Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 161208 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
708 AM EST Sat Dec 16 2017

.UPDATE...
Issued at 708 AM EST SAT DEC 16 2017

The stratocu deck has cleared most locations, with only some
scattered clouds affecting portions of the area over the next
hour or two. Have mainly freshened up the hourly temperature curve
through the morning to account for the latest trends in
observations.

&&

.SHORT TERM...(Today through Sunday)
Issued at 339 AM EST SAT DEC 16 2017

Surface high pressure is centered across the Gulf Coast States,
with ridging nosed up into the Tennessee and Ohio valleys. Aloft,
a split flow pattern is in place, with a trough axis shifting off
the New England Coast within the northern stream, while an upper
level low spirals across northern Mexico within the southern
stream. Stratocu cloud cover is finally relenting across eastern
Kentucky, with clearing skies working in from the west. Return
flow out of the south southwest has ensued, with cooler valleys in
the east reporting in the lower 20s, while upper 20s are noted on
the ridges.

The models are in good agreement through the short term, with the
upper level low across northern Mexico ejecting out and dampening
as it moves northeast. This feature will reach the Arklatex
region by tonight, and then moves through the Ohio Valley by late
Sunday, bringing a round of light precipitation to the region.

Today will feature a mostly sunny day, as ridging sharpens up
across the Mississippi Valley in response to the approaching short
wave from the southwest. Temperatures will rebound to around the
50 degree mark for most locations today, with southwest winds of
around 10 mph, and a few gusts of 15 to 25 mph for some locations
during peak mixing.

Mostly clear skies will be in place initially tonight, as the
ridge axis moves overhead. This will allow for the valleys to drop
off quickly in the east, with some mid 20s expected. Clouds will
thicken and lower for the second half of the night, likely
stifling the diurnal drop through dawn. The short wave will then
move through the region on Sunday, bringing light precipitation
to the area. Most of the guidance has generally trended slower
with the precipitation onset, ensuring that this will fall as
light rain. Did stay on the cooler side of guidance for the highs,
with generally mid 40s expected, given the wet-bulbing.

.LONG TERM...(Sunday night through Friday)
Issued at 357 EST SAT DEC 16 2017

The period will begin Sunday night with a flattening upper level
shortwave moving across the Ohio Valley. Therefore think the
period will begin with a chance of rain showers before weakening
and drying out through the night. The caveat here is lower level
moisture becomes trapped below 850mb inversion layer based on the
forecast soundings and consequently we will keep lower cloud deck
through Tuesday afternoon. This lead to lowering afternoon high
temperature produced by the blends closer to a raw blend of model
data. Do think these may need to be lowered further as confidence
increases on lingering stratus deck

The next system of interest is a southern stream wave that ejects
out of Texas Tuesday into Wednesday. The models have trended
slower with this wave but the models have also diverged in
solutions. This wave is more dampened in the in the GFS compared
to the ECMWF and this leads to differences in the surface low
development and evolution. The blended data would suggest a
compromise between the two models and felt like this was
reasonable at this point in the forecast process. The next system
of interest is a higher amplitude longwave trough that works into
the middle of the CONUS to round out the long term period. This
will help to send a cold front across the Ohio and Tennessee
Valleys by Friday, but there remains some differences in timing
and therefore capped POPs at chance for now.

It remains difficult to add too many larger ridge/valley
temperature splits given the uncertainty seen in much of the
period. The period will be presented with afternoon highs near to
above normal and morning lows mostly above normal for this time of
year.

There remains quite a bit of chatter regarding the longer term
model trends as we move closer to the Christmas holiday weekend.
Given the above mentioned higher amplitude longwave trough a good
portion of the western and central CONUS stand a reasonable chance
for below normal temperatures based on the CPC and CIPS extended
analog guidance. That said, both pieces of guidance would suggest
a good portion of the Ohio and Tennessee Valleys would remain in
between the colder and warmer anomalies leading to near normal
temperatures moving into this period. In terms of the
deterministic models, they remain unreliable with big differences
in location and evolution of the baroclinic zone from run to run
and, this will have major implications on sensible weather for
this period. All of this to say it is way too early to nail down
any specifics for this period of time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
ISSUED AT 708 AM EST SAT DEC 16 2017

VFR conditions will prevail through the period, as an upper level
ridge of high pressure builds in from the west. Some scattered to
broken stratocu ranging from 4-6k feet agl will be seen over the
next few hours across portions of the area, before dissipating.
Skies will remain mostly clear into this evening, with an increase
in mid and high level cloud cover expected after midnight, as an
upper level disturbance approaches from the southwest. Southwest
winds of 5 to 10 kts, will increase to around 10 kts, with gusts
of 15 to 20 kts at times during the afternoon. Winds will diminish
and back more out of the south after dusk.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...GEOGERIAN
SHORT TERM...GEOGERIAN
LONG TERM...DJ
AVIATION...GEOGERIAN



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