Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 211910

National Weather Service Jackson KY
310 PM EDT Thu Sep 21 2017

Issued at 145 PM EDT THU SEP 21 2017

Did a quick update to allow for some convection even over the
northern parts of the CWA given the cellular nation of the cu
there and some returns starting to show up on radar. Have also
fine tuned the T and Td grids per the latest obs and trends. These
have been sent to the NDFD and web servers.

UPDATE Issued at 1015 AM EDT THU SEP 21 2017

14z sfc analysis shows a fairly persistent area of weak high
pressure through eastern Kentucky. This again led to mostly clear
skies last night (excepting a rogue thunderstorm that moved south
into the far eastern parts of the area). This fostered
radiational cooling resulting in a ridge to valley temperature
split along with dense fog in the river valleys. The fog has just
about cleared out with mostly sunny skies on tap through the rest
of the morning. Do still see a small potential for showers and
storms to develop just about anywhere across the area this
afternoon owing to some lingering energy aloft and weak high
pressure at the sfc. Currently readings are in the upper 60s and
lower 70s with moderately moist dewpoints in the low to mid 60s
along with light and variable winds. The forecast was updated
mainly to take out the fog, adjust sky cover, and also to tweak
temperatures and dewpoints per the latest obs/trends. These have
been sent to the NDFD and web servers. A freshened set of zones
and HWO have been sent, as well.

UPDATE Issued at 730 AM EDT THU SEP 21 2017

Have freshened up the hourly temperatures and dew points through
this morning to account for the fog dissipation. Otherwise the
forecast remains on track.


.SHORT TERM...(Today through Friday)
Issued at 350 AM EDT THU SEP 21 2017

Surface high pressure rules across the Ohio and Tennessee valleys.
Aloft, an upper level ridge is expanding across the Mississippi
valley, with some troughiness lingering across the Eastern

The models are in good agreement with the ridge gradually
strengthening across the Ohio Valley through the short term,
bringing above normal temperatures and drier weather.

For today, eastern Kentucky will start out foggy once again in the
valleys. Some short wave energy passing south on the leeward side
of the ridge axis will allow for a threat of mainly isolated
convection across our area today. Convection will die off quickly
into this evening, with dry weather to follow as the ridge builds
in stronger through Friday.

High temperatures will average in the mid 80s each day, with
Friday a degree or so warmer than today. Lows tonight will
average in the low to mid 60s, with areas of dense valley fog
setting up.

.LONG TERM...(Saturday through Thursday)
Issued at 310 PM EDT THU SEP 21 2017

Models are in good agreement through most of the extended. Mid
level ridge centered over the Great Lakes will eventually weaken
and dissipate as it shifts eastward into New England by the end of
the period. Further aloft, a rather large center of "relative"
low pressure over the Atlantic Coast will retrograde westward into
the deep south before lifting back out of the region into the
Atlantic. A more significant trough will be moving into the Upper
Midwest and Great Lakes region at the end of the extended period.
At the surface, high pressure centered over the upper Ohio Valley
will remain anchored until Dy6 Wednesday when it erodes under the
influence of Maria, currently forecasted to ride northward just
offshore of the east coast, and an approaching storm system from
the west. The storm system moving in from the west will bring a
cold front into the region towards the end of the forecast window,
or possibly just beyond. Timing of this surface boundary into our
area is in question since the 12Z GFS has trended slower with it`s
parent trough approaching from the west. The 0Z ECMWF suggests a
couple of shortwave impulses rounding the base of the main trough,
which may bring multiple surface boundaries (front/troughs) and
reinforcing shots of cooler air through the area. The 0Z ECMWF is
also about 18-24 hours faster than the 12Z GFS solution with the
main trough. A brief glance at the 12Z ECMWF shows a similar trend
in the timing of the system (slower) into the Ohio Valley, just
not as pronounced as the GFS. The ECMWF is now more consolidated
with the smaller scale features, or shortwave energy rounding the
base of the main trough as well.

Sensible weather remains quiet, dry, and warm through the bulk of
the extended. Temperatures will generally continue to run above
normal with daily highs in the 80s and overnight lows generally in
the 60s. Conditions remain favorable for patchy/areas of fog most
nights. As already mentioned the only potential of rain will come
at the end of the forecast window as a cold frontal system
approaches from the west. With uncertainty in timing and details
of this storm system will only advertise slight PoPs on the last
day of the extended. Suspect future runs will tend to keep
Thursday dry and hold off on higher PoPs until beyond the range of
the current forecast package. Eventually however, once this storm
system manages to clear the area, single digit H850 temperatures
will be ushered into the region providing what may be a true shot
of much cooler autumn air.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)

Scattered cumulus ranging from 3 to 5k feet agl are found through
the area this afternoon and a stray shower or storm may cross over
a TAF site. However, these should be isolated enough to keep a
mention out the TAFs. Skies will clear out once again tonight,
with LIFR or worse fog forming mainly in the valleys, but will
allow for some visibility restrictions from this late tonight at
the LOZ, SME, and SYM sites. Winds will be light and variable
through the period.





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