Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250533
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1133 PM CST Wed Dec 24 2014

.UPDATE:
Issued at 831 PM CST Wed Dec 24 2014

Already updated the fcst once this evng to account for the
lingering precip across the area and it looks like another update
will be necessary as the pesky light rain and drizzle will be slow
to exit the area. The precip has slowly been transitioning from
measurable to trace over the past several hrs. So that will be
the flavor thru midnight...lingering patchy drizzle. Temps have
also been slow to fall off and have been holding steady so far
this evng despite the CAA. Still think lows for tonight look good
with upstream obs across wrn MO in the low 30s. All other elements
look reasonable with only minor adjustments to account for short
term trends. A new update will be out shortly.

2%

&&

.SHORT TERM:  (Through Tonight)
Issued at 325 PM CST Wed Dec 24 2014

An area of light rain possibly mixed with a few wet snowflakes
currently centered through St. Louis was moving east around 25-30
kts. This precipitation is occurring in association with a trailing
vort max within the cyclonic flow on the backside of the exiting
upper trof. Present indications are it should have exited or just
about exited the eastern CWA counties in south central/southwest IL
at 00z. There might be some lingering patchy drizzle for a few hours
into early evening. Satellite imagery has stratus hanging tough and
still well back into eastern KS and eastern OK at 21z. Trends along
with the RAP and NAM 925 mb RH suggests the clearing trend will be
slow and won`t commence in our CWA until well after midnight, and
the back edge of clouds around 50 miles west of STL at 12z Thursday.
The clouds will help keep temperatures a tad warmer overnight than
the previous forecast.

Glass

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 325 PM CST Wed Dec 24 2014

(Thursday through Saturday)

Primary focus thru this period will be precip chances late in the
period.

Mdls remain in good agreement thru Fri. Sfc winds quickly become sly
by tomorrow as ridge quickly moves off to the E. Expect current
cloud deck to move ewd tonight and shud be along/near the MS River
around 12z Thurs. These clouds shud be on the ern boarder of the CWA
by 18z. However, mdls suggest these low clouds will probably be
replaced with CI which shud hinder heating somewhat. Have therefore
kept temps twd the cooler guidance, esp across ern portions of the
CWA will clouds will linger. For Fri, have trended slightly warmer
as strong sly winds will be in place. However, with questions
regarding cloud cover, did not go as high as warmer MOS for now.

Cut off low over srn Rockies begins to open and eject into the
Plains by Fri evening. However, the GEM is a slower soln and
actually redevelops an upper low after the initial s/w is reaching
the nrn Plains. As this soln is an outlier, have trended away from
this soln and twd the other mdls which remain in good agreement.
Have kept PoPs in lower chance category for now as mdl soundings
suggest much of the QPF may be more DZ than RA. Not to mention that
the NAM and local WRF suggest the precip bifurcating with a branch
to the N and one to the S. However, with the trof approaching and
some low level forcing, light RA will be possible. Question becomes
p-types on Sat and Sat night as the cold air filters into the
region. While the GFS soundings suggest precip will end as a brief
period of IP, then SN, setup is typical of this area where precip
will likely end as RA just ahead of the surge of cold air. This is
more likely given the sfc ridge center is currently progd to be over
wrn KS. Regardless, close attention shud be paid to this time period
as mdls come into better agreement.

(Sunday through Wednesday)

Focus quickly turns to temps thru this period.

Will continue trends twd a GFS/ECMWF compromise due to issues with
the GEM mentioned above. The DGEX also becomes a fast outlier as the
next trof ejects into the Plains sometime on Wed.

Relatively cold air moves in place behind the cdfnt on Sat. A
secondary cdfnt is expected to push thru the region Sun night into
Mon ahead of the next system approaching the region late Mon night
into Wed. Have trended temps twd cooler guidance for Tues and beyond
placing temps nearly 20 degrees below seasonal avg.

A number of questions arise with the system on Tues/Wed. The GFS
cuts off an upper low over the swrn U.S. which it is known to do.
The ECMWF ejects this trof into the Plains, but given the weakening
jet upstream of this trof, am not convinced this soln is correct.
With both mdls suggesting very low QPF, will keep only slight chance
PoPs going for now with plenty of time to adjust. The one item both
mdls do agree on is the precip is expected to be all SN.

Tilly
&&

.AVIATION:  (For the 06z TAFs through 06z Thursday Night)
Issued at 1132 PM CST Wed Dec 24 2014

Back edge of SC deck is making its way east across western MO as
of 0530z. KCOU should begin to scatter out around 08z Thursday,
while KUIN and metro area tafs should scatter out between 11z-12z
Thursday. Otherwise, west winds to back to the south by daybreak
and pickup by mid morning to between 10 and 15kts and remain that
way through rest of forecast period.

Specifics for KSTL:
Back edge of SC deck is making its way east across western MO as
of 0530z. Metro area should scatter out between 11z-12z Thursday.
Otherwise, west winds to back to the south by 12z and pickup
by 18z to between 10 and 15kts and remain that way through
rest of forecast period.

Byrd
&&

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

$$
WFO LSX




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