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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KJKL 221116
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
716 AM EDT Tue Aug 22 2017

.UPDATE...
Issued at 657 AM EDT TUE AUG 22 2017

Forecast seems to be on track so far this morning. Loaded in the
latest observations to make sure the near term forecast for temps,
dew points, and winds were on track with current conditions. Fog
remains confined to the deeper valleys and should dissipate over
the next couple of hours as temperatures quickly heat up. All
changes have been published and sent to NDFD/web.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 445 AM EDT TUE AUG 22 2017

A cold front will be the main culprit during the short term
portion of the forecast. Aloft, troughing and NW flow across the
region will allow a cold front to push SE towards the state
throughout the day. It is then expected to traverse southward
across eastern Kentucky starting between 0 and 3Z this evening,
and exiting to our SE between 12 and 15Z Wednesday. As models
come into a more fine tuned agreement of this system, so has the
POP forecast. As the front moves across the state, strong surface
high pressure to our north will begin to sink southward. Winds
will quickly shift to a more northerly direction and allow much
more stable and dry air to filter in. This will likely result in a
fairly sharp cut off of precip behind the frontal passage tonight
and early Wednesday morning, though upslope flow could allow it
to linger a bit longer into Wednesday afternoon along the Virginia
border. Elsewhere, mostly clear conditions will be in play by
Wednesday, with continued NW flow aloft and Nrly flow at the
surface keeping temperatures below seasonable normals, in the
upper 70s with low humidity.

Latest CAMs are all in agreement that convection will move
through in a line structure ahead of the frontal boundary, with a
secondary line possibly developing along the actual front just
behind. The first round will affect eastern Kentucky through the
afternoon, dissipating the farther it gets east. The second line
then looks to pose the biggest punch as it moves across during the
late afternoon and early evening, weakening as we head into the
overnight. Also can`t rule out a few individual cells developing
out ahead of these lines. Forecast soundings continue to support
decent instability parameters as these lines move through the
region. 0 to 6km sheer still looks to be between 30 and 40 kts
through the afternoon which again is decent, but not overly
impressive. In context, we are located quite a distance from the
actual low center, so the overall best lift and sheer will be
located farther to our NE where more sustained/larger QLCS`s are
expected. SPC did not mention the concern of spin ups along the
line in our region, and reasonably so.

Freezing levels are quite high, and given the progressive nature
of the linear system, hail, while possible, is not the main
concern. Instead, still looking at more of a momentum transfer
wind event as mixing occurs in the heavier showers/storms and
along the leading edge of the line. Heavy rain will also be
possible, with PWATS peaking around or just above 2 inches during
the late afternoon/evening time frame. Thankfully we have had a
decent period of drying across much of eastern Kentucky, so this
should not prompt much of a flooding concern, especially given the
progressive nature of the system. That being said, can`t rule out
some isolated instances in low lying areas, urban areas, or
locations that may experience several rounds.

Overnight, the loss of heat and instability should weaken the
system and cut off severe potential, along with general thunder
potential. Dropped thunder down to chance and then slight chances
in the prefrontal environment through the overnight. Once the
front does pass through, convection will quickly be cut off all
together.

.LONG TERM...(Wednesday night through Monday)
Issued at 359 AM EDT TUE AUG 22 2017

The models are in good agreement with a more amplified long wave
pattern to hold on through the beginning of this weekend. Troughing
will mainly rule in the East, with ridging in the West. Details
concerning the evolution of the pattern by this weekend and early
next week become more murky, especially with regards to the now
remnants of Harvey. As such, did not deviate from the blended
guidance.

Dry weather will be on tap for eastern Kentucky through Friday, as
high pressure controls to our north. A weak inverted trough will
then nose in from the southwest, bringing just a slight chance of
showers to our Virginia border counties on Saturday. POPs will
diurnally increase into next week as the inverted trough appears
to sharpen up more; however, much will depend on the outcome of
Harvey.

Temperatures will average below normal through the period, with
highs mainly in the mid to upper 70s, with lows in the mid to
upper 50s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
ISSUED AT 716 AM EDT TUE AUG 22 2017

VFR conditions and light winds will prevail at the TAF sites
through early this afternoon, though clouds will be on the
increase. An approaching cold front will then bring scattered
thunderstorms this afternoon, with the storms becoming more
numerous by late afternoon/evening. As such, kept VCTS starting
around 16 to 17Z, increasing to predominate TSRA by 22 to 23Z.
Some of these storms could be severe, with damaging winds the main
threat. Torrential rainfall and frequent lightning will be
secondary threats. We lose instability heading into the overnight,
but showers will continue to be numerous through the first half
of the night (-SHRA VCTS around 3Z). Coverage will then quickly
taper off to scattered showers and isolated thunderstorms as the
front moves through late (went VCTS by 6Z). Any shower or
thunderstorm during the TAF period could reduce visibilities
quite dramatically as they pass over a TAF site, however given
the uncertainty of impacts and specific timing, went prevailing
MVFR at this time. Winds will generally be from the SW ahead of
the cold front, becoming more westerly just behind the frontal
passage late in the period.

&&

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

$$

UPDATE...JMW
SHORT TERM...JMW
LONG TERM...GEOGERIAN
AVIATION...JMW