Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 212005
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
405 PM EDT Thu Sep 21 2017

.SHORT TERM...(This evening through Friday night)
Issued at 405 PM EDT THU SEP 21 2017

19z sfc analysis shows weak high pressure through the area. This
has not been strong enough to keep the convection at bay as mostly
pinprick showers and a stray thunderstorm have developed this
afternoon and are drifting south. In general, the more
substantial showers are favoring the Cumberland Valley while
mainly sprinkles are falling from any other cells. The small
increase in cloud cover and limited pcpn coverage this afternoon
did not slow the temperature rise much as they are topping out in
the mid 80s most places while dewpoints are generally in the mid
60s. Winds through east Kentucky are mainly light from the
northeast.

The models are in good agreement aloft through the short term
portion of the forecast. They all agree that ridging will hold
over the area into the weekend. Early on, though, some stuck
energy will be around the area before fading out tonight. This
will be more of smaller scale issue so that the higher resolution
models will be better at capturing the sensible weather. For that
reason, while a general model blend was favored, the near term
CAMS guidance was leaned on most heavily for the first part of
the forecast, along with some persistence for the late night with
respect to fog.

Sensible weather will feature a few showers and a possible storm
into the first part of the evening. Again expect the convection to
die out and the clouds to clear this evening setting the stage for
radiational cooling and the development of fog - dense in the
river valleys - toward dawn. The fog will burn off by mid morning
Friday and with less support from aloft expect a convection-free
day and more warm conditions as we cross into autumn late that
afternoon . Similar conditions as to tonight for Friday night will
lead to areas of fog developing in the valleys - locally dense
near the rivers and lakes.

Used the CONSShort and ShortBlend as the starting point for the
grids with some adjustments to the spot temperatures at night
given the anticipation of ridge to valley splits. As for PoPs,
kept the elevated through 00z this evening before dropping them
below mention for wx later tonight and then kept them dry into
the weekend.

.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)
ISSUED AT 200 PM EDT THU SEP 21 2017

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.

&&

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

$$

SHORT TERM...GREIF
LONG TERM...RAY
AVIATION...GEOGERIAN/GREIF



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