Area Forecast Discussion
Issued by NWS Jackson, KY

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167
FXUS63 KJKL 061510 AAB
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
1010 AM EST Tue Dec 6 2016

.UPDATE...
Issued at 1010 AM EST TUE DEC 6 2016

The forecast remains on track so far today, with periods of
widespread light to moderate rain showers expected across the
area. Current temperatures should not change much during the
course of the day, due to the ongoing precipitation and widespread
cloud cover. Will not be updating the zone forecast text product
just yet as the current forecast is good as is. An update will be
required around noon today, to remove morning wording from the
forecast text product and to make any other changes that may be
necessary by then.

UPDATE Issued at 728 AM EST TUE DEC 6 2016

Light to moderate rain continues across much of eastern KY,
especially the southern half of the CWA. This seems to jive well
with the ongoing POP and weather forecast. Still expecting rain to
continue across the area throughout the day, so no updates needed
to this portion of the forecast at this time. Temperatures will
only increase by a few degrees throughout the day given the
ongoing cloud cover and rain. Did update the forecasts for the near
term temps, dew points, and winds, however, to make sure they were
on track with current observations. All changes have been
published and sent to NDFD/web.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 438 AM EST TUE DEC 6 2016

A strong upper level low was in place across TX 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 TN and eastern
KY 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 TN and
heading towards the eastern KY, with an expected direct impact to
the CWA between 15 and 18Z today, before quickly shifting northeast
out of the state.

Fairly good wind sheer is taking place, with light E to SE winds
ahead of the surface low, and strong W 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 CWA 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 W to WSW 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 TUE DEC 6 2016

The models are in good agreement concerning the amplified long
wave pattern in place across the CONUS 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 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 12Z TAFS through 12Z Wednesday morning)
ISSUED AT 736 AM EST TUE DEC 6 2016

Rain continues across much of eastern KY, with heaviest coverage
currently across KSME and KLOZ. Cigs range across the CWA,
anywhere from MVFR to VFR. Tried to trend to most sustained
values, though there will continue to be some bouncing around
throughout the day. Overall trend should be for deteriorating CIGS
throughout the day, reaching IFR conditions by this afternoon,
generally between 15 and 18Z. Rain may also cause some VIS
restrictions, generally MVFR. The surface low pressure system
which is spawning this rain will continue to move across eastern
KY through this afternoon before exiting east of the region.

Dry air will quickly filter in across eastern KY in the mid and
upper levels behind this surface low. This will cut off best rain
potential by this evening and into the overnight. However a strong
llvl inversion will set up, trapping abundant moisture near the
surface. As such, depth of moisture may be enough that drizzle
could occur through about 6Z at most TAF sites. After 6Z expect
slowly improving conditions for both the VIS and CIGS as rain
chances exit the area for good. Winds will generally remain under
10 knots throughout the period, however some of the heaviest rains
could mix gusts between 10 and 20 mph to the surface during the
day today.

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 CIG 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.
&&

$$

UPDATE...AR
SHORT TERM...JMW
LONG TERM...GEOGERIAN
AVIATION...JMW



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