Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230028

628 PM CST Mon Dec 22 2014

.SHORT TERM: (Tonight and Tuesday)
Issued at 330 PM CST Mon Dec 22 2014

Batch one of rain is currently located from east central MO into
south central IL, and lifting northeast. The next batch of rain
is evolving across western MO in advance of a short wave trof
currently extending from the ND upper low into OK. The rain will
overspread the area this evening as the associated short wave
rotates northeastward in advance of the southeastward moving
upper low. Present indications are that it should be a quick shot
with most if not all the rain out of the CWA by midnight. Mid
level drying and large scale subsidence then spread across the
area overnight in the wake of the shortwave trof. West to east
clearing will proceed as well overnight as the cold front advances
and the winds veer to more westerly. Tranquil day on tap for
Tuesday with remaining low clouds clearing but mid and high clouds
generally overspreading the area in the increasingly southwesterly
flow aloft associated with deepening of the upper trof through the
Plains/central U.S..


.LONG TERM:  (Tuesday through Next Monday)
Issued at 330 PM CST Mon Dec 22 2014

Attention then turns to the storm system that will impact the area Tuesday
night into Christmas Eve day. The guidance is in general agreement that
a closed low will develop within the deepening trof across OK by Tuesday
evening, and then lift northeastward Tuesday night into Wednesday night.
In response a surface low will develop on the initial cold front across
the lower MS Valley and then track northeast through the TN Valley
and into the OH Valley. While there is general consensus in the pattern,
there is considerable model spread in the details of the structure
and position and track of the important features - much greater spread
than we normally see inside at T-60 hours. If anything there has
been a slight general west shift in the track of the mid/upper low.
Worrisome is the SREF mean and GEFS means are further west than the
deterministic solutions, meaning there are quite a few members well
to the west. At this point this forecast can be characterized as
high uncertainty and low confidence. That said the current forecast
is a uptick of the previous one with greater weighting to the
Global models - ECMWF, operational GFS and GEM. The scenario is
that precipitation will spread from AR northeastward across southeast
MO and southwest IL on Tuesday night as a deformation zone develops
in response to large scale forcing/ascent, with precipitation
along this axis on Wednesday morning and then shifting to the
northeast on Wednesday afternoon in association with the lifting
system. The precipitation should be all rain through at least 06z,
and then mix with and changeover to snow on the western fringe
early on Wednesday morning, changing to all snow by Wednesday
afternoon as the cold air continues to deepen. This looks like a
wet snow with marginal surface temperatures in the lower-mid 30s
and low SLRs. The St. Louis area will be on the western fringe of
the precipitation band. I currently have 1-2 inches forecast along
and axis from Ellington MO to Ramsey IL. This may or may not be
conservative, and will depend greatly on feature tracks and cold
air availability. A secondary short wave rotating on the back side
of the upper low may also bring a threat of light snow to western
portions of the CWA.

The snow that does fall won`t last long however. Low level warm advection
gets underway by late Christmas Eve and ramps up on Christmas day
with above average temperatures into Friday. Another system could
potentially bring another round of accumulating snow Saturday
night into early Sunday across the southeast third of the CWA,
however the models diverge greatly on the upper pattern at that
time. Stay tuned.



.AVIATION:  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 541 PM CST Mon Dec 22 2014

A cold front extending south across wrn MO out of a sfc low over
wrn IA will push thru the terminals prior to midnight. There is
another batch of rain that will move thru the terminals this evng
prior to FROPA. The front will bring winds around to the SW/WSW
overnight. IFR CIGs and MVFR VSBYs will slowly raise overnight
after FROPA with low level CIGs dssptg prior to 12Z. There is a
chance that KUIN will not break out of the MVFR CIGs tomorrow...but
most guidance indicates that they will. There is another batch of
MVFR CIGs poised to move into the area towards the end of the prd
that was not accounted for with this issuance.

Specifics for KSTL:

IFR CIGs and MVFR VSBYs will continue for the next several hours
with this next batch of precip. CIGs and VSBYs will begin to raise
overnight after FROPA with VFR conditions expected to continue for
the remainder of the prd. Another area of MVFR CIGs are expected
to move into the terminal just beyond then of the fcst prd.





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