Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 050130
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
830 PM EST Sun Dec 4 2016

.UPDATE...
Issued at 830 PM EST SUN DEC 4 2016

WSR-88D radar indicating that while narrow the next band of rain
does expand south to cover much of the region. This hour this
narrow band of rain was along the western portion of the CWA and
therefore expanded numerous POPs along this narrow band. Using a
combination of short term models and hand edits bringing this
band from west to east through 5Z before exiting the CWA. Other
changes were minor at this time to add latest obs.

UPDATE Issued at 711 PM EST SUN DEC 4 2016

WSR-88D radar picking up on some light rain and drizzle across
much of the region this hour. We are stuck between the southern
stream overrunning and the northern stream trough swinging into
the midwest. The northern stream system will bring best chances of
rain to the northern half of the region over the next few hours
and then exits east by later tonight. Given this did raise POPs
generally along and north of the Mountain Parkway with the
aforementioned trough. Otherwise most spots will see light
rain/drizzle/fog overnight into Monday morning. Upstream obs and
forecast soundings would suggest this stratus sticks around till
sometime Monday afternoon. That said felt the clouds were
clearing too soon in grids and much of the guidance overnight.
Therefore increased clouds through the night to better match up to
other offices and model soundings.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 300 PM EST SUN DEC 4 2016

Active pattern on tap in the short term. The initial weather maker
for eastern Kentucky will be an area of low pressure aloft, that
is expected to move across the region this afternoon through the
overnight period. Scattered to at times numerous light rain
showers will move across eastern Kentucky through early this
evening. The rain should finally begin to taper off after 4Z
tonight, as the upper low moves off to our east. Eastern Kentucky
should be precipitation free by around 11Z Monday. Rainfall
amounts with this first system should be very light across eastern
KY tonight, generally a tenth of an inch or less. After several
hours of quiet weather, another area of low pressure is forecast
to approach the region from the southwest Monday afternoon and
evening. Rain showers associated with this second weather system
should have overspread the entire area by around 6Z Tuesday. This
second weather system will provide a more robust shot of rain. We
could see rainfall amounts Monday night into early Tuesday morning
rain from around 0.4 inches in our far north, to between 0.5 and
0.8 inches in our far south.

Temperatures should be running at or slightly below normal during
the day on Monday, with highs ranging from the mid 40s in our
northern areas to around 50 in our southern areas. Tonights lows
should bottom out with above normal values in the mid to upper
30s, due to the influence of cloud cover and precipitation.
Tomorrow nights lows look to be even warmer, as winds shift from
the east to south and increase in strength and cloud cover and
precipitation also increase in coverage. Monday nights lows are
expected to fall into the low to mid 40s, with a non diurnal
hourly temperature pattern also expected due to cloud cover and
wind influences.

.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
east.

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 00Z TAFS through 00Z Monday evening)
ISSUED AT 621 PM EST SUN DEC 4 2016

Tricky TAFs for the period as we see southern stream system exits
east this hour and northern stream system approaching from the
west. Overall we will see TAF sites drop through the night with
rain and drizzle aiding in restricting VIS. Many sites will
approach if not exceed airport mins overnight into early Monday.
We will see improvements as the rain exits by Monday, and
possibly VFR by the afternoon before another system approaches.
Winds will be light and not a concern through the TAF period.

&&

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

$$

UPDATE...DJ
SHORT TERM...AR
LONG TERM...HAL
AVIATION...DJ



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