Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261224
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
624 AM CST Wed Nov 26 2014

.SHORT TERM: (Today and Tonight)
Issued at 310 AM CST Wed Nov 26 2014

Clipper system over Nebraska/Iowa border will continue to drop
southeast today, bringing the area some light snow/rain. Short
range guidance including the HRRR, RAP, NAM and GFS are in good
agreement that light precip will overspread the area through the
morning hours, reaching the Mississippi River between 16-18Z.
Primary lifting mechanisms continue to look like low level
moisture convergence, frontogenesis, and broad scale lift
generated by the parent shortwave. Precipitation will initially
have to fight some pretty dry low level air as it moves into the
area as dewpoints are in the upper teens and low 20s. Evaporative
cooling as well as favorable vertical profiles would indicate that
initially most if not all of the precipitation will fall as
snow...especially over central and northeast Missouri. Insolation
through later in the morning and afternoon, combined with
southerly flow along and east of the Mississippi river (which will
be east of the low track) will probably cause the snow to mix with
rain across a good portion of the area for at least part of the
afternoon. Between the mix, above freezing temperatures, and warm
ground conditions (2 inch soil temperatures range from 32 to 40
degrees across the area), expect only minor accumulations of and
inch or less today. The tail end of the clipper will continue to
affect the area this evening with additional minor accumulations
possible...generally along and east of the Mississippi in
southeast Missouri and southwest Illinois. The lingering
precipitation should end before 06Z this evening.

Carney

.LONG TERM:  (Thursday through Tuesday)
Issued at 410 AM CST Wed Nov 26 2014

Quiet period of weather for Thanksgiving Day through Saturday as the
pattern transitions from high amplitude to more zonal.  After a cold
day on Thanksgiving with highs struggling into the upper 20s to mid
30s, the trof currently over the eastern CONUS will shift offshore
by early Friday morning which will start a moderating trend.  Low
level southwest flow will bring temperatures up to near normal in
the low 50s, and temperatures will warm further on Saturday into the
upper 50s and low 60s.  Continuing to keep a dry forecast on
Saturday in spite of the GFS and ECMWF which continue to spit out
light QPF.  Still think this is due to the rapidly moistening
boundary layer and the model`s tendency to make precipitation when
this occurs.  Sunday`s highs are still in question as a 1035mb high
dips down into the Northern Plains and Upper Midwest.  The resulting
cold front will be moving into northern sections of our area during
the day which makes high temperature forecasts for Sunday quite
difficult.  Should be mild south of the front with highs in the 50s
and 60s...possibly warmer.  North of the front it will be sharply
colder.  As there will be frontal-scale lift as well as weak low
level moisture convergence, have kept low chance PoPs in the
forecast south of the front in the warm sector.  High will shift
eastward Monday and Tuesday.  Medium range guidance agrees that
Monday should be cool and dry under the influence of the high, with
the potential for temperatures rising back above normal for Tuesday
as southerly flow returns to the area.

Carney

&&

.AVIATION:  (For the 12z TAFs through 12z Thursday Morning)
Issued at 615 AM CST Wed Nov 26 2014

Surface low of clipper appears to be near ADU in western IA at
11z, with N-S band of precip in zone of WAA ahead of system
stretching from east of DSM to DMO. The precip should rotate east
during the morning, with the band of precip setting up along the
Mississippi River around midday and then spiralling its way into
southern IL and western KY during the aftetrnoon as low tracks
into the lower Ohio Valley by evening.  Have attempted to time
arrival and decrease of precip with a blend of SREF, HRRR, and
local WRF.

Latest forecast soundings indicate that precip should mainly be
snow in UIN (arriving in the 16-17z time frame), with occasional
IFR cigs/vsbys in the bands of heavier snow. Further s, it still
appears that the precip could go either way (rain or snow) as
evaporative cooling attempts to offset low level warming. In this
transition area, gone with prevailing MVFR cigs in rain/snow mix,
with the thinking that IFR conditions will occur for a time as the
stronger lift produces heavier precip, with the resultant evaporative
cooling lowering the freezing level so that it falls as all snow.

Any significant precip should taper off to flurries heading into
the evening, primarily along the Mississippi River. It appears
that considerable low cloudiness will work south across the region
in the wake of the clipper, so have maintained MVFR cigs 2-3kft overnight.

Specifics for KSTL: Band of precip currently over western MO
should work into STL area between 16-18z. Thermal profiles still
suggest a mix of rain/snow, and have gone with prevailing
cigs/vsbys in the MVFR range, with occasional IFR for pockets of
heavier precip where evaporative cooling will cause the precip to
fall as all snow. Precip should taper off to flurries this
evening, with MVFR cigs lingering throughout the night.

Truett

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     39  28  34  27 /  70  60   0   0
Quincy          38  20  27  22 /  80  20   0   0
Columbia        39  23  34  27 /  70  20   0   0
Jefferson City  40  25  35  27 /  70  20   0   0
Salem           40  29  33  24 /  70  60   0   0
Farmington      40  28  36  26 /  70  60   0   0

&&

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

$$

WFO LSX





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