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Area forecast discussion 
National Weather Service Jackson Kentucky
505 am EST Tue Dec 6 2016

Short term...(today through wednesday)
issued at 438 am EST Tuesday Dec 6 2016

A strong upper level low was in place across Texas as of this past
evening, and has quickly transitioned northeast while weakening into
an open wave, now located across Mississippi as of 9z this morning.
This will continue to track northeast, moving across Tennessee and eastern
Kentucky by this afternoon while dissipating even further, quickly exiting
by 0z Wednesday. Meanwhile, the surface low pressure system is
outracing the upper level wave, already pushing across Tennessee and
heading towards the eastern KY, with an expected direct impact to
the County Warning Area between 15 and 18z today, before quickly shifting northeast
out of the state.

Fairly good wind sheer is taking place, with light east to southeast winds
ahead of the surface low, and strong west to SW winds aloft pulling in
moisture from the Gulf of Mexico ahead of the upper level wave.
There was a pretty sharp cutoff, as was forecast, between the high
clouds that were across the region, and the time at which these
clouds built down and rain began. A sharp llvl inversion is also in
place given the wind/temp disparity. As of now, much of the County Warning Area has
become saturated, with light to moderate rain reaching the ground.
This rain is expected to continue throughout the day as the center
of the surface low crosses over the region. As llvl winds begin to
shift to a more southerly and then easterly direction on the
backside of the surface low, midlevel winds will also become more
easterly, generally after 15z. Once this wind shift takes place,
much drier air will be able to filter into the region, with the
upper levels drying out by 21z, and the mid levels following suit by
0z this evening. This process may be delayed by an hour or two in
the far east. The dry air entrainment will essentially cut off best
rainmaking processes as well, though will leave a layer of moisture
near the surface, stuck below a strong llvl inversion that will
setup overnight.

This set up looks very similar to that of the last rain system we
had, which leans itself to low clouds and drizzle potential through
much of the overnight instead of fog. Went ahead and added patchy
drizzle into the forecast, generally tapering off from west to east
as dry air filters in near the surface throughout the overnight.
Temperatures are expected to fall to near freezing in many
locations. While this may raise some concerns for freezing drizzle,
it appears as most of the drizzle will taper off with the cooler
drier winds moving in, before temperatures reach the freezing mark,
thereby alleviating concerns.

Dry air will continue to infiltrate the region through the day
Wednesday as a broad area of surface high pressure sets in. Aloft,
strong west to west-southwest flow will remain in place, also pulling drier air in
from the Central Plains. This will help skies will clear throughout
the late morning/early afternoon. Surface winds, while light, will
generally be more northerly in direction. This will ultimately
result in temperatures just below seasonable normals for afternoon
highs on Wednesday, generally in the mid 40s.

Long term...(wednesday night through monday)
issued at 505 am EST Tuesday Dec 6 2016

The models are in good agreement concerning the amplified long
wave pattern in place across the Continental U.S. During the middle of the
week. This regime will transition to more zonal flow through the
first part of the weekend, before amplifying once again through
early next week. There is more confidence in the smaller scale
features through Saturday. Confidence then drops off through the
rest of the extended portion of the forecast due to timing and
evolution differences.

A deep upper level low will shift east from Ontario to
Quebec through Thursday, enabling a short wave trough to swing
through the Great Lakes and Ohio Valley regions. This will bring
the coldest air thus far this season into eastern Kentucky. Winds
will shift to the northwest late Wednesday night into Thursday,
with some light snow, although impacts looks to be minimal given
the shallow depth of the moisture available and most of the better
upper level support passing by further north. Highs on Thursday
will be in the low to mid 30s at best, with temperatures likely
dropping through the afternoon.

Despite lingering cloud cover into Thursday night, lows will drop
off into the teens for most locations, with highs on Friday only
in the 20s. The center of the high pressure looks to pass through
the region Friday night into Saturday morning. Clouds should be a
bit thinner, as the flow becomes more zonal, allowing at least for
some mid teens, if not lower potentially.

Highs will recover back to the mid to upper 30s on Saturday. The
next system will affect the area from Sunday through Monday, as a
trough digs in across the plains and then moves east, dragging a
cold front across the commonwealth. The GFS is more progressive
with this system compared to its mean, with the European model (ecmwf) slower
still. Have therefore allowed for a slightly slower onset of the
precipitation compared to that of the blended guidance. Consequently,
this also resulted into hanging onto higher pops deeper into Monday
and beyond. Did allow for a small chance of a rain/snow mix for
early sunday; however, given the likely slower trend, any
foreseeable impacts look too low confidence to mention at this
point. Temperatures will be moderating back closer to normal
through early next week, with highs back near the 50 degree mark
by Monday.

&&

Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 114 am EST Tuesday Dec 6 2016

Conditions are beginning to deteriorate at taf issuance,
especially across the southern portion of the state. Ksme has now
fallen to MVFR, and kloz will be soon to follow over the next
hour or so. High clouds still persist at the remaining taf sites,
but should begin falling quickly to MVFR generally after 8z at
kjkl and closer to 9z at ksym. Rainfall will be on the heals of
the ceiling drop, expected to start to ksme and kloz over the next
couple of hours as well. Ceilings will likely lower further to IFR
and possibly LIFR by Tuesday morning as rainfall becomes more
widespread. Visibilities will also decrease to MVFR and
potentially IFR as rain falls. Should see the bulk of the rain
taper off by late afternoon, allowing visibilities to increase to
VFR criteria. However, IFR ceilings look to be a good bet through
the end of the taf period (6z). Winds will increase to 5-10 knots
through the rest of tonight, while veering southeasterly. Will see
these veer further to the west/southwest this afternoon, with
speeds increasing to near 10 knots with higher gusts during the
latter portion of the afternoon. All this as a cold front gets set
to move through the Ohio Valley and commonwealth. Additionally,
southeasterly low level wind shear near 35-40 knots may become an
issue tonight across far southeast Kentucky.

&&

Jkl watches/warnings/advisories...
none.
&&

$$

Short term...jmw
long term...geogerian
aviation...jmw

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