Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240012

612 PM CST Tue Dec 23 2014

.SHORT TERM: (Tonight through Wednesday Night)
Issued at 251 PM CST Tue Dec 23 2014

The main forecast issue remains the impending storm system and
potential for accumulating snowfall. The models have come into
much better agreement with mass fields than they exhibited 24
hours ago. Yet, there are still some large issues with thermal
fields, qpf, and speed of the system that are resulting in lower
than normal confidence and higher than normal uncertainty.

The overall scenario is unchanged. The upper low is in the
development process within the southern portion of the deep
positively tilted trof across OK. The guidance is fairly well
clustered lifting the upper low northeast into southwest MO by
daybreak Wednesday to near St. Louis at 18z and into the lower
Great Lakes region by 00z. In response the surface low currently
located along the front in LA will deepen and move through the TN
and OH Vallies over the next 24 hours. Initially the warm conveyor
belt and mid level frontogenetical forcing will be the key players,
with rain presently across AR increasing in coverage and spreading
northeast into southeast Missouri this evening and northeastward
into far east central MO and southwest IL overnight. Model consensus
indicates the thermal profiles should begin cooling sufficiently
on the western fringe of the rain shield after 09z for the rain to
mix and begin changing to snow. After 12z Wednesday the large
scale forcing/ascent really ramps up in association with the
ejecting mid/upper low and this should result in an upswing in
coverage and intensity of precipitation from east central MO into
south central and central IL, and accelerated changeover to snow.
All of the CWA should be entirely snow by 18z, as the deformation
zone precipitation begins shifting away from the CWA. Snow amounts
are still a tough call and there is lots of room for error. The
mean track of the H85 low would place the most-favored axis of
highest precipitation from around Belleville to Decatur, but
during some of the time the rain will be changing to snow. Also
surface temperatures will be in the 32-35F range and SLRs are
expected to be lower than normal with a wet snow expected. The new
forecast has the highest amounts along the aforementioned corridor
with a general 2-3 inches. I wouldn`t be surprised to see locally higher
totals, especially as you head northeastward into central IL.
Given potential the impacts of slushy/slippery roads and reduced
visibility on holiday travel, we have gone ahead an issued a winter
weather advisory even though these amounts are generally below
those we typically consider for an advisory.

Some residual non-accumulating light snow/flurries may linger from
east central MO into IL during the first part of the evening,
however this will be a short window. The system continues its
quick departure with ridging aloft already by 12z Thursday and low
level warm advection underway after 06z.


.LONG TERM: (Thursday through Next Tuesday)
Issued at 251 PM CST Tue Dec 23 2014

Pronounced warm advection and backing flow aloft in response to
new upstream amplification will dominate Christmas day. The snow
cover will likely be short-lived as temperatures moderate above
normal Thursday into Friday.

A deep and slow progressive long wave trof will then dominate the
upper air pattern into the weekend featuring separate northern and
southern stream components. Present indications are we should see
a cold front passage on Friday night with an attendant threat of
precipitation. An overrunning pattern could then set-up Saturday
into Sunday with snow possible in the deeper cold air, however the
models are quite variable in their depictions of the important features.



.AVIATION:  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 536 PM CST Tue Dec 23 2014

This is a low confidence fcst...esp at the STL metro terminals wrt
precip timing, type and assoc VSBYs. A an ill defined cold front is
mvng thru the area this evng setting the stage for a tricky fcst for
tomorrow. CIGs lower progressively behind the front eventually bcmng
IFR by mrng. An upper level disturbance will eject out of the sthrn
plains overnight and move NE thru tomorrow to near the Grt Lks by Wed
evng. A sfc low across the deep south will deepen and track into the
TN vly tonight and on into the OH vly tomorrow. An area of precip is
expected to dvlp to our south overnight and move towards the STL metro
area terminals by early Wed mrng. Current indications are that the STL
metro area will be on the wrn fringes of the precip tomorrow mrng. This
poses a couple of issues...first, whether the precip actually impacts
the metro area. KCPS has a better chance than KSUS. Secondly, if the
terminals are not in the heart of the precip, then they will not be
collocated under the best upper level forcing needed to switch the precip
over to snow. Given this scenario, there is a chance that precip may
mix with snow at times but not switch over completely. If this occurs,
then the VSBYs for tomorrow mrng are too low. The possibility exists
for KCPS to see the precip switch over to all snow with some accums
and KSUS to not see much precip at all. It is a tough call attm and
will require updates to capture the short term trends. Any precip should
exit to the NE by 18Z with just some lingering lght precip for the aftn.
KUIN is not expected to be impacted by this system other than an outside
chance at some very lght precip late tomorrow mrng. Cntrl MO may have
a batch of lght precip move in from wrn MO during the aftn in response
to another upper lvl disturbance. This also looks like a mix with no
significant impacts expected. The IFR CIGs should hang tough thru the
end of the prd. Kept the basic flavor of the previous fcst and will
wait for addntl guidance before making significant adjustments.

Specifics for KSTL:

Low confidence fcst into Wed aftn. An upper level disturbance will pass
over the region tomorrow and drive a sfc low into the OH vly by tomorrow
evng. Many questions surround this upcoming system. There is significant
model spread on where the wrn edge of the precip shield will set up
tomorrow. Best guess now is somewhere in the STL metro area. This poses
an addntl problem, if the terminal is not in the heart of the precip
then it will not be collocated under the best forcing needed to switch
the precip over to snow. If that is the case, then the precip may not
mix with or change over to snow at all meaning VSBYs for tomorrow mrng
are too low. If the precip materializes, it should move to the NE by
18Z leaving only lght precip in its wake for the remainder of the aftn.
The IFR VSBYs are expected to remain in place thru the end of the fcst



     Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis MO.

     IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-St. Clair IL-
     Washington IL.



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