Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270939

339 AM CST Sat Dec 27 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 338 AM CST Sat Dec 27 2014

Don`t be fooled by the mild conditions at daybreak; temperatures
will fall during the day as a cold front moves across the region. At
08z, the cold front stretched from SWRN MO through NERN MO, and was
just about to move through KIRK.

It looks like there will be a break in the rain this morning in many
areas before the next batch moves northeastward out of OK. Although
the vort max over Mexico in the base of the trough is forecast to
move through TX and remain south of MO/IL, an elongated shear axis
develops between that vort max and the one currently over NE/IA,
which will be located over SERN Ontario by 00z tonight. Despite the
slightly confluent flow aloft downstream of the vort max and shear
axis, pcpn across the southeastern half of the CWA is expected to be
supported by isentropic ascent along the 300-305 K sfcs (especially
after 18z), frontogenesis between 900-800mb (local WRF model fields
at 09z had a fair handle on the precip in eastern OK), and PW values
between 0.8" and 1" (+2SD for LSX in December).


.LONG TERM:  (Tonight through Friday)
Issued at 338 AM CST Sat Dec 27 2014

(Tonight through Tuesday)

Main forecast problem tonight is how much precip to hold onto over
southeast sections of the CWA.  00z NAM certain appears to be the
outlier in ending almost all of the precip by 00z, while all of
the other synoptic scale models (GFS, ECMWF, and GEM) keep precip
over about the SE half of the CWA through 06z, with some lingering
in our far SE counties past midnight. Given that the trof axis
will still be well to our west by this evening feel that the
slower solutions are the way to go. Going PoP trends for tonight
still look pretty good (main tweak was to raise PoPs a bit over
our far SE counties in the 00-06z time frame to near 50%), but
anticipate that day shift will need to fine-tune these a bit more
once precip trends going into the mid-late afternoon become more
apparent. Will also mention of the rain/snow mix on the back edge
of the precip shield as the low level cold air deepens and
freezing levels drop.

The cold high pressure building into the region in the wake of
today`s front will push into the area overnight and control the
weather across the area on Sunday with more seasonable and dry late
December conditions.

All 00z guidance is in good agreement that E-W trof will be dropping
from Canada into the CONUS during the start of the new work week.
The resultant lowering of heights will allow Arctic air to build
into the northern Plains and upper Mississippi Valley on Monday, and
then spread south from the southern Plains into the Ohio Valley by
Tuesday. This Arctic AMS appears to be fairly robust, with model
consensus indicating 850mb temps of at least -10 to -12C over the
northern half of the FA on Tuesday. Due to the intensity of the
cold air, it may be that hourly surface temps will have trouble
rebounding too much from their morning lows. Will also add a few
flurries to northern sections of the CWA on Monday night, tied to
the arrival of the Arctic air.

(Wednesday through Friday)

Medium range solutions are still suggesting the evolution of a split
flow regime heading into the latter half of the week.  Energy on the
western side of the aforementioned E-W trof is forecast to break
off and lead to the development of a cut-off low over the west
coast.  Meanwhile, eastern extension of trof rotates into the
eastern U.S. and is followed by additional shortwaves rotating
around cut-off low near Hudson`s Bay that produces broad trofing
over the eastern half of the U.S.

In the initial stages of this evolution, the weather over our area
should be relatively quiet as dynamics over the region remain fairly
benign.  The cold weather of Tuesday should continue into Wednesday
as the Arctic air remains locked over the area, with some modest
moderation in temps by Thursday.

However, the weather across the region may become increasingly
active heading into the last part of the week as the cut-off low
over the west coast begins to push east.  While there is agreement
in this overall trend the specifics between the different models
vary greatly and in an important way; the GFS takes the system in a
generally eastward track and brings it across the central Plains by
Friday night, while the ECMWF is much slower and much further south
with the upper system, taking in into w TX by 12z Sat.  Either
solution would suggest precipitation chances increasing across parts
of the CWA in the Thursday night/Friday time frame, but obviously
extent of the increase and exact winter ptype will be dependent on
which solution is more correct.  At least through Friday night gut
hunch is that ECMWF may be more correct as models are notoriously
too fast ejecting cut-off lows out of the western U.S. So for now
have kept PoPs in chance category late Thursday night and Friday
and have gone with a simple rain/snow mix...obviously details
will be refined with time. System bears watching so stay tuned...



.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1043 PM CST Fri Dec 26 2014
Area of light rain should exit the KSTL metro area by around 08Z,
with only occaisional sprinkles thereafter through the night. MVFR
ceilings should hold through the night and into the morning on
Saturday at all TAF sites. Ceilings will gradually start to lift
to VFR range by mid morning at KUIN, then at KSTL by mid
afternoon. Cold front will reach KCOU by around 12Z, and KSTL by
around 16Z with winds veering to 280 degrees behind the front.

Specifics for KSTL: Main area of light rain should exit KSTL TAF
site by around 08Z. Only isolated very light showers or sprinkles
are expected for the remainder of the night. MVFR ceilings will
last through the night and into early afternoon on Saturday, then
gradually lift to VFR range before sunset. Ahead of an
approaching cold front, winds will veer to 230 degrees around 14Z,
then to 280 degrees after frontal passage around 16Z.





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