Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201746
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1146 AM CST Sat Dec 20 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 320 AM CST Sat Dec 20 2014

Dirty ridge remains over the area today with abundant low level
moisture trapped beneath an inversion around 850mb.  Flurries are
currently falling at WFO LSX out here in Weldon Spring, so have
updated the forecast to include flurries along and west of the
Mississippi for a few hours this morning.  Forecast soundings
indicate that the lower atmosphere should warm sufficiently to
preclude any frozen precipitation by around 15Z, and weak low level
lift should be subsiding by mid-day as the upstream shortwave which
is currently over northwest MO/eastern NE moves into Illinois.  Was
a little too cool on yesterday`s highs...and today`s temperatures
may be tricky as well with variable cloud heights and thicknesses
across the region.  Used a blend of MOS guidance, but tried to lean
toward the warmer numbers except over northeast Missouri and west
central Illinois where clouds should generally be thicker allowing
for less insolation.

Carney

.LONG TERM:  (Tonight through Friday)
Issued at 320 AM CST Sat Dec 20 2014

(Tonight)

Southerly flow is expected to take some toll on the rather extensive
area of low clouds currently entrenched across the area.  The areas
that stand the best chance of seeing some clearing will be south and
east of STL metro and went cautiously optimistic with the forecast.
Any clearing should be short-lived with mid and high clouds
increasing heading into Sunday with the next system.

Prefered the warmer MET MOS for areas north and west of STL metro
where clouds should hang tough, but a MOS blend preferred for areas
in the south and east where some clearing looks likely.

(Sunday - Wednesday)

The beginnings of what will be a deep upper level TROF is expected
to develop later on Sunday over the central CONUS and then take at
least until Wednesday before finally shifting east.

An initial impulse will begin to lay the groundwork for this to
happen late Sunday night and Monday...with PoPs primarily confined
to the northern part of MO and central IL.  The highest PoPs were
reserved for Monday Night and Tuesday with a very favorable synoptic
setup with a more mature deep TROF, upstream, and a significant
impulse rotating around the base of the TROF.  Pcpn-types thru
Tuesday afternoon look to be all liquid, however.

Things begin to get interesting Tuesday night into Wednesday with
most model solutions closing the system off and with the cold air
thus generated from this closing off, the pcpn will turn to snow.
Where and when this system closes off will be critical.  The current
model consensus favors this to occur late Tuesday night almost
directly overhead of our region.  If this stands, snow will be
possible at the end of the event later on Tuesday night and
transition to snow showers Wednesday with light accums possible.
Something to watch closely...because...an earlier closing off and/or
a more western location for the closing off of the system (per the
DGEX and 06z NAM) would mean a substantial increase in snowfall
potential for parts of our area...such as what the consensus
solution currently shows for parts of eastern IL.

Temps will be at or above normal thru Tuesday, with a return to
below normal values on Christmas Eve in the wake of the system.

(Christmas)

Models continue to advertise a transitional pattern this day which
is a good sign for quiet wx with near seasonal temps.

(Friday)

Models all diverge significantly on how to handle the next system
and so very low confidence on not just PoPs but also pcpn-types with
a rather large spread in temp profiles.  A lot of how this pans out
will depend on exactly how the Tuesday-Wednesday system develops.

TES

&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1145 AM CST Sat Dec 20 2014

Seeing some holes in the clouds today, some making their way
towards STL metro area. But for the most part continue to see MVFR
cigs most locations, though did add tempo mention of IFR cigs at
KUIN for a few hours this afternoon. Winds to persist from the
south through the forecast period.

Specifics for KSTL:
Seeing some holes in the clouds today, some making their way
towards STL metro area. But kept MVFR cigs for now. Winds to
persist from the south through the forecast period.

Byrd
&&

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

$$
WFO LSX





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