Area Forecast Discussion
Issued by NWS Jackson, KY

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928
FXUS63 KJKL 021733
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1233 PM EST Fri Dec 2 2016

.UPDATE...
Issued at 1039 AM EST FRI DEC 2 2016

Stratus deck in far northern KY has drifted south as low level
wind flow has shifted from west to west northwest. Cloud deck just
now into Fleming County at the northern end of forecast area.
Current trends and latest short range model guidance pointing
towards a trend of low level winds continuing to veer to more
northwesterly. This complicates the near term portion of the
forecast with respect to forecasting advance of low cloud deck.
Have modified sky cover to bring more clouds into the northern
part of the forecast area, with a resulting decrease in afternoon
maximum temperatures.

UPDATE Issued at 652 AM EST FRI DEC 2 2016

Stratus deck hovering along the Ohio River this morning with
isolated patches of lower clouds streaming across portions of
eastern Kentucky. May see this shield shift south a bit this
morning as temperatures climb toward the mid 40s near the greater
cloud cover to low 50s farther south.

&&

.SHORT TERM...(Today through Saturday)
Issued at 352 AM EST FRI DEC 2 2016

A stacked low pressure system across Quebec will continue to result
in cyclonic flow into the Ohio Valley, producing rounds of clouds
moving across eastern Kentucky. Despite the greatest concentration
of stratocumulus remaining north of the Interstate 64 corridor,
cirrus will stream in downstream of an upper ridge attempting to
build north across the Great Plains as an upper low digs into the
Gulf of California. However, aforementioned Canadian upper low
will limit height rises, subsequently keeping high temperatures in
the mid 40s to low 50s as a low level cold dome remains locked in
place into the Tennessee Valley. Surface ridging stretching from
the northern plains to the northern gulf coast will make greater
headway toward the Commonwealth today. Enough of a pressure
gradient will remain in place to allow for west/southwest winds to
still increase to 5-10 mph later this morning through this
afternoon.

This surface ridge looks to nose into southern Kentucky this evening
and tonight, producing calm surface winds. Portions of the Big Sandy
region through northeast Kentucky will likely see enough low level
moisture to keep radiational cooling from taking full effect. The
remainder of eastern Kentucky will likely also be somewhat limited
in this regard as the cirrus shield thickens and becomes more
widespread. Nonetheless, valley locations should cool into the mid
20s while ridges remain near 30 degrees by daybreak Saturday.

Upper ridging looks to finally move overhead by later in the day on
Saturday with accompanying height rises. However, top-down
moistening and high pressure overhead, making for slow retreat of
the lingering cold dome, will in all likelihood result in below
normal temperatures once again to start the weekend, as highs top
out in the mid 40s to low 50s.

.LONG TERM...(Saturday night through Thursday)
Issued at 356 AM EST FRI DEC 2 2016

While deamplified ridging is expected across the area through the
first part the weekend, a small upper level shortwave is expected to
begin pushing eastward toward the state from the upper Mississippi
River Valley through the day Sunday. This shortwave should pass,
mainly to our north, through 12Z Monday morning before exiting to
our northeast. Zonal flow will take hold once more during the day
Monday. However, attention will then turn to a closed upper level
low over southern TX as of 12Z Monday, as it begins to propagate NE,
reaching western TN and western KY by 12Z Tuesday. The low will
dissolve slightly into a wave as it passes over eastern KY Tuesday
afternoon, before getting absorbed into the overall longwave
troughing pattern establishing itself across the western 2/3 of the
conus. Through this point, but the GFS and ECMWF are in very good
agreement with one another. Things start to break down a bit as the
longwave trough begins to shift eastward toward the Mississippi
River Valley and eventually the Ohio River Valley mid week. The
ECMWF is a bit slower with the axis of the trough reaching the
state, while the GFS has it directly across KY by 18Z Thursday.
Given the uncertainty, will stick to model blend for precip and
other variables during this time.

The first shortwave to impact the region will bring with it a surge
of upper level support, including a strong jet max. However, at the
surface, high pressure should generally still be in control, though
moving east of the region, with very light winds and little llvl
moisture support. Most of this system will be mid and upper level
dynamically driven, as winds should start feeding off of the
stronger system to our SW over TX, pulling moisture off the Gulf of
Mexico and northward. As such, this heightened pull of moisture
should eventually overcome the stable air at the surface, and allow
precip to fall through the dry layer. All models are in agreement
that precip should start entering into the CWA from the south
between 6 and 12Z, quickly increasing as the shortwave nears closer
to the state throughout the day Sunday. Temperatures will be at or
just above freezing Saturday night and into early Sunday morning,
before quickly warming. Despite this, temperatures aloft during this
time, in the moist layer, will be quite a bit colder. As such, as
precip begins to overcome the dry air, it is possible that snow may
mix with the rain, before changing over to all rain for the
remainder of the day. This should not cause any actual snow
accumulations.

The precip and upper level dynamics should finally shift east of the
region Monday, allowing for a brief respite in the precipitation.
However, it won`t be long before the next upper level low begins to
push northeast from TX, and brings with it another wide swath of
precipitation. Both GFS and ECWMF are once again in good agreement
with this system, bringing the front end of precip into the region
between 0 and 3Z Tuesday, then quickly increasing to categorical
coverage by 6Z and continuing throughout the morning and early
afternoon as the surface low and upper level wave pass nearly
overhead. As the wave moves northeast of the state and gets
absorbed into the upper level flow during the day Wednesday,
forcing will lesson and precip will taper off. The GFS does keep
some residual moisture and pops across the CWA for the entire day
Wednesday so did leave some slight chance pops in, though QPF should
be minimal.

Model differences start to appear in timing from here on out. The
longwave pattern across the western 2/3 of the US will begin to
shift eastward, bringing impulses/shortwaves across KY. The GFS
shows this as early as Wednesday evening/night, while the ECWMF is
almost 12 hours behind in timing. Regardless of the timing, they
both agree that this could be a rather potent system, with very
tight pressure gradients pulling deep southerly flow from the Gulf
of Mexico and producing decent QPF values. Went with a blend of the
two models for POPs and QPF, which resulted in an average of chances
for both Wednesday night through Thursday, but no doubt that we will
see precip at some point during this period. If the GFS pans out,
the cooler overnight temps could lead to an onset of snow,
transitioning to rain during the day. The ECMWF should be late
enough, however, that most precip should fall as all rain. Either
way, based on the combination of the two producing chance pops
Wednesday night, this will also result in some snow chances. While
some light accumulations are possible, generally less than an inch,
as snow changes over to rain during the morning hours, all snow
should quickly melt.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
ISSUED AT 1233 PM EST FRI DEC 2 2016

Deck of stratocumulus has continued to sink southeastward across
the forecast area, with MVFR ceilings now expected to prevail
across the northeastern one half of the forecast area into the day
Saturday. The MVFR cloud deck is expected to persist at SYM, SJS
and JKL through the forecast period. It is still uncertain how far
southwest the cloud deck will eventually extend, but there are
some indications it could reach LOZ and SME by daybreak Saturday.

&&

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

$$

UPDATE...SBH
SHORT TERM...GUSEMAN
LONG TERM...JMW
AVIATION...SBH



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