Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 280247
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
947 PM CDT Fri Mar 27 2015

.UPDATE:
Issued at 945 PM CDT Fri Mar 27 2015

Regional radar loop indicates a very narrow NNW-SSE oriented
band of fragmented precip stretching from the Red River Valley
along the ND/MN border to north of STJ near the MO-IA border.
Trajectory of current echoes would keep precip west of our CWA,
but there has been a subtle eastward shift in the precip axis over
the past few hours, and believe that this eastward nudge will
continue overnight due to the strong and persistent WAA ahead of
shortwave diving into the region. 00z soundings indicate low
level AMS (sfc to at least 5kft) is quite dry, but believe that
lift/saturation caused by the WAA will be enough to get some
precip through this dry layer, primarily in the form of snow.

Haven`t made any major changes to the PoPs overnight and have
maintained likely PoPs during the predawn hours over mid MO, but
I did try to tighten up the PoP gradient to the east of the COU
area as all models strongly indicate there will be a very sharp
eastern edge of the precip during the overnight hours.

Truett

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 325 PM CDT Fri Mar 27 2015

The stratocumulus cloud deck will gradually dissipate and advect
southeast of our forecast area late this afternoon and into the
early evening hours due to increasing subsidence as the surface
ridge extending from northwestern WI southwest into central OK
shifts southeastward into our forecast area, and as an upper level
trough along with cold mid level temperatures shift east of our
area.  Any flurries or sprinkles should end by late afternoon.  Mid
level cloudiness will spread into our area later this evening and
overnight as low-mid level warm air advection increases over the
western portion of the forecast area on the nose of a westerly low
level jet ahead of an approaching northwest flow shortwave.  A band
of snow is forecast to drop southeastward into central MO late
tonight due to low-mid level frontogenesis.  The GFS is stronger and
further east with the area of frontogenesis and hence has more QPF
and has its placement further east compared to the NAM.  The ECMWF
model is a compromise between the NAM and GFS.  For now will go with
a compromise between the ECMWF and the GFS model, the NAM appears a
little too far west with its narrow nw-se band of QPF.  Looking at
the thickness forecasts, model forecast soundings and surface
temperatures the precipitation type should be snow.  For now will go
with about one half inch of snow accumulation late tonight for the
COU and JEF area.

GKS

.LONG TERM:  (Saturday through Next Friday)
Issued at 325 PM CDT Fri Mar 27 2015

(Saturday through Monday)

Mdls continue to show a strong NW/SE oriented band of frontogenesis
along the wrn CWA border. The NAM seems to be a wrn outlier and have
trended twd the GFS/ECMWF/local 4km WRF. Still appears heavier SN
will remain W of the CWA.

The sfc ridge will slide just to the E of the area bringing sely
winds to the region for Sat night. This shud help keep temps up
slightly and have trended twd the warmer guidance.

Sun will bring a brief warm up with a cdfnt pushing thru the region.
Have increased PoPs with good low level forcing with a fairly strong
s/w and good ULJ support. Main question will be how far S precip
will develop with weaker support, but have kept at least low PoPs
across the srn portions of the CWA.

Despite the cdfnt mentioned above, thermal ridge builds over the
area with swly to wly flow shud help push temps into the 60s for
Mon.

(Tuesday through Friday)

Mdl solns begin to diverge quickly with the ECMWF/GEM solns bringing
a weak cdfnt thru the area while the GFS keeps the fnt N and pushes
the thermal ridge across the region. While both mdls have pushed
this fnt further S compared to yesterday, have cooled temps only
slightly for now compared to the prev forecast.

The ECMWF and GEM have height rises across the region with the GFS a
split flow bringing RA to the CWA. Have kept low chance PoPs due to
the uncertainty and to keep some persistence with the prev forecast.
Mdls are in better agreement with a cdfnt pushing into the area on
Thurs, but the more zonal ECMWF stalls this fnt further N while the
GFS is more progressive. Have raised PoPs where mdl guidance agrees,
but kept some lower PoPs going into Fri due to uncertainty.

Tilly

&&

.AVIATION:  (For the 00z TAFs through 00z Saturday Evening)
Issued at 637 PM CDT Fri Mar 27 2015

Mid and high clouds moving in ahead of next shortwave which will
slide through tonight. Best chances of light snow will be over
central MO and areas to the west, so only have it mentioned in
KCOU taf between 09z and 13z Saturday with mvfr cigs/vsbys. Rest
of taf sites to remain vfr through forecast period, though a few
flurries not out of the question late tonight/early Saturday
morning. As for winds, to veer to the east then southeast towards
end of forecast period.

Specifics for KSTL:
Mid and high clouds moving in ahead of next shortwave which will
slide through tonight. Best chances of light snow will be over
central MO and areas to the west, so metro area to remain dry,
though a few flurries not out of the question. Otherwise, taf to
remain vfr through forecast period. As for winds, to veer to the
east then southeast by 17z Saturday.

Byrd

&&

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

$$

WFO LSX






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