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000
FXUS63 KLSX 071805
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1205 PM CST Sat Dec 7 2013

.UPDATE:
Issued at 1203 PM CST Sat Dec 7 2013

Updated grids and zones as temperatures are a few degrees higher
than originally forecast. Otherwise, everything looks good for
this afternoon under mostly sunny skies.

Byrd
&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 340 AM CST Sat Dec 7 2013

Unseasonably cold weather expected today despite full sunshine as
strong 1043 mb surface ridge centered over South Dakota and ern
Nebraska shifts ewd.  The 850 mb temperatures at noon will only be
about -8 to -11 degrees C over our forecast area.  Will go below the
MOS temperature guidance across sern MO and swrn IL where there is
significant snow cover.  The high temperatures today will be at
least 20 degrees below normal for early December.

GKS

.LONG TERM:  (Tonight through Friday)
Issued at 340 AM CST Sat Dec 7 2013

A longwave TROF will begin this period situated to our west, but
will gradually propagate east across the CONUS during early-mid
week, with the axis passing thru our region Monday night.  By
mid-late next week, a much flatter flow pattern is expected to be in
place.  Overall this signals the very cold air will continue thru at
least the middle of next week with some moderation to take place
afterwards.

The highlights will feature a couple of quick-hitting systems.  The
first will be later tonight and into early Sunday night but will
track much further to the NW than this past system.  It will begin
as a WAA/frontogenetical dominated region of lift pushing up from SW
MO and expanding N-NE towards the I-70 corridor late tonight.  The
lift will be sufficiently deep with this first phase, and enough ice
in the cloud, for resultant pcpn to be snow.  Despite what should be
a brief first round, could see light accums to a half inch thanks to
high anticipated liquid-to-snow ratios from the very cold
atmosphere.  The models all diverge and move out-of-phase the
various lift from this first round by late Sunday morning and
re-focus with the main system that will pass thru further NW.  Pcpn
with this will again be all snow for mainly NE MO and W IL and
should continue into the afternoon before pulling away to the N-NE.
Elsewhere for much of Sunday, we are looking at a lot of lo cloud in
place with very little above it and some weak lo level lift, which
unfortunately, still points towards FZDZ potential.  Have maintained
FZDZ in the forecast as patchy coverage and with the very cold
surfaces already in place, what FZDZ that does form should have
little trouble forming a light glaze.  This should all come to an
end with a cold front sweeping thru later Sunday night and
reinforcing the cold air.

The other item of interest is centered around Monday night.
Broadscale lift is expected to increase with passage of the longwave
TROF axis thru our region, and combined with favorable quadrant from
a 180kt h300 jet streak and intensifying frontogenesis to focus the
lift, EC/GFS/SREF and now 06z NAM are pointing at potential for
narrow band(s) to form across central-southern MO and southern IL.
Something like this is hard for the models to get a good handle on 3
days out and will take a cautious approach and introduce flurries
for now.  But something to watch.

TES
&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1203 PM CST Sat Dec 7 2013

Surface ridge to begin moving off to the east later today, so
winds to veer to the northeast then east through tonight. Then
warm front to develop along MO/AR border by daybreak and slide
north through forecast area on Sunday. Decent lift along boundary,
as well as waa and some moisture so will see mid cloud deck develop
this evening and lower over time to MVFR. Will see snow begin
after 12z Sunday over central MO and spread north and northeast
through 18z Sunday. Could see cigs lower to IFR with main area of
pcpn, but too far out at this time so only have it mentioned in a
prob30 group for KCOU between 15z and 18z Sunday.

Specifics for KSTL:
Surface ridge to begin moving off to the east later today, so
winds to veer to the east by 03z Sunday. Then warm front to
develop along MO/AR border by daybreak and slide north through
forecast area on Sunday with winds veering to the southeast by
18z Sunday. Decent lift along boundary, as well as waa and some
moisture so will see mid cloud deck develop by 03z Sunday and
lower over time to MVFR by 14z Sunday. Will see snow begin after
14z Sunday, tapering off by 18z Sunday as it will be a fairly
quick shot of light snow for metro area. Could see cigs lower to
IFR with main area of pcpn, but confidence is low and did not
mention at this time.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$
WFO LSX