Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 291141
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
541 AM CST Mon Dec 29 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 345 AM CST Mon Dec 29 2014

Today should be another typical late December day with the CWA
between two different systems. The previous cold front that pushed
Thur the region on the 27th is still lingering across the sern US
with continued precip. Of importance to our area is the lingering
stratus that never completely cleared out of the sthrn zones last
night. Now that the low level flow has become erly on the backside
of the sfc ridge, this cloud cover is pushing back into the ern
Ozarks from sthrn IL and the MO bootheel early this mrng. Not
completely sure how far NW the clouds will make it before winds turn
nrly this aftn and push it back to the south. The other system is an
Arctic cold front that currently stretches from the lower tip of Lk
MI to the IA/MO border and west along the KS/NE border. This bndry
denotes the wind shift and this should push thru the CWA today. The
truly cold dry Arctic air lags behind this bndry considerably and
should arrive tonight.

2%

.LONG TERM: (Tonight through Sunday)
Issued at 345 AM CST Mon Dec 29 2014

(Tonight-Wednesday)

Guidance remains in fairly good run to run and model to model
agreement with the timing and intensity of the Arctic airmass that
will arrive tonight and remain over the region into midweek.  Other
than a few minor tweaks to hourly temps tonight and into Tuesday
that try to reflect the impact of the Arctic air (falling temps
through at least mid morning Tuesday, followed by a very minimal
warmup in temps during the afternoon) haven`t made any significant
changes to going forecasts.  Certainly...the coldest temps will be
Tuesday night when overnight lows dip into the single digits and
teens...with highs on Wednesday only rebounding into the 20s.  Have
continued mention of a few flurries with the arrival of the cold air
overnight tonight and into Tuesday morning.

(Thursday-Sunday)

Medium range concerns remain focused on cutoff low that forms over
the west coast over the next few days, and its evolution and
movement as it pushes into the central CONUS by the end of the week.
As expected, each cycle of the medium range guidance continues to
slow the eastward progression of this system, with consensus now
suggesting the upper low will be over the TX panhandle by midday
Saturday...a full 12-18 hours slower than the solutions of yesterday
at this time.

In spite of the slow down, there still appears to be a threat of
precip across the area late Thursday night and Friday (as depicted
in current forecast), tied to the initial stages of WAA as the
Arctic airmass begins its retreat.  The 00z GFS wants to keep most
of this precip south of our CWA, while the ECMWF pushes the northern
edge of the precip shield into northern sections of the CWA by
Friday.

Other than this initial shot of precip well ahead of the system
(sometimes referred to as a "throat-clearing" for storms coming out
of the SW), lots of uncertainty remains with the eventual impact of
this storm on our CWA.  Trying to pick a particular solution at this
early juncture would primarily be speculation and/or "wishcasting"...
so plan to continue with a very broad brush approach with both PoPs
and ptype for now. The only real adjustment to ptype will be to
trend them a bit more to rain over southeast sections of the CWA
after Friday morning into Saturday night as both GFS and ECMWF depict
a decent warmup of the AMS over this area. It will probably be several
more days (at least) before we get a better handle on this system.

Truett

&&

.AVIATION:  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 524 AM CST Mon Dec 29 2014

VFR fcst thru the prd. Area is between systems today. The MVFR stratus
from yesterday never really pushed out of the area overnight and is
expanding back to the west over the ern Ozarks early this mrng. Not
sure how far to the NW the low CIGs will make it before nrly winds
this aftn begin to push the clouds back to the south. Of more concern
is a strong cold front that will push across the area thru the prd.
The wind shift will progressively occur from late mrng at KUIN to
the aftn along I70. This will not be a sharp change of direction
but rather gradual over several hrs. The real push of Arctic air
doesn`t arrive until this evng when wind speeds increase to near
10kts. Some models are indicating that there will be a band of
MVFR clouds overnight but not enough model consensus to include
attm. There may also be some flurries assoc with that band of
stratus/stratocu. This is not expected to be a significant impact
with MVFR visibilities at the worst.

Specifics for KSTL:

VFR fcst thru the prd. Winds should gradually become nrly this
aftn as an Arctic cold front works thru the area. Winds increase to
near 10kts overnight as the real push of the colder air arrives.
There may be a band of MVFR CIGs assoc with the push of Arctic air
with some flurries also possible.

2%

&&

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

$$

WFO LSX





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