Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 181205 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
805 AM EDT Fri Aug 18 2017

.UPDATE...
Issued at 745 AM EDT FRI AUG 18 2017

Did a quick update to fine tune the PoPs and Wx grids through
Noon and also to adjust the T/Td grids per the latest obs and
trends. These have been sent to the NDFD and web servers.

&&

.SHORT TERM...(Today through Saturday)
Issued at 400 AM EDT FRI AUG 18 2017

07z sfc analysis shows a broad and fairly deep area of low
pressure over the Great Lakes with a cold front stretching south
and southwest through the Ohio Valley. This front is just west of
the JKL CWA and pressing eastward with clusters of showers and
embedded thunderstorms out ahead of it. A few of these clusters
are exhibiting training and will have to be monitored for
excessive rains through the next few hours. Clouds have overspread
most of the CWA early this morning ahead of the boundary and with
the convection, though some clear patches are found in the far
east where the best shot at seeing some fleeting, but locally
dense, fog exists. Temperatures are running in the very mild low
70s most places with dewpoints equally high. A few of the
convective cells have also been able to bring down some higher
wind gusts - up to 30 mph from the southwest, otherwise winds are
generally light and from the south throughout eastern Kentucky
early this morning.

The models are in good agreement aloft building a trough over the
region through the short term portion of the forecast. They all
depict a decent short wave passing through the northern parts of
the area this morning as height falls spread through Kentucky. The
troughiness over the region will be reinforced on Saturday as a
substantial wave moves across the Ohio Valley with further height
falls as the main trough bottoms out. This features moves by
quickly, though, with heights starting to rebound by evening.
Given the good model agreement have favored a general blend with
greatest consideration to the radar trends and higher resolution
HRRR in the near term - NAM12 thereafter.

Sensible weather will feature showers and potential thunderstorms
passing through the CWA for the first part of the morning with
much of this taking place in the pre-first period time frame - so
will run the ZFP formatter with that in mind. Drier air and
clearing skies then move into the region this afternoon with not
so high dewpoints on tap. As the air mass changes today, a cooler
but still mild, night will ensue but cold to neutral advection
will limit terrain differences, though some valley fog can be
anticipated under a regime of mostly clear skies and light winds.
For Saturday, slightly more comfortable humidity levels will not
prevent temperatures from climbing into the seasonable norms of
mid 80s for highs under partly sunny skies. Given the trough push
aloft, though, cannot rule out a stray shower or storm, mainly in
the far north during the day, but limited moisture availability
should make this a minor concern.

Once again the CONSShort was used as the starting point for the
grids through the short term portion of the forecast with only
small adjustments to temperatures for terrain distinctions. As
for PoPs, mainly adjusted them up this morning given the ongoing
radar trends and beefy HRRR through dawn. Also trimmed them back
quicker than guidance later this morning.

.LONG TERM...(Saturday night through Thursday)
Issued at 445 AM EDT FRI AUG 18 2017

The axis of a shortwave trough will be exiting to our east to
start the extended portion of the forecast, with rising heights
expected through the day Monday as a high pressure ridge
establishes itself along the far southeast. Heights will quickly
lower, however, as a upper level low dive bombs to the SE through
eastern Canada Tuesday, continuing across Ontario and then Quebec
Wednesday. A secondary low will develop and strengthen the
longwave trough across the eastern U.S. to finish out the forecast
period.

As for sensible weather... High pressure will be in control at
the surface to start the first part of the forecast period, being
reinforced by the upper level ridge. This will boost temps above
normal in the upper 80s both Sunday and Monday. There was some
discrepancy about a few afternoon diurnal showers/thunderstorms in
the CWA Monday. However, latest models are continuing to go more
dry. The GFS does put a spot or two over us in the afternoon in
the high terrain, misplaced from the previous run. Given its weak
and isolated nature, and the inconsistencies between model runs,
capped pops at 14 so that no mention of precip was in the
forecast.

As the ridge breaks down aloft, a surface low is expected to move
eastward north of the state. From this a cold front will sink
southward, traversing Kentucky through the day Wednesday. This
will bring chances for showers and thunderstorms to the region as
early as during the day Tuesday, increasing to likely coverage
during the day Wednesday just ahead of the frontal passage. Chance
of precip will remain in place Wednesday night, tapering off
from north to south during the day Thursday. Given the lack of
post frontal instability, kept only rain mentioned from Wednesday
night on. Temperatures will remain in the upper 80s during the day
Tuesday as warm moist air is advected in ahead of the frontal
passage. However the trend thereafter will be a cooling one.
Wednesday will see highs in the low 80s, Thursday right upper 70s
to right around 80, and by Friday, only mid and upper 70s.
Overnight temperatures will be much cooler during the last part of
the work week as well.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
ISSUED AT 755 AM EDT FRI AUG 18 2017

The main cluster of showers and a few embedded thunderstorms is
pushing through far eastern parts of the CWA this morning with
some IFR conditions. Following the departure of the convection
conditions will improve at all TAF sites with VFR returning.
Winds, outside of any storm, will be light and variable through
mid morning before picking up from the west at 5 to 10 kts post
fropa.

&&

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

$$

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


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