Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241744
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1144 AM CST Wed Dec 24 2014

.UPDATE:
Issued at 1135 AM CST Wed Dec 24 2014

Expecting predominately light rain/drizzle this afternoon across
eastern MO and into western IL as the next vort max rotates
through the area in the wake of the upper low. The latest MDCRS
sounding from KSTL still showing the freezing level is near 1,800
ft AGL and surface temps are in the 35-40 range so any snow will
be hard to come by and brief. Temps holding nearly steady this
afternoon with solid low overcast and weak cold advection.

Glass

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 351 AM CST Wed Dec 24 2014

As greatly advertised, a deep upper level TROF does indeed exist
over the central CONUS early this morning. However, there really
is no true closed circulation at this time anywhere. Rather a
sharply defined shear axis is creating a quasi-circulation over
northwest MO. The strong disturbance that was supposed to create a
new southern LOW center by this time is over southern AR.
Satellite has filled in with cloudy skies across the forecast
area, but not too long ago, there existed a region of mostly clear
skies back into northeast and central MO which allowed sfc temps
to cool into the lo-mid 30s. Other areas remained in the upper 30s
and lo 40s with the warmest air over in the Salem IL area. Radar
mosaic shows the system pcpn finally showing its hand, with banded
pcpn and pcpn reaching the ground forming from Farmington, MO to
Metro STL East to just west of Litchfield, IL. Some additional
echo is forming still further to the west over south-central MO in
response to the approaching shear axis but no pcpn being reported
but rather this is mid cloud.

The models still are anticipating the energy and focus on this
complex system to eventually transfer to the south and east, but
the current model consensus does not expect this to occur until
late this morning if not midday. By this point, the pcpn will be
exiting our region and there will be very little impetus to push
the pcpn back westward into the colder air until it is too late.
As a result, we are looking at a more easterly QPF axis and less
of a push to create snow as a pcpn-type.

Have forecasted less pure snow and instead now believe a rain-snow
mix will be the best it can do even with accounting for dynamical
cooling processes. Without a clear closed circulation center
before it is by our area, it is just going to be too tough with
several other factors already fighting it, such as warm ground and
an initially warm boundary layer.

Snow amounts have been scaled back to an inch or less and that
could be overdone...with the western edge cutting thru roughly
from Litchfield, IL to Belleville, IL. Factoring in also the
previous forecast, have decided to continue the Winter Wx Advisory
for a smaller region which includes Metro East and parts of
southwest IL, with the Advisory cancelled for STL City and MO
sections. If snow falls in the western Metro it should not
accumulate on roads with minimal to none on grassy surfaces with
rates not expected to be much.

Add to all of this some scattered -SHSN/-SHRA with region of deep
cyclonic flow passing into our region later this morning and into
the afternoon from western MO, but accums if any will be
negligible.

Went on the cool side of MOS, but that should still yield maxes in
the upper 30s or so.

TES

.LONG TERM:  (Tonight through Tuesday)
Issued at 351 AM CST Wed Dec 24 2014

Any lingering light snow in southwest IL will taper off tonight as
the low pressure system continues moving away from the area.

Sun-starved inhabitants can look forward to a present from the
atmosphere on Thursday, as skies will likely be partly to mostly
sunny for at least part of the day. Daytime high temperatures will
be around 5-10 degrees above average for this time of year. In
addition, there should be a bit of a SW or S breeze due to the
tightening pressure gradient ahead of the next low pressure
system, which is tentatively expected to affect MO/IL between
Friday night and Sunday night.

Ahead of this system, WAA will lead to daytime temperatures of
around 10-15 degrees above average on Friday. The associated cold
front will then move through the region on Friday night and
Saturday. The initially warm temperatures will keep the ptype as
pure rain on Friday night, with snow becoming possible on Saturday
once temperatures have cooled within the column.

A new H5 trough develops over the western CONUS early next week,
but there are differences between the GFS and the ECMWF on how the
vorticity maxima eject out of the trough, leading to different
depictions of the large-scale pattern between Monday and Tuesday
night.

An Arctic intrusion is anticipated during the middle of next week
around the beginning of the new year. Such an intrusion is
consistent with recent teleconnection signals.

Kanofsky

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1135 AM CST Wed Dec 24 2014

The back edge of IFR flight conditions was near a KVFR-KFSW line
late this morning with MVFR conditions extending well west into
the Plains. IFR flight conditions with light rain/drizzle and
possibly a few snowflakes will be predominate through the afternoon
across eastern MO into western IL including the St. Louis area
terminals. Conditions will be improving to MVFR during the afternoon
in central MO including KCOU. MVFR conditions will prevail much of
tonight at all terminals with gradual clearing and improvement to
VFR from late tonight into just past daybreak on Thursday.

Specifics for KSTL:

IFR flight conditions with light rain/drizzle will prevail through
the afternoon. I can`t rule out a few snowflakes but nothing of
significance is currently anticipated. Conditions will be improving
to MVFR by early evening and then prevail tonight. Clearing and
improvement to VFR is currently expected just past daybreak on
Thursday with increasing southerly winds during the day on Thursday.

Glass

&&

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

$$

WFO LSX






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