Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 180744

National Weather Service Jackson KY
344 AM EDT Wed Oct 18 2017

.SHORT TERM...(Today through Thursday)
Issued at 344 AM EDT WED OCT 18 2017

Early this morning, rather zonal upper level pattern was in place
across the Conus. An upper level ridge was centered southwest of
the Baja of California and extended into Mexico and the southern
Plains. Meanwhile, the shortwave upper ridge was moving through
the Appalachians with a weak shortwave trough approaching from
the west. At the surface, a ridge of high pressure was centered
over the central Appalachians and was in control of the weather
across East KY. A few high thin cirrus are passing overhead while
fog has developed in the river valleys and near some of the area
lakes and along some larger creeks. Temperatures were generally in
the upper 30s to around 40 in the valleys with temperatures on
ridges generally ranging through the 40s as of 7Z.

The axis of the shortwave upper level ridge will move east of the
area today as the weak shortwave trough moves into and across the
OH Valley through tonight. At the same time, upper level ridging
will begin to build across the western Gulf of Mexico and MS
Valley with height rises over the lower OH Valley on Thursday.
Also during this time frame, an upper low and associated surface
low pressure system will be tracking well north of the area across
Canada and the northern states. Locally, the axis of the surface
ridge of high pressure will shift south of the area today
resulting in light winds veering to average out of the south to
southwest. With the upper level ridge building toward the end of
the period, surface high pressure will become centered over the
southern Appalachians, but remain in control of the weather into
Thursday as well.

The main forecast challenge during the period is overnight lows
tonight and the extent to which mixing will lead to dewpoints
dropping off this afternoon and on Thursday afternoon despite
south to southwest low level flow. Temperatures will moderate
further today with temperatures topping out near or just above the
70 degree mark in most locations. Weak low level flow today and on
Thursday should limit the extent of mixing although have opted to
undercut superblend dewpoints a few degrees in the afternoon
leaning toward the lower MAV MOS values a bit. As for lows
tonight, opted to go just above the coldest valley temperature
compared to COOP MOS. With high pressure in place, ridgetop
temperatures were warmed a few degrees from superblend guidance
and lowered a few degrees in the valleys to account for the
expected persistence of a nocturnal inversion and ridge/valley
temperature split development. Some valley fog should again

.LONG TERM...(Thursday night through Tuesday)
Issued at 250 AM EDT WED OCT 18 2017

Models in generally good agreement on maintaining the mid/upper
level ridge across the area through the upcoming weekend. This
will mean cool mornings and mild afternoons through the weekend.
We will also continue to see low afternoon RH with dewpoints
staying below guidance. 12z ECMWF becomes more amplified with the
ridge late in the weekend as a sharp trough develops across the
plains. GFS is much weaker with this feature, so amplification is
much more scaled back. These differences translate into large
amount of uncertainty heading into early next week regarding
precipitation amounts. ECMWF is very wet with a much stronger
system crossing the region, while the GFS is much weaker.
Confidence is increasing we will see a decent chance of rain on
Monday, just large variance in overall amounts. Thus, rain chances
have been increased to categorical for Monday. The pattern will
turn more active after a substantial cool down behind the early
week system with several systems possible impacting us into the
middle of next week. The ECMWF with the stronger system has a
stronger push of cold air, but both models generally support it
cooling down. Thus, while the pattern looks like it will shift to
a cooler one, it also looks to be favoring a wetter one as well.
Certainly with the colder pattern, we will become more susceptible
to frost/freeze conditions as we through next week.


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

High pressure will remain in control of the weather through the
TAF period. Mostly VFR conditions are expected to persist through
the period, with the exception being in the river valleys and near
larger creeks and lakes, where fog that has already developed or
will be developing over the next few hours could become dense in
some locations, especially between 8Z and 13Z. Some of the fog
could advect into SME overnight with this less probable at the
other TAF sites. The fog will lift and should dissipate between 13
and 14Z. Winds through the TAF period will remain light averaging
out of the southeast to south.




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