Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 100527
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1127 PM CST Fri Dec 9 2016

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 324 PM CST Fri Dec 9 2016

Surface ridge will be overhead tonight, allowing winds to become
light and variable. Only issue will be temperatures tonight
depending on cloud cover. Strato cu to our northeast is slowly
eroding this afternoon, in the meantime mid and high clouds are
streaming in. So for now will see lows in the mid to upper teens.

On Saturday, surface ridge moves off to our east allowing southerly
winds to return once again. But with more clouds over the region as
next system approaches region, temperatures will remain just a bit
below normal in the 30s.

Byrd

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 324 PM CST Fri Dec 9 2016

(Saturday night through Monday)

Focus for this period remains the system impacting the region Sun
and Sun evening.

Mdls have come into better agreement thru this period with respect
to mass fields, but also thermal profiles, tho to a lesser degree.
Period begins with sfc ridge building ewd out of the region allowing
sely flow to return to the area. The lee cyclogenesis as the s/w
ejects into the Plains will help winds become more sly on Sun and
increase WAA across the region.

Expect min temps Sat night to be below freezing, however, this shud
occur early in the period with temps holding or rising overnight.
There remains a fair amount of uncertainty regarding p-type Sun
morning. Part of the uncertainty is the start time of the precip.
All mdls depict rather strong WAA over the area. However, the
airmass in place is quite dry in the boundary layer. The struggle
will be if the boundary layer can saturate before sfc temps warm
above freezing. Have therefore kept low chances of FZRA Sun morning.
With such strong WAA, expect temps to warm above freezing despite
the diabatic cooling from precip. Expect p-type to remain all RA
thru the remainder of the event with the precip dissipating before
enuf cold air arrives to change p-types.

(Tuesday through Friday)

Mdl solns quickly diverge beyond Mon with differences in timing as
well as overall evolution. Temps on Mon thru Tues will be relatively
warm with a weak sfc ridge quickly pushing thru the area.

Have kept low PoPs for Tues night and Wed with the GFS suggesting a
snow event. There is some support from the GEM, tho timing is
different. The ECMWF suggests a deeper upper trof and stronger sfc
ridge, delaying this system. This event may potentially have some
quite high SLRs based on progd soundings from the GFS. However,
would like mdls to be in better agreement before worrying too much
about this event.

A very cold airmass, 1032+ mb at the sfc with -10C or lower 850mb
temps on Thurs, moves into the area Thurs. Have trended twd the
cooler guidance for Wed and beyond. These temps may need to be
altered depending on how potential snowfall on Wed evolves.

Tilly

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1127 PM CST Fri Dec 9 2016

VFR conditions and dry wx will prevail at the TAF sites thru the
valid period. Otherwise, light and variable surface winds will
become southeasterly on Saturday and strengthen somewhat heading
into Saturday night.

TES

&&

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

$$

WFO LSX



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