rest of the day.

Winds to diminish Saturday night as gradient weakens and surface
ridge builds in. Cooler and drier wx for last half of weekend. Then
surface ridge moves off to the east by Monday, allowing southerly
winds to return and temperatures to moderate for the for the first
part of the work week. Extended models have some timing differences
with cold front that swings through on Tuesday, but both agree that
main dynamics to stay over Great Lakes with frontal passage dry in
our area. Weak surface ridge builds in once again by Wednesday.

Byrd

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1126 AM CST Thu Nov 16 2017

VFR conditions are expected today into the first part of the
overnight hours. However, some uncertainty in the forecast is
present tonight as some IFR cigs/light fog may develop across
portions of IL. If this can develop, it may make a run at
STL/SUS/CPS/UIN. Will leave out any mention for now given the
uncertainty, but will continue to monitor in future forecasts.

Higher confidence in reduced cigs tomorrow as low-level moisture
increases. MVFR cigs will become common by the mid-morning hours
at all sites, with even some patchy/areas of drizzle from time to
time especially at COU/UIN. Winds will be on the increase tomorrow
as well, with ESE winds gusting upwards of 20 knots at times.

SPECIFICS FOR KSTL:
VFR conditions are expected today into the first part of the
overnight hours. However, some uncertainty in the forecast is
present tonight as some IFR cigs/light fog may develop across
portions of IL. If this can develop, it may make a run at STL.
Will leave out any mention for now given the uncertainty, but will
continue to monitor in future forecasts.

Higher confidence in reduced cigs tomorrow as low-level moisture
increases. MVFR cigs will move in by the mid-morning hours, with
even some patchy drizzle from time to time. Winds will be on the
increase tomorrow as well, with ESE winds gusting upwards of 20
knots at times.

KD

&&

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

$$

WFO LSX



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