Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211738
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1138 AM CST Sun Dec 21 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 340 AM CST Sun Dec 21 2014

A full latitude trof is moving through the Great Plains this
morning; and although the pattern is relatively low amplitude, the
trof will begin deepening today as the strong shortwave over the
Pacific Northwest moves rapidly across the Rockies and dives into
the Great Plains.  This will result in increasing southerly flow in
the low levels as the trof deepens over the Plains.  Unfortunately,
plenty of moisture will remain trapped under the persistent
inversion which has been locked in over the CWFA for the past few
days.  Present indications are that the sky will remain cloudy
across most of the area today; but there is potential for some
breaks in the overcast during the afternoon along and south of the
I-44 corridor.  Highs today will likely be near or just a few
degrees warmer than yesterday`s temperatures.

Carney

.LONG TERM:  (Tonight through Saturday)
Issued at 340 AM CST Sun Dec 21 2014

(Tonight - Wednesday)

There remains good model consensus on a deep longwave upper level
TROF becoming established over the central CONUS by Monday...right
over our region...and then taking until Wednesday before exiting.
It is the specifics on the last 24 hours that remains elusive and
probably will for at least another day or so until a critical
component can finally come ashore.

The initial phase tonight and Monday will feature some deepening of
the TROF to our west with most of the pcpn staying to the N and W of
STL metro.  Pcpn-types here will be all liquid.

The middle phase Monday night and Tuesday will then feature
significant deepening and amplification of the TROF with higher
levels of moisture and additional disturbances riding northward.  At
this time, it looks like much of the moisture will get shunted off
to the east and so have the best PoPs generally for STL metro and
areas to the S and E.  Pcpn-types once again will be all liquid.

The final phase for Tuesday night and Wednesday, arguably the most
important one, is where the models still have significant diffs on.
All solutions continue to trend slower with bringing the TROF axis
thru our region, but differ on exactly how to do that.  The GFS is
the most extreme in closing off a LOW over western AR and then
tracking it northeast into the lower OH valley.  This would paint a
solution of accumulating snow for a good chunk of our region late
Tuesday night into Wednesday (Christmas Eve).  The EC and GEM
continue to lead the opposition by maintaining an open wave with
this southern disturbance and maintaining a LOW to the north.  This
would end up leaving very little pcpn back in the cold air with what
edges us from the deformation zone well to the north and scattered
SHSN in its wake.  Spotty dustings of snow would be the likely
result here.  Both the 00z and 06z NAM have somewhat more of a
compromise between these two camps.  Vertical temp profiles favor
rain changing to snow for most areas Tuesday night, especially west
of STL city, continuing to trend snowier for Wednesday.  Sufficient
cold air should be aloft to not have to worry about mixed pcpn,
especially if the GFS verifies, with upper LOs notorious for
manufacturing their own cold air, and will almost exclusively come
down to temps in the lowest 2kft of the atmosphere, with decent
correlation to surface temps:  where surface temps are mid 30s or
below, this should favor mostly snow, with liquid for upper 30s and
up.  Have maintained the going course of preferred snow for the
pcpn-types but spreads are much too large to properly speculate on
amounts or impacts at this time, but with surface temps mostly above
freezing and recent mild air, it will take strong snowfall rates to
successfully achieve decent accums anyway and would need the GFS
solution to win out.

Mild temps forecast thru Tuesday, with below normal temps for
Wednesday.

(Christmas)

Models continue to advertise a transitional pattern this day with
quiet wx and temps returning to above normal levels.

(Friday - Saturday)

There is better model agreement than 24hrs ago, with a
re-establishment of a longwave upper TROF over the central CONUS and
an area of sheared vorticity moving thru.  Coupled with a cold front
on Friday, it is worth a PoP mention but amounts look sparse at this
time.

TES
&&

.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1132 AM CST Sun Dec 21 2014

Southeast flow at the surface is helping to bring in drier air, so
cigs have lifted to low end VFR at taf sites along I-70. KUIN
should see cigs lift to low end VFR by 00z Monday. Otherwise,
winds to persist from the southeast to south through forecast
period. Next weather system to approach region tonight, will see
rain develop over western MO and spread east, moving into KUIN and
KCOU by 14z Monday and metro area by 18z Monday. When rain begins
to move in, will see cigs/vsbys lower to mvfr.

Specifics for KSTL:
Southeast flow at the surface is helping to bring in drier air, so
cigs have lifted to low end VFR in metro area. Otherwise, winds
to persist from the southeast to south through forecast period.
Next weather system to approach region tonight, will see rain
develop over western MO and spread east, moving into metro area by
18z Monday. When rain begins to move in, will see cigs/vsbys lower
to mvfr.

Byrd
&&

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

$$
WFO LSX




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