Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 240551
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1151 PM CST Tue Dec 23 2014

.UPDATE:
Issued at 821 PM CST Tue Dec 23 2014

Digesting the 00Z NAM and HRRR runs and combined they continue to
support the going forecast with a band of rain developing
overnight along and east of the Mississippi River, then mixing/changing
to snow from west to east during the morning hours. Accumulation
will be most likely under the heaviest and most persistent band of
snow which looks to roughly align from KBLV to KTAZ. Only minor
changes were made to the forecast as the placement of the
mesoscale snowband and final adjustments to the forecast will be
a nowcasting exercise by the next shift.

Issued at 650 PM CST Tue Dec 23 2014

Quick update this evening to lower POPs, as it appears that other
than some areas of drizzle across southwest Illinois, the better
shower activity will not develop until after midnight. Still
appears that the majority of the precipitation should fall as rain
tonight, perhaps mixing with snow very late on the western fringe
of the precipitation that develops overnight.

Hope to have another update out by 10 PM after some of the 00Z
model guidance arrives regarding any changes needed to the
snowfall forecast for tomorrow.

CVKING

&&

.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.

Glass

.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.

Glass

&&

.AVIATION:  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1124 PM CST Tue Dec 23 2014

Tricky low confidence fcst for Wed. A cold front has pushed thru
all the terminals this evng. CIGs should slowly lower to IFR by
Wed mrng and then remain in place thru the end of the fcst prd. An
upper lvl disturbance will force a sfc low across the deep south
to strengthen overnight as it moves to the ern Grt Lks by tomorrow
evng. An area of precip will dvlp and move towards the STL metro
area late tonight. There is some uncertainty about how far west
the precip shield will extend. Current guidance indicates that the
precip should mix with and potentially change over to snow for a
few hours Wed mrng, though new model solutions this evng are
beginning to trend away from this scenario. There is some question
about when or where the changeover to snow will occur. It is
possible that KSUS may not see much precip at all while KCPS has
accumulating snow during the late mrng into early aftn. Am not
confident that any terminal in the STL metro area will see snow.
If that occurs, then VSBYs are too low. Am not inclined to change
the fcst until the precip dvlps. There is another upper lvl
disturbance that will track into cntrl MO Wed mrng and then push
east thru the aftn producing some addtnl VFR -RASN. Any precip
should come to an end by evng.

Specifics for KSTL:

An area of precip is expected to dvlp south of the area and move
north into the terminal prior to sunrise. Precip is expected to mix
with and potentially switch over to all snow during the late mrng.
Am not confident in this scenario as new model guidance this evng
has trended away from this solution but, am inclined to leave the
basic details intact until the precip dvlps and details become
better defined. If the switch over to snow does not occur then
VSBYs are too low tomorrow aftn. The main precip shield should
lift north of the terminal by early aftn. Occnl VFR -RASN should
continue thru the remainder of the aftn in the wake of this system
and as another upper level disturbance approaches from cntrl MO.
IFR CIGs should hang tough thru 6Z.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR
     Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis MO.

IL...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR Bond
     IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-St. Clair IL-
     Washington IL.

&&

$$

WFO LSX






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