Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 071743
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 AM CST Wed Dec 7 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CST Wed Dec 7 2016

Main concern will be chance for light snow today and this evening.
Weak shortwave trough currently seen in shortwave imagery over the
central Rockies will race eastward today in zonal flow.  There will
be some weak ascent develop ahead of the trough across central and
southern Missouri into southern Illinois. There is very little
moisture for this system to work with, so will go with only low
chance PoPs or a chance of flurries/sprinkles.  Forecast soundings
still suggest that this will be mainly a snow event.  Given the
limited amount of moisture and fast motion of the system, little if
any snowfall is expected across central and southeast Missouri.

The upper trough will move quickly east tonight.  This will cause
skies to clear and winds to turn out of the northwest.  With the
clouds and cold air advection, went with highs closer to the cooler
SREF mean temperatures today.  Lows tonight will be closer to the
cooler MAV MOS lows.

Britt

.LONG TERM...  (Thursday through Tuesday)
Issued at 335 AM CST Wed Dec 7 2016

Thursday and Friday continue to look cold, especially in comparison
to the mild Fall season we just had.  This is due to the first real
Arctic airmass spilling across the Great Plains and Midwest. Daytime
highs will struggle to reach the mid and upper 20s with morning lows
in the low to mid teens.  This should average out to around 15
degrees below normal for December 8th and 9th.

The ridge axis will move east of the Mississippi into the Ohio
Valley by Saturday morning which will allow lo level flow to turn
back to a southerly direction.  This will allow temperatures to
finally warm back up above freezing Saturday afternoon...though
highs will probably top out a few degrees below normal in mid 30s
to around 40.

Attention then turns to the next chance for precip which will begin
Saturday night.  A fast-moving shortwave will deepen over eastern
Colorado Saturday afternoon/evening forcing lee-side cyclogenesis
according to the GFS.  The low level wave will move across southern
Iowa late Saturday night and into northern Indiana by Sunday
evening.  GFS generates widespread QPF in the warm advection wing of
the storm.  Some of this model generated precip is likely due to the
GFS`s tendency to generate precip in strong return flow situations,
but there probably will be some light rain or snow.  Forecast
soundings that far out are inconclusive, but lean toward a mix of
rain and snow along and south of i-70 with snow further north.
Precipitation should be ending late Sunday afternoon or early
evening with another shot of cold high pressure building back over
the area Sunday night.

Of course, that`s the "World According to GFS" solution.  The ECMWF
continues to be slower in the medium range with a flatter wave
moving off the Rockies on Sunday.  Precip is delayed somewhat and is
less widespread Saturday night.  Additionally the precip continues
through Sunday night.  When the front does finally go through on
Monday the airmass behind it isn`t quite as cold as what the GFS
depicts cold airmass is still up over the Northern Plains. In the
end, I stuck pretty close to ensemble guidance given the timing
uncertainties.

Both the GFS and ECMWF turn even colder on Tuesday next week, but
the ECMWF is more aggressive with the cold air bringing another
Canadian 1040+ mb high down through the Plains.  Trended a bit
colder than ensemble guidance for this period...but there`s still a
while to watch this next potential cold outbreak.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1123 AM CST Wed Dec 7 2016

Have continued a VFR forecast thru this TAF period. There is an
area of SN over ern MO, with a small band stretching into central
MO currently. With the dry low level air in place, expect no more
than flurries at KCOU, with the forcing pushing south this
afternoon. Cigs shud lift and sct out later this evening. KUIN
should only see a bkn to ovc mid deck thru this evening.

Strong and gusty wnwly winds will pick up Thurs morning and
continue thru the afternoon hours.

SPECIFICS FOR KSTL/KSUS/KCPS: Continued a VFR forecast thru the
period. Expect cigs to lower this afternoon as a system approaches
the area. However, currently only some sct flurries are expected.
Strong and gusty wnwly winds will pick up Thurs morning and
continue thru the afternoon hours.

Tilly

&&

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

$$

WFO LSX


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