Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 050903

National Weather Service Jackson KY
403 AM EST Mon Dec 5 2016

.SHORT TERM...(Today through Tuesday)
Issued at 403 AM EST MON DEC 5 2016

As the mid/upper level wave that impacted the area yesterday
continues to shift northeast of the region, more zonal flow is
taking hold with surface high pressure moving eastward across the
state. That being said, MVFR clouds continue to plague much of the
region under a pretty steep inversion that has set up. Dry air will
continue to filter in aloft and in the mid levels throughout the
day, eventually working its way down to the surface and allowing the
low cloud deck to mix out, generally after 15 to 18Z.

Attention then turns to a strong upper level low positioned over
southern TX this afternoon. At the surface, the center of low
pressure will be located just off the TX/Louisiana coast, but will
continue to push northeastward throughout the day following the
upper level low, quickly reaching Mississippi by this evening, and
TN late tonight, before traversing eastern KY and eastern TN during
the day tomorrow. This will push the center of high pressure
currently across the state to our northeast, and allow deeper
moisture to quickly begin filling into the state from the south,
reaching our southern CWA between 21Z and 0Z this afternoon/evening.
That being said, upper level winds will continue to pull moisture
into the region well ahead of this system, so even as the low clouds
begin to mix out today, high clouds will already begin building in
ahead of this system. Overall, it is not likely that we will see
much of a break in the clouds throughout the day as a result.

According to the latest forecast soundings from the NAM12, the
transition from high clouds to moisture build down and precip will
be very quick. Only slight chance pops are in the forecast along the
TN border before 0Z, but widespread rain is expected to encompass
the entire southern half of the CWA by 6Z, and and the entire
eastern portion of the state by 12Z. A steady rain is expected to
continue throughout the day Tuesday. Still not seeing enough support
for instability, even aloft, to include thunder at this time.

The upper level low is expected to decrease in strength, becoming an
open wave by the time it moves over eastern KY. As the surface low
pressure center moves across the CWA in the afternoon, drier air
will be quick to filter in behind, weakening the surface low and
cutting off best rain potential. As such, rain is likely to end just
as quickly as it started, with only low end chances expected across
far eastern KY by 0Z Tuesday.

While temperatures today will be near seasonable normals despite
cloud cover, the surge of warmer moist air into the region will
actually boost temps well into the 50s for the day tomorrow.

.LONG TERM...(Tuesday through Sunday)
Issued at 434 PM EST SUN DEC 4 2016

There are three potential systems to deal with, and the unusual
situation where the greatest uncertainty is in the middle of the
forecast period.

System number one will be here as the period begins. Surface low
pressure is expected to be over middle TN at 12z Tuesday, with an
affiliated shortwave aloft just a bit further to the southwest.
The low should be tracking northeast along the western edge of the
Appalachians during the day while coastal development occurs over
the Carolinas. Rain is expected, mainly Tuesday morning. It looks
as if cold air advection will bring a nondiurnal temperature,
with temperatures starting to drop off Tuesday afternoon. The
coastal low will take over as the main low while the system is
pulling out Tuesday night, and surface ridging will build in just
to our north and bring quiet weather lasting into Wednesday.

The effects of the model controversy come Wednesday night into
Thursday for our area. It involves a shortwave moving east from
the central Rockies Tuesday night into Wednesday, and its
interaction with an upper low over southern Canada. The ECMWF
holds a substantial portion of this upper low over southwest
Ontario Wednesday, while the GFS allows it to take off to the
east and leaves a weaker portion behind. The aforementioned
shortwave phases with whatever of the upper low is left behind.
This results in the ECMWF having a stronger system than the GFS.
The ECMWF has weakened with its related surface system and is
slightly more progressive with it when compared to earlier runs,
which is a trend toward the GFS. The stronger and slower system
of the ECMWF would briefly pull in milder air and result in rain.
the GFS would have much less precip, but would allow for snow or a
mix. At this point, have continued to use a blend. The resulting
forecast has high chance pops Wednesday night into Thursday,
mainly in the form of rain, but does mention snow.

Better agreement comes to finish the period. Cold air advection
and northwest flow brings scattered snow showers Thursday night,
followed by the coldest air of the season so far as strong high
pressure drops into the central CONUS from Canada and then heads

Both the GFS and ECMWF show warm air advection and isentropic lift
bringing a potential for precip on Sunday, with borderline temperatures
allowing for rain or snow.


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

Tricky TAFs for the period once again. Upper level wave is
continuing to exit northeast of the region, with surface high
pressure moving in across the state. In most situations the
lingering moisture and high pressure center would suggest fog,
however in this situation, we are still seeing thick llvl clouds
across much of the region, including well upstream of the TAF
sites. As such, redid TAFs to remove mention of fog, as this will
likely be a low stratus event, with generally MVFR CIGS expected.
Can`t rule out some IFR at times however, though it seems like the
majority of the obs upstream are within the MVFR range. Tomorrow,
low clouds will likely persist into the afternoon, before finally
starting to scour out. About this time, another system will begin
approaching from the southwest. This will allow for high clouds to
enter back into the region, and quickly build down after 0Z, with
rain chances soon to follow. Went ahead and included VCSH at most
TAF sites between 3 and 4Z, except for the far northern KSYM,
where chances will likely occur closer to the end of the forecast
period. Overall winds should remain light and variable, though
more gusty conditions may continue at KSYM through the next
several hours before finally dissipating.




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