Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 151148

548 AM CST Mon Dec 15 2014

.SHORT TERM...(Today through Wednesday)
Issued at 350 AM CST MON DEC 15 2014

Timing issues will likely predominate precipitation coverage
today. Short and medium range guidance still verifying relatively
well on orientation and initial timing of the movement of the
expected weather across the WFO PAH forecast area today and
tonight, although the medium range guidance was holding onto the
precipitation coverage far too long in Southwest Indiana and the
Pennyrile region of West Kentucky tonight.

Boosted the coverage of differential PoPs for thunderstorms closer
to the GFS solution (based on elevated lifted indexes near 850 mb)
for today. This is reasonable given the trajectory of the elevated
instability currently moving northeast through Southwest and
Central Arkansas at 09z Monday. Still kept coverage near isolated
levels at this time.

Although 100% PoPs would be ideal this morning, spatial variations
and evolutionary changes in the system as it move through the WFO
PAH forecast area this morning and this afternoon may lead to some
intermittent breaks in the precipitation shield, so hinted at that
through slightly lower PoP numbers today.

The NAM-WRF deterministic guidance and SREF guidance seemed to
have a generally good handle on thermal, precipitation (mass),
wind fields and was used as background template for any changes
associated with the short term forecast with this package.

.LONG TERM...(Wednesday Night through Sunday)
Issued at 350 AM CST MON DEC 15 2014

Well, in a nutshell, confidence remains quite low in the long term.
Really looks as though there will be two systems to deal with, a
lighter one Wed night into Thu AM, then a stronger signal Friday
into Saturday.

Weak overrunning/isentropic upglide should get going over the
southern half of the forecast area Wednesday night, over a cold dome
of high pressure at the surface. The latest 00z operational ECMWF
has backed off on qpf amounts with this system, and now keeps most
locations below a tenth of an inch liquid qpf, as does the 00z GFS.
Precip type is still a question mark. Forecast soundings for PAH
snake right along the 0C line from the sfc right up to about 10K FT.
Also, by the time we saturate down to the sfc, the -10C to -20C
layer has dried out, suggest little if any ice nucleation. For now,
will continue to go with a mix, since anything seems possible at
this time.

The bigger precip potential will come later Friday and Friday night
as a potent mid level wave lifts ne out of the srn Plains. However,
the timing and placement of this wave, as well as the
placement/movement of the associated sfc low, is still very much in
question. The latest 00z GFS/ECMWF and GEM runs are stubbornly
similar to yesterdays runs, yet distinctly different from each other
at the same time. The GFS conts to suggest the upper wave will lift
out in a much farther nw track compared to the other models, and is
quicker also. This would suggest a warmer solution, with only rain
all locations and maybe even a few thunderstorms over portions of
western KY along the path of the sfc low. The ECMWF still suggests
a colder/snowy solution with a sfc low track through MS/nrn AL. In
fact, the latest op ECMWF would suggest a significant winter event
over much of the region Friday night into Sat. In contrast, the
latest GEM actually takes the sfc low across the nrn Gulf of Mexico,
and keeps most of the significant precip south of our region.
Ensembles also suggest a large spread in possibilities. Figured the
best way to minimize error at this point would be to take a blended
approach, which would be a track fairly close to the 00z operational
ECMWF. Will not go hog wild on the winter qpf at this point though
do to all of the uncertainties. hopefully, model solutions will
begin to merge in the next couple of days.

In any event, cold Canadian air will likely follow the system next
weekend, with highs only in the 30s for the most part. Would be
colder, of course, if some places were to get a layer of snow. Will
side toward the colder EC MOS numbers for this package. GFS MOS just
look too warm given the expected pattern.


Issued at 548 AM CST MON DEC 15 2014

Ceilings associated with the main precipitation band approaching
KCGI around 12z Monday will move from VFR to MVFR and IFR within
2-3 hours after onset of the rain. This will likely be the case for
the subsequent downstream WFO PAH TAF sites. Visibilities will
move from VFR to MVFR within the precipitation shield with post
rain visibilities holding in the 2-4 statute mile range until the
front passes through each taf site. MVFR to local IFR ceiling
conditions will also prevail at each TAF site following the
passage of the rain.




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