Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 181950
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
250 PM EST Sat Nov 18 2017

.UPDATE...
Issued at 1248 PM EST SAT NOV 18 2017

Some light returns are starting to pop up on the radar over the
last hour, mainly across the southwestern CWA. Will go back and
increase pops slightly so that isolated pops show up across more
of the CWA to cover any sprinkles/light rains that may accompany
these developing returns. Otherwise forecast continues to be in
shape, with a small update to make sure temps, dew points, and
winds were on track with the current observations. Continuing to
monitor the wind/wind gust situation, but no updates have been
needed thus far other than in the very near term. All updates have
been published and sent to NDFD/web.

UPDATE Issued at 953 AM EST SAT NOV 18 2017

Precip ahead of the main system has dissipated across the CWA
leaving dry conditions, though some light rain is still ongoing
across central Kentucky. Overall forecast pops seem to be in good
shape so far this morning. Kept in the slight chances in the
northern CWA just b/c a light shower or two redeveloping cannot be
ruled out. Loaded in the latest observations for temps, dew
points, and winds to make sure they were on track with the current
conditions. Continuing to monitor wind gusts across the region,
will make any updates as needed as we head throughout the day.
All updates have been published and sent to NDFD/web. A new
forecast package is not needed at this time, but a timing graphic
was sent out to show the estimated arrival times of the heavier
line of showers and thunderstorms late this afternoon/evening.

UPDATE Issued at 645 AM EST SAT NOV 18 2017

Did a quick update to the grids to fine tune the PoPs early this
morning. Also, adjusted temperatures as the valleys have just
about mixed out. These have been sent to the NDFD and web servers.

&&

.SHORT TERM...(Today through Sunday)
Issued at 310 AM EST SAT NOV 18 2017

07z sfc analysis shows a tightening sfc pressure gradient through
the Ohio Valley ahead of a rapidly developing low pressure system
over central Missouri. This is resulting in breezy conditions
just off the sfc and a few southerly wind gusts to 20 mph for our
western counties at the moment, but lighter winds are found in the
east with the valleys calm under an inversion. In those valleys,
despite increasing high clouds through the evening, many manged to
effectively decouple and radiate down closer to dewpoint
temperatures in the lower 30s. These are slowly starting to warm
back up, but mid to upper 30s are still noted. Elsewhere, the
better mixed sites are reporting mid 50s. Dewpoints vary from the
upper 20s in a few of the higher elevation points to the northeast
while values near 40 exist to the west and south. On radar, a few
light returns are noted but not much is likely making to the
ground given the drier conditions of the boundary layer.

The models are in good agreement aloft through the short term
portion of the forecast and the dynamic system set to move across
the area today and tonight. A large trough will quickly
consolidate over the Southern Plains this morning moving into the
Mid Mississippi Valley region and then Tennessee Valley by
nightfall. As it does so, plenty of energy will run through the
bottom of the trough and right across Kentucky punching through
the JKL area between 7 pm and midnight. The accompanying height
falls will pass through by 06z with heights rebounding a bit in
its wake into Sunday morning. A trailing shortwave does move over
the area Sunday afternoon and this will help to usher the system
out of here. Just minor differences are noted with the models as
this impactful system passes so confidence in most of the details
is relatively high. As such, have favored the short term model
blends while leaning most heavily on the highest resolution ones
like the HRRR and NAM12.

Sensible weather will feature lowering clouds and winds picking up
from west to east this morning with advisory level winds possible
by early afternoon. These windy conditions from the south to
southwest will continue through the evening as a cold front
approaches and passes through eastern Kentucky. There should be
just enough instability ahead of the boundary for a convective
line to form with some embedded thunderstorms. The strongest of
these could tap into the strong wind field aloft to bring down
damaging winds. As a result, much of the JKL CWA is in a marginal
risk for severe weather per SPC. Will keep this highlighted in the
forthcoming HWO and social media graphics. A little more than a
half an inch of rain can be expected for most of the area as the
front moves through. Winds will then switch to the west and
northwest staying strong and at wind advisory level through
midnight. After that, though, the winds will start to subside with
colder air advecting into this part of the state with falling
temperatures and upslope flow into Sunday morning. This is
expected to result in a mixing of light pcpn with snow showers or
flurries for a time before drier air takes over from west to east
later in the morning ending all but the last lingering traces in
the far east by afternoon. With the snow potential only the
eastern ridges will have a shot at accumulations though warm
ground temperatures will negate much of this before it ends. As
such, just have a small amount - less than a 1/2 inch on the
highest points (but close to 1" on top of Black Mountain) down to
a trace on the lower elevation ridges. A chilly day will follow
for Sunday under mostly cloudy conditions and continued CAA on
northwest winds.

Used the CONSShort and ShortBlend as the starting point for all
the grids, though did make significant changes to beef up the
winds and gusts today into the evening. Also, kept the PoPs maxed
out as the front crosses this evening rather than having them
drop back into the 80 percentile range per the blends tonight.
Finally, for Sunday lowered the highs a notch and allowed for more
lingering light pcpn (ra/sn) than implied by the model blends.
Will continue the wind advisory as outlined by the day shift.

.LONG TERM...(Monday through Saturday)
Issued at 250 PM EST SAT NOV 18 2017

There is good model agreement with the evolution of the overall
mean upper air pattern, but there are considerable differences in
the details. This is not surprising considering we will be in a
fairly progressive pattern through the coming week. The progressive
pattern will result in a roller coaster pattern for temperatures
from Monday through Saturday. Moisture will remain limited, with
the next chance for rain not coming until next Saturday.

The week will begin on a cold note, with an upper level trough axis
to our east, weak upper ridging over the MS valley, and surface high
pressure over the central and southern Appalachians. As the surface
high shifts east from Monday into Tuesday and weak upper ridging
builds over the area in advance of the next upper trough, we`ll see
a quick warm up into Tuesday. That will be short lived as the next
trough moves east and drives another cold front across the area
Tuesday night. There will be very limited moisture with this front,
and no precipitation is expected, but it will bring colder
temperatures back to the area for Wednesday and Thanksgiving Day.
Another warm up begins late in the week that will last into next
weekend. Rain chances will return next Saturday as yet another cold
front approaches the area. Looking just beyond the forecast period,
indications are for another shot of cold air late next weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
ISSUED AT 100 PM EST SAT NOV 18 2017

A cold front continues to approach the region, resulting in strong
and gusty southerly flow. sustained winds between 10 and 20 knots
with gusts between 35 and 50 mph will be possible at all TAF
sites this afternoon and evening. Isolated showers are also
starting to pop up across eastern Kentucky, but should pose little
concern with continued VFR VIS and CIGS. CIGS are expected to
continue lowering to MVFR as the cold front nears the region late
this afternoon/evening. Along the actual front, more widespread
showers (and possibly an isolated thunderstorm) will occur which
may lower VIS to MVFR (or possibly lower at times), passing from
west to east generally between 21Z and 3Z. However light rain
could linger through 6Z. Rain will continue to taper off through
the rest of the overnight as winds become more westerly and
entrain in drier air. Wind speeds will also quickly decrease
behind the frontal passage. Dry but MVFR CIGS are expected to
continue into the day Sunday.

&&

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

Wind Advisory until 4 AM EST Sunday for KYZ085>088-107-109-110-
112-113-115>120.

&&

$$

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



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