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000
FXUS63 KLSX 091136
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
536 AM CST Mon Dec 9 2013

.SHORT TERM:  (Through Late This Afternoon)
Issued at 255 AM CST Mon Dec 9 2013

Early this morning:  longwave TROF axis was over the High Plains
with WSW flow downstream into our region.  Another Canadian high
pressure, centered over the northern High Plains was in the process
of building in, with W flow over our region.  Weak cyclonic flow
remained, especially towards central IL.  Extensive area of lo
clouds is eroding from the NW, with clearing skies already in UIN
and COU areas, but already to the southwest, higher level cloud is
developing over KS and advancing rapidly northeast.  Radar is PPINE,
but due to lo clouds and weak cyclonic flow lingering, cannot rule
out a stray snow flurry or brief incidence of FZDZ over the next few
hours, but additional impact should be nil.  Temps fall off quickly
into the mid teens with clearing, but stay up in the mid-upper 20s
underneath the blanket of lo clouds elsewhere.

The upper level flow out of the W-SW will persist thru late this
afternoon.  Models continue to be in good agreement with a small and
narrow region of intense h850-700 frontogenesis moving into SW MO by
early this afternoon and snaking its way east into the eastern
Ozarks by late this afternoon.  While a lot of the initial shot of
the most intense lift will go into rectifying the rather dry
atmospheric column that will be in place, models generally are a bit
too slow, like with WAA events, in developing pcpn.  Measurable pcpn
threat should still hold off until after 00z, but have added in
flurries for late this afternoon for a good chunk of the eastern
Ozarks.

Otherwise, lo clouds will continue to erode from the NW this
morning, shunting off to far SE MO and S IL by early this afternoon,
but by and large, this will merely be an exchange of one layer of
clouds for another as thick mid cloud overspreads most of our region
by this time, and so will probably only yield a brief period of
sunshine this morning.  Some temp recovery can be expected for areas
to the NW of STL, but will also see the coldest start, while areas
that are the last to lose the lo cloud to the SE will be lucky to
rise a few degrees thanks to persistent CAA thru the day.  MOS max
temps not a bad guess.

TES

.LONG TERM:  (Tonight through Sunday)
Issued at 405 AM CST Mon Dec 9 2013

A shortwave will move quickly ewd through srn MO and nrn AR late
tgt and Tuesday mrng. The models were depicting relatively strong
upper level divergence across srn portions of MO and IL tgt into
Tuesday mrng ahead of an approaching upper level trough and in the
right entrance region of an upper level jet streak moving newd
through the Great Lakes and Ohio Valley regions. It appears that a
fast moving frontogenetical band of snow will move through
portions of sern MO and swrn IL tgt. Prefer the slightly further
south qpf forecast depicting this sw-ne band of the GFS and ECMWF
models tgt over the more nrn qpf forecast of the NAM. The city of
St Louis should be on the nrn edge of the snow with the main band
through the FAM area where at least 1 inch of snow accumulation is
expected. With still some uncertainty as to the exact track of the
snow band will be a little too broad with the qpf and pop forecast
tgt as a sharp nrn gradient of the snow is expected with cold and
dry low level conditions just north of the best forcing. There
will likely be a very narrow axis of around 2 inches of snow
across a small portions of sern MO and swrn IL. A shortwave will
move sewd through the nrn Plains and Great Lakes region late
Tuesday ngt and Wednesday sending a cold front sewd through our
area Wednesday mrng. It appears that this will be a dry cold
frontal passage with the snow remaining north of our forecast area
ahead of the shortwave, although could not rule out flurries
across nern MO and w cntrl IL Wednesday mrng. A strong 1040-1042
mb sfc ridge will build sewd into our area on Wednesday behind
this front leading to unseasonably cold conditions for Wednesday
ngt. Will see warmer temperatures heading into Friday as southerly
flow returns ahead of an approaching upper level trough and
associated sfc low and trailing cold front. Precipitation should
spread into much of our area on Friday and continue through at
least Friday ngt, tapering off by Saturday. Prefer the ECMWF model
over the GFS which seems to be suffering from feedback issues
during this time period. The ECMWF model is a little colder than
the GFS, but would still favor mainly rain over most of our area on
Friday, possibly changing over to snow from nw to se Friday ngt as
the precipitation tapers off as colder air filters sewd into the
area behind the sfc/850 mb low.

GKS

&&

.AVIATION:  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 535 AM CST Mon Dec 9 2013

Lo clouds in the process of clearing out of STL metro sites,
otherwise VFR conditions are anticipated to prevail at the TAF
sites for the remainder of the valid period. A narrow band of snow
is expected to form late this afternoon and exit after midnight,
but should be south of the terminals, being the closest to CPS.
However, the snow band should still be too far south for mention
of precipitation or impact from lo ceilings, with lowest cloud
ceilings remaining above 5kft. Otherwise, NW surface winds will
back SW late tonight and Tuesday morning as a ridge of high
pressure slides thru. LLWS conditions may be attained very late
tonight, especially around UIN, but will late day shift take
another look before adding to TAF.

Specifics for KSTL: VFR conditions will prevail thru the period. A
narrow band of snow is expected to form late this afternoon and
exit after midnight, but should be south of the terminal--too far
south for mention of precipitation or impact from lo ceilings,
with lowest cloud ceilings remaining above 5kft. Otherwise, NW
surface winds will back SW late tonight and Tuesday morning as a
ridge of high pressure slides thru. LLWS conditions may be
attained very late tonight, especially further to the north, but
will late day shift take another look before adding to TAF.

TES

&&

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

$$

WFO LSX