Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231801
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1201 PM CST Tue Dec 23 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 330 AM CST Tue Dec 23 2014

A nearly stacked LOW pressure system was over southwestern IA early
this morning, with a dry slot on its southern flank pushing thru MO
and southern IL.  Precipitation has effectively been shunted way to
the north as a result; near the I-80 corridor.  Meanwhile, a strong
upper level disturbance was moving into western TX already forming
what will become our next precipitation event later today and into
Wednesday.  At the surface, a weak cold front has pushed thru much
of our region, getting thru STL metro within the past hour or so,
allowing temps to drop thru the 40s and upper 30s.  Still mild by
late December standards.

The stacked LOW pressure system is expected to head northeastward
into the western Great Lakes today and weaken, as it seeks to rejoin
the main flow...while the strong disturbance in TX will take full
advantage of the situation and drive into eastern TX and southern OK
and begin to close off and become the dominant LOW within the deep
TROF.

For much of the day, our region will be in-between systems and will
instead see increasing and thickening clouds and southwest winds in
an otherwise dry and mild day.  Max temps between 45-50 for most
seems reasonable.

The main change, however, will be that frontogenetical forcing from
the new dominant southern system will reach our region a bit sooner
and is expected to result in earlier pcpn chances during the
afternoon for parts of southeast MO and southern IL.  Already radar
is showing pcpn echo approaching the ARK-LA-TX region and new model
data seems to have this well in hand.  Pcpn-types for today are
expected to be rain thanks to too warm surface temps and boundary
layer conditions.

TES

.LONG TERM:  (Tonight through Monday)
Issued at 330 AM CST Tue Dec 23 2014

(Tonight - Christmas Eve)

A good model consensus continues for this system with a small closed
LOW expected to track from eastern OK northeastward into the
Michiana region by nightfall. And so while there is expected to be a
lot of pcpn development over the OH valley with the WAA component, a
full band of deformation zone pcpn is anticipated to develop later
tonight and continue thru the day on Wednesday that is located
further back in the cold air that could result in a significant
winter event.

The model consensus for the axis of this band of deformation pcpn is
from the area midway between UNO and POF in southeast MO thru Metro
STL East to just east of PIA in central IL.

However, trends between iterations continue to nudge this axis a bit
further west and ensemble means would also support this as well
having been consistently to the west over the past 24-36hrs.

QPF in this band should be stout too, with very strong
frontogenesis, a weak TROWAL, and a compact but very dynamic upper
level system that is more reminiscent of a late winter/early spring
system by some of the levels of moisture it will be able to tap into
thanks to the highly amplified flow out ahead of it.

The problem is that there is no real quantity of cold air to tap for
this and despite sufficient cold air aloft, boundary layer and
surface temps will be where the pcpn-types will be determined, and
it will be a rain versus snow question, fortunately, with sleet and
freezing rain not supported, and if they do occur, should be very
brief.  The very strong dynamical nature of this system, however,
should allow for decent cooling of the boundary layer with NAM and
GFS both supporting temps between 32-35F underneath the max pcpn
axis.  With temps such as these, pcpn-types will transition from
rain to snow and this should occur from later tonight for areas to
the north and west of STL, and to some degree into parts of the
eastern Ozarks of southeast MO as the band gets setup, with a
changeover to snow in STL metro during Wednesday morning and lastly
in parts of southwest IL during early afternoon.  The timing for all
of this is about a few hours after the band first sets up into these
particular areas.  Once the changeover occurs, it looks like there
will be about 4-5 hours of snow before it moves on.

Putting it all together, the warm ground and above freezing surface
temps will result in a wet snow with low snow-to-liquid ratios.
Where rates get heavy enough, you will see accumulation on grassy
surfaces.  Where you get rates heavy enough and persistent enough,
you will probably also get accums on roads as well.  The most
probable snow accum forecast looks to be 1-2" from the eastern
Ozarks of SE MO thru STL metro and into central IL.  This discounts
mesoscale effects, which will not be able to be realistically
forecast with any degree of confidence until probably just before or
during the initial onset of the event.  When one considers this, the
potential for a much heavier snow forecast does loom.

(Christmas)

A transitional pattern will be in place this day with surface winds
turning from the south and strengthening.  Should see a quiet wx day
with temps returning to above normal levels.  A rare moment of
sunshine is thus expected and any snow that survives into this day
will probably not make it to the end.

(Friday - Monday)

The upper TROF becomes re-established over the central CONUS early
in this period and then persists into early next week.  The
orientation of the TROF is such that a W-SW mid level flow will
result which will provide our region with some opportunities for
pcpn, and some of them should be snow with cold air favored to be
able to drop down in this setup and receive reinforcements
periodically.  As it stands now, the models favor much of the pcpn,
and best PoPs, to get shunted to our south and east, however.
Heading into the middle of next week, the long rumored Arctic air
intrusion seems to be on track as we approach 2015.

TES
&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1201 PM CST Tue Dec 23 2014

Mid and high clouds over forecast area through early this evening,
then cigs begin to lower as next system approaches region. Will go
down to MVFR by 03z Wednesday for STL metro area while KCOU/KUIN
will not lower til around 09z Wednesday. KUIN and KCOU will be on
the northern fringes of this system with precipitation mainly in
the form of light snow by 14z Wednesday. As for STL metro area,
precipitation to begin as rain around 06z Wednesday, changing to a
mix of rain and snow by 12z Wednesday, then all snow by 15z
Wednesday. Once snow kicks in cigs to lower to IFR all locations
and persist through rest of forecast period. As for winds, south
winds to veer to the west then northwest by this evening and
pickup a bit by Wednesday morning.

Specifics for KSTL:
Mid and high clouds over forecast area through early this evening,
then cigs begin to lower as next system approaches region. Will go
down to MVFR by 03z Wednesday for STL metro area. Precipitation to
begin as rain around 06z Wednesday, changing to a mix of rain and
snow by 12z Wednesday, then all snow by 15z Wednesday. Once snow
kicks in cigs to lower to IFR and persist through rest of forecast
period. As for winds, south winds to veer to the west then
northwest by 03z Wednesday and pickup a bit by Wednesday morning.

Byrd
&&

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

$$
WFO LSX




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