Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 170910 CCA

National Weather Service Jackson KY
510 AM EDT Tue Oct 17 2017

.SHORT TERM...(Today through Wednesday)
Issued at 354 AM EDT TUE OCT 17 2017

Early this morning, a surface ridge of high pressure extends from
the OH Valley and Appalachians to the Southern Plains. This high
is expected to remain in control of the weather across the region
through the period as it eventually becomes centered off the east
of the area. Skies were clear overnight and valley fog has
developed in many of the river valleys and along larger creeks and
streams. The fog is evident on GOES 16 10.3-3.9 fog product and
GOES 16 nighttime cloud microphysics. AWOS sites at BYL and I35
are both reporting dense fog at 7Z. As of 3 AM, valley temperatures
were generally in the upper 30s with some low 40s on some

Fog will continue to develop through the remainder of the night
and some patchy frost may develop in valley locations that do not
experience fog. Otherwise, the fog should begin to lift and
dissipate around sunrise, before finally mixing out toward 10AM.

Thereafter, sunny skies are expected with milder conditions
expected today with surface high pressure in place and as a
shortwave ridge moves across the area. Temperatures should average
within 5 degrees of normal for this time of year. 1000 to 500 mb
layer rh is progged by the models to drop into the 20 to 25
percent range. Mixing into this dry air aloft of which models
have the driest of the air above 850 mb should lead to a bit lower
dewpoints, especially across the east and southeast part of the
area, compared to Monday with more mid 30s common for a few hours
in the afternoon. When combined with the higher temperatures,
rh will bottom out near 30 percent in several locations this
afternoon with some locations bottoming out in the 25 to 30
percent range.

The surface high will continue to dominate the weather tonight and
into the day on Wednesday as noted earlier. A weak 500 mb
shortwave will approach the area late tonight and then work into
the area on Wednesday. This system will have very limited moisture
and the only noticeable affect may be an increase in high clouds.
Winds will also shift around to the southeast tonight and then
the south on Wednesday as the center of the sfc high moves off to
the east and northeast. Nocturnal inversion development should
lead to a ridge/valley temperature split that is moderate in
magnitude especially in the east. Valleys should reach the mid to
upper 30s while ridgetops bottom out in the lower 40s. Went more
toward the Coop MOS side of guidance for the valley low temps
tonight. With the dewpoints expected to have dropped off a bit
more today and high pressure remaining in place tonight, the
potential will again be there for patchy frost in some of the
deeper valley locations along with some fog as well. The frost
potential will continue to be highlighted in the HWO.

Only some passing high clouds are expected on Wednesday.
Temperatures should moderate further with weak southerly flow in
place as sfc high pressure centered to the east of the area
continues to dominate. Temperatures should top out in the lower
70s. Even with some slight recovery in dewpoints as well with the
southerly flow, rh will again bottom out near 30 percent if not
below during the afternoon.

.LONG TERM...(Wednesday night through Monday)
Issued at 310 AM EDT TUE OCT 17 2017

The models are in fairly good agreement aloft for the bulk of the
extended portion of the forecast. They all depict the weak shortwave
moving through the Ohio Valley Wednesday night. This will
temporarily suppress the heights through the Ohio Valley as it
passes with some weak energy accompanying the broader wave. Heights
will rebound quickly in its wake, though, as ridging will build
north from the Central Gulf Coast up into Kentucky Thursday. This
ridge then strengthens as it holds court over the Southeast part of
the nation through the first half of the weekend. More model spread
is found to the west at this time step with plenty of energy and
troughing spilling through the Rockies and into the Plains. The
stubborn ridge will hold over eastern Kentucky into Sunday even as a
Full latitude trough heads toward the Mississippi River. At this
point the model agreement breaks down with the 12z ECMWF curling the
southern portion of its trough into a deepening closed low over
eastern Texas while the GFS maintains a progressive trough into
Kentucky Sunday night into Monday. Another difference will be the
location of the core energy and troughiness from the models into
Monday with the ECMWF concentrated with its Southern Plains closed
low and the GFS keeping more to the north for the great Lakes. The
fresh 00z version of the ECMWF does come closer to the GFS in the
latter periods lending more confidence to their composite forecast.
With the uncertainty late in the period will generally stick with a
blended solution - not ready to throw in with any specific model

Sensible weather will feature sfc high pressure and a dry air mass
leading to a continuation or renewal of the warm and dry weather
that has characterized the start of autumn thus far. Even the mid
level weakness at the start of the extended will be marked by little
more than some extra high and mid level cloudiness. Given this
pattern will focus mainly on hitting the ridge to valley temperature
differences at night and the midday mix-down of drier air in the
dewpoint grids. Will also allow for some showers moving into the
area late Sunday into Monday as the heights will likely be falling
with more energy intrusion regardless of which model ends up being
more correct.

Throughout the forecast made the typical terrain based adjustments
to the grids most nights, aside from Sunday night into Monday as
moisture will be higher with more uncertain conditions for an
inversion. As for PoPs, basically kept them minimized through Sunday
morning and then allowed them to move into the high chance category -
in line with the 00z model consensus.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)

High pressure is in place across the OH Valley this morning. Some
valley fog has already developed in East KY and this should
become dense in a few valley locations during the next few hours.
At this point, the TAF sites are expected to remain MVFR or
better. The exceptions may be KSYM where fog is evident in the
nearby Licking River Valley and possibly KSME. Guidance has
generally trended toward less fog for both sites, however, so
confidence was not high enough to go with IFR at this time. Valley
fog will lift and dissipate by 14Z, with VFR conditions in all
locations from then until the end of the period. Winds will be
light and variable through the period with high pressure in place.




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