Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 180650 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
150 AM EST Sat Nov 18 2017

.UPDATE...
Issued at 150 AM EST SAT NOV 18 2017

Did a quick update to add a touch of PoPs into the grids with a
few of the showers and sprinkles able to reach the ground late
tonight/early this morning. Also, incorporated the latest obs and
trends into the T and Td grids. These have been sent to the NDFD
and web servers.

UPDATE Issued at 1044 PM EST FRI NOV 17 2017

There have been some light precip returns, but none have reached
the ground yet. The advancement of the precip line forecasted
is still reasonable, therefore have not adjusted PoPs. With the
advance of the warm front and clouds, the trend of the
temperatures is to become more steady and/or increase with time.
Adjusted the temp grids slightly to reflect current trends in
observations. Otherwise forecast remains on track.

UPDATE Issued at 654 PM EST FRI NOV 17 2017

Updated sky grids to slow advancement of cloud cover. Dry air and
calm winds in the valleys this evening have allowed temps to
cool past the forecasted lows for tonight. Adjusted temps in the
eastern valleys based on these current trends. Once clouds move in
to the area, the temps should become more steady, if not warm
through the rest of the night.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 415 PM EST FRI NOV 17 2017

High pressure at the sfc and aloft is beginning to shift east of
the area. Meanwhile a mid level trough with a few embedded
shortwaves is beginning to approach from the west. The individual
shortwaves will begin to phase as the trough near the MS River on
Saturday with a near neutral tilt. The trough will take on more of
negative tilt on Saturday night. At the same time, the surface low
is expected to track from the Central Plains to the Mid MS Valley
tonight and then deepen rather quickly in response to the mid
level trough deepening from late tonight through Saturday night as
it tracks from the Mid MS Valley and to the mid MS and toward the
Adirondacks to St Lawrence valley area pending model differences.
The GFS is generally deepest and further west compared to NAM or
ECMWF runs. WPC guidance is generally a compromise between the two
with position and strength of surface low at the end of the
period.

As the high moves east and the low and associated front and upper
trough begin to approach, winds aloft will increase as the
pressure gradient increases across the lower OH Valley region. A
low level jet will also increase overnight across the lower OH
valley region. In addition, initially high clouds will begin to
thicken and lower between 0Z and 6Z with mid level clouds possible
late as a warm front approaches. Some showers or sprinkles will be
possible across the northwest late as the front nears with
possibly a stray rumble of thunder, but downslope flow should
preclude any showers or sprinkles in the southeast. With light
winds and high pressure initially in place, eastern valleys
should decouple prior to sunset and drop 15 degrees or more
through late evening before the clouds and pressure gradient
increase leading to a leveling off of temperatures if not a rise
for those valley locations.

Winds fields aloft will remain rather strong ahead of the cold
front on Saturday with some mixing down of momentum possible in
the morning behind the warm front with some limited solar
insolation. Then, showers and even a few embedded thunderstorms
ahead of the front will be capable of mixing down the higher
momentum. SPC has place the western portion of the area in a
Marginal Risk for Severe Thunderstorms with this. The highest
winds aloft are generally in the late afternoon and evening. Time
height sections indicate that theta e and saturated equivalent
potential temperature lines become nearly vertical along and or
just behind the cold front during the evening. This is the period
when the threat for widespread wind gusts to around 40 mph or
higher appears most likely.

Lingering low level moisture may be sufficient with cold air
advection occurring, 850 mb temperatures falling to below 0C by
the end of the period and a west to west northwest upslope low
level flow for some rain or snow showers in the higher terrain
before sunrise on Sunday.

.LONG TERM...(Sunday through Friday)
Issued at 437 PM EST FRI NOV 17 2017

Forecast starts off Sunday morning with an exiting cold front and
precip tapering off from west to east. A strong shot of post frontal
cold air could result in some snow mixing in with the rain as it
tapers off Sunday morning, though not expecting any accumulations as
ground temperatures will lag. The exception will be the highest
terrain, mainly Black Mountain, but precip will be so light by
Sunday morning, only a trace or so will likely occur before it mixes
with rain and then tapers off soon after.

High pressure is expected to move in overhead Sunday night. As was
the case yesterday and last night, there is still some question as
to if enough llvl moisture will become trapped under the subsidence
inversion to keep clouds across the region as we head into the
overnight. This could have a dramatic impact on the expected low
temperatures. GFS shows the clouds scattering out across the region
generally 3Z and after, while the NAM is a bit slower, showing the
scattering occurring after 6Z. Went ahead and did a bit of a ridge
valley split for Sunday night/Monday morning, however did not go
that aggressive given the uncertainty on if these clouds will
stick around or not. Return flow on the backside of the exiting
high pressure will take hold by Monday night. Light southerly flow
could provide a better environment for decoupling, so did drop
valley temps (mainly the sheltered eastern ones) down quite a bit.

An upper level trough is expected to shift east through the Upper
Great Lakes during the day Tuesday, with a surface system expected
to follow in its path. This will drag a weakening cold front
southeast across the region Tuesday night into Wednesday, however
all indications point at this frontal system being dry as it moves
across the Ohio Valley, with the only impact being a slight uptick
in clouds and a reinforcement of cold northerly flow. Temperatures
are expected to remain below normal through the rest of the forecast
period as this northerly flow continues ahead of another approaching
large high pressure system.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
ISSUED AT 130 AM EST SAT NOV 18 2017

Low level wind shear looks to be underway across the area this
night per the VWP at JKL showing winds 2k feet above the sfc at
45 kts from the southwest compared with light sfc winds.
Meanwhile, VFR cigs will continue lower through the night, ahead
of an approaching cold front from the west. A few returns on radar
may reach the ground, but will be immaterial for aviation
purposes. Southeast to south winds at around 10 kts can be
expected for most sites through dawn with some gusts up to 20 kts
for our more western airports. South southwest winds will then
increase to 10 to 20 kts, with gusts of 25 to 35 kts during the
day on Saturday. Some scattered shower activity will be possible
between 06 and 18z, with more widespread showers and potential
thunderstorms moving in after 18z. Ceilings/visibilities will
lower down to MVFR by the end of the period for most sites, as the
cold front moves on across eastern Kentucky and winds switch to
the northwest equally strong for a time.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 7 AM this morning to 1 AM EST Sunday for
KYZ044-050>052-058>060-068-069-079-080-083-084-104-106-108-111-
114.

Wind Advisory from 10 AM this morning to 4 AM EST Sunday for
KYZ085>088-107-109-110-112-113-115>120.

&&

$$

UPDATE...GREIF
SHORT TERM...JP
LONG TERM...JMW
AVIATION...GREIF


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