Area Forecast Discussion
Issued by NWS Jackson, KY

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099
FXUS63 KJKL 170523
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1223 AM EST Sun Dec 17 2017

.UPDATE...
Issued at 735 PM EST SAT DEC 16 2017

Hourly grids have been updated based on recent observations. The
eastern valley KY Mesonet locations and KEKQ have been dropping
off faster since around sunset compared to the inherited forecast.
Hourly grids have been adjusted accordingly to lower hourly
temperatures for the next few hours and in some cases min T was
lowered. Otherwise, no changes were needed at this time.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 339 PM EST SAT DEC 16 2017

Current conditions across the area feature high pressure under
control across the region with clear skies over eastern Kentucky
and afternoon high temperatures around 50. Heading into tonight,
models have slowed the approach of the next system moving in from
the southwest significantly. Thus, will see a decent ridge to
valley temperatures difference setting up tonight as skies will
likely be clear through much of the night with only some thin
cirrus creeping in late tonight.

Heading into tomorrow, the forecast challenge will be when the
profile saturates allowing precip to reach the surface which at
this point with the model trends, looks to be by 18Z. Once the
precip reaches the surface, the moisture present will be enough
for the entire area to measure but due to the progressive pattern
and duration, QPF looks to be around a tenth. In fact, most of the
rainfall will exit by early Sunday night with low level moisture
hanging across the area into Monday morning. So will be dealing
with a lingering stratocumulus layer likely lasting into the day
on Monday. Thus will leave low temps Sunday night into the upper
30s with the lingering ceilings keeping the surface somewhat
insulated.

.LONG TERM...(Monday through Saturday)
Issued at 220 PM EST SAT DEC 16 2017

The models are in good agreement aloft at least through the first
half of the extended portion of the forecast before starting to
diverge toward the end of the week. They all depict stout ridging
over the Southeast with fast and nearly zonal flow through the
Ohio Valley carrying just weak packets of energy above us through
Tuesday morning. While the northern stream remains active through
the Great Lakes, Kentucky`s next weather maker will come from a
trough in the southern stream moving out of New Mexico rolling
east during the day, Tuesday. This wave does dampen as it
approaches by Wednesday with the GFS slightly weaker and faster
than the ECMWF. Its passage that night will allow heights to
rebound on Thursday ahead of a much stronger and full latitude
trough digging into the Four Corners region. This is where the
model spread becomes rather large with the ECMWF bringing the core
of its trough east into the Southern Plains quicker than the
stronger GFS Friday morning. The ECMWF also releases a good
portion of its energy into the northern stream that then quickly
spreads through the northern Ohio Valley while the GFS has hardly
any hints of this action - keeping its core energy well to the
southwest. The model split magnifies heading into the weekend with
strong ridging from the GFS pushing well into Kentucky from the
Southeast while the ECMWF is spreading lower heights east into the
lower and mid Mississippi Valley. Needless to say, confidence is
low for the Christmas weekend weather given these large model
discrepancies. For now will not deviate too far from the model
blend at these time steps until there is a clearer break toward
one solution or another.

Sensible weather will feature near normal conditions to start the
work week, though lingering stratus will be a concern through the
day Monday (and well into Tuesday) limiting the warming potential
even as high pressure slides off to the east. This will likely
set up a larger than normal temperature gradient across the area
from southwest to northeast into Tuesday. Warm air will move more
effectively into the area for Tuesday afternoon but a mostly dry
frontal boundary will then shift south enough to renew the
temperature gradient across the area into Wednesday as a southern
wave passes just south of the area - though potentially strafing
our southern tier with some pcpn in the form of light rain. Quiet
wx takes hold through the rest of the work week with another
gradual warm up occurring for all of eastern Kentucky into Friday.
Low pressure running northeast to our west and then passing
Kentucky by the northeast will then set up a baroclinic zone
through the Ohio and Tennessee Valley later that night and into
Saturday with significant uncertainty as to the placement of the
colder and warmer air throughout the area. The GFS shows strong
overrunning across this boundary from an open Gulf leading to
ample pcpn for the JKL CWA Friday night and through the weekend
split by the cold air intrusion making for significant concerns of
winter wx across our area. Meanwhile, the ECMWF slips its cold
front right on through the state and well to our southeast taking
with it any real potential for significant winter weather lacking
anywhere near the overrunning signals of the GFS with its
baroclinic zone well off to the southeast of the state.
Accordingly, will downplay any of the more extreme aspects of
this potential situation in favor of a more benign interpretation
of the blends. Given the high level of uncertainty and dynamic
potential, though, this will have to be watched closely in
upcoming model runs.

Only made some minor adjustments to temperatures through the
period - even at night given different advection patterns
throughout the extended and mostly plentiful cloud cover. As for
PoPs, have downplayed the potential for midweek in the far south
and again Friday into Saturday given the high degree of
uncertainty at this point.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
ISSUED AT 1223 AM EST SUN DEC 17 2017

We are starting the period off VFR and this will remain the trend
for much of this TAF cycle. The feature of interest for the TAF
period is an upper level shortwave which is currently located
across the Southern Plains tonight. This feature will move
northeast and weaken as it does so. This will bring a shot of
light rain to the area through the day on Sunday. The impacts will
be minimal till we get closer to the end of the cycle when MVFR
CIGS will become the norm across most TAF sites. The winds are
expected to remain light out of the south and southwest through
the TAF period.

&&

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

$$

UPDATE...JP
SHORT TERM...SHALLENBERGER
LONG TERM...GREIF
AVIATION...DJ



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