Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 161418

National Weather Service Jackson KY
1018 AM EDT Fri Mar 16 2018

Issued at 1018 AM EDT FRI MAR 16 2018

Have updated NDFD for latest observational trends. Main change was
to speed up the rate of dewpoint fall as drier air advects in from
the northeast.

UPDATE Issued at 715 AM EDT FRI MAR 16 2018

The winds have started to stir up from the north to northeast at
JKL and other locations around the CWA and as a result helped to
mix out much of the ridge to valley distinctions as well as
advecting in colder air from the northeast. Have updated the near
term grids to account for this as well as to tweak sky cover for
the next few hours. These grids have been sent to the NDFD and web


.SHORT TERM...(Today through Saturday)
Issued at 455 AM EDT FRI MAR 16 2018

08z sfc analysis shows high pressure over the Upper Midwest and
nosing into northeast Kentucky. At the same time, low pressure is
coming out of the Southern Plains and is starting to bolster a
boundary lying across southern parts of the area with increasing
clouds. Before they fill in very much, clear skies (and light to
calm winds) have allowed a fairly large ridge to valley
temperature split to develop, primarily in the east, ranging from
the lower 40s on the hills to the upper 20s in the sheltered low
spots. To the west, conditions are more uniform due to better
mixing most of the night. Meanwhile, dewpoints are generally in
the low to mid 20s throughout the area.

The models are in better agreement aloft for the short term
portion of the forecast. They all depict a flattening ridge
slipping over and through Kentucky today in the face of a
dampening mid level closed low spreading east along the Ohio
River later tonight and into Saturday. The energy associated with
this will start to impact the area after midnight with several
separate impulses passing through eastern Kentucky during the day
Saturday. The NAM is slightly stronger with these than the other
models and was preferred. In general, a model blend was favored
on account of the improving agreement, with a lean toward the
higher resolution NAM12 and HRRR in the near term.

Sensible weather will feature a cooler day than yesterday as the
high pressure exerts its fleeting influence over the region.
Meanwhile, a boundary to the southwest will become more of a warm
front, one with with increasing and lowering clouds, that will
start to lift northeast by the end of the afternoon. This will
become more activated this evening with showers and possible
thunderstorms expected to move through eastern Kentucky ahead of
the potent sfc low cutting east-southeast through the northern
parts of the state overnight into Saturday morning. Luckily, the
front will bring enough warmer air aloft and at the sfc for the
JKL CWA to prevent a concern for freezing rain or sleet at its
onset. The low`s movement will keep much of eastern Kentucky in
the system`s warm sector with at least some instability building
along with a good wind profile for storm development. In fact,
some locations in the Cumberland Valley could reach 70 degrees
(while upper 40s for highs are found near and north of Interstate
64 where the warm front will stall out). Accordingly, SPC has
placed much of the area in an outlook for a marginal threat of
severe weather on Saturday with large hail and potentially
damaging wind gusts the main threats. Have included thunder in the
wx grids for this and highlighted the severe potential in the
HWO. This core of the sfc low will pass through eastern Kentucky
by 00z Sunday with cooler conditions to follow.

Used the CONSShort and ShortBlend to kick start the grids with
some significant temperature and dewpoint adjustments in the near
term per the current conditions reflecting a strong ridge to
valley difference for the eastern valleys. As for PoP, did bring
some small chances in a tad quicker this evening and into the
overnight as the sfc low gets closer.

.LONG TERM...(Saturday night through Thursday)
Issued at 515 AM EDT FRI MAR 16 2018

A bit of an active upper level pattern expected through the
extended, as multiple shortwaves move across the region. The first
of these will occur through the night Saturday, with ridging returns
for the day Sunday. This ridging will quickly become dampened as a
closed upper level low quickly gains strength and pushes into the
southern Plains by Monday morning. From here it will change paths,
quickly shifting eastward and across the state of Kentucky Monday
night. While it is expected to exit NE of the state Tuesday
afternoon, it will merge with another closed low system and result
in overarching troughing in place across the region through Thursday
night. Rising heights will be the story for Friday as both models
agree on an area of ridging moving eastward into the state.

As for sensible weather...With each passing upper level disturbance,
expect impacts in our weather. Therefore, will say the extended
portion of the forecast does look generally wet with varying
temperatures and weather types. Saturday night will feature an
exiting low pressure system and a shift to northerly winds. Ongoing
rain chances at 0Z Sunday will start to exit to the east and dry out
from the west during the overnight, with all rain chances exiting
the CWA by 12Z. Light N to NE flow and high pressure will remain in
control for the day Sunday, with abundant sunshine allowing temps to
rise back into the 50s across the CWA. The high pressure system will
then dissipate as it quickly shifts eastward Sunday night ahead of
an approaching low pressure system over the southern Plains,
correlating to the above mentioned closed upper level low in the
same location.

As the upper level closed low shifts eastward into the day Monday
and traverses Kentucky into Monday night, so the surface system will
follow. According to the latest GFS, the surface low is expected to
track along the southern half of Kentucky starting between 0 and 6Z
Tuesday and continuing into eastern Kentucky by 18Z Tuesday before
getting hung up in the mountains by 0Z Wednesday. It should finally
start shifting away from the state by Wednesday afternoon. Precip
associated with this system will begin to affect the southern CWA
late Sunday night, expanding across the entire CWA by Monday and
Monday night. Likely precip chances will then remain in the forecast
for much of the CWA through the day Tuesday and into the day
Wednesday before the system shifts eastward Wednesday afternoon.
Pops will be slow to taper off in the far eastern portion of the
state with upslope flow aiding in moisture release. Chance pops
won`t fully taper off here until Thursday afternoon.

As for precip types during this period... There was some discussion
of possible thunder Monday afternoon with the approaching low and
increasing precip chances. However, looking at soundings, would
agree with previous day shift`s decision to remove thunder from the
CWA. Lapse rates and shear are good in the low levels in the
southern half of the CWA where surface southerly flow is in place,
but a mid level inversion and lack of instability (CAPE and LIs) are
both factors that lowers confidence in its occurrence. In addition,
soundings don`t look very saturated, and forecasted afternoon QPF is
under 0.1 inches, so there may not be enough rainfall/precip to
instigate lightning. After Monday, forecast looks more certain for
only rain. However, as the bulk of precip starts to move eastward
Tuesday night into Wednesday, colder NW flow could result in
overnight temperatures in the low to mid 30s in some locations
(mainly north). This may enable some snow to mix in with rainfall,
though the ground should remain too warm during the short period of
time it could mix in to produce any accumulations. As such, took out
snowfall accumulations that the forecast builder had added in.
Temperatures will be even colder Wednesday night into Thursday
morning under continued NW flow (upper 20s to around 30), so there
will be a better chance of any lingering moisture changing over to
snow this night/early morning. However, QPF will be quite light by
this point, so not expecting significant accumulations or impacts
this far out in the forecast.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)

VFR conditions will continue to be the rule through the bulk of
the forecast period as some mid level clouds build to the south
with a developing warm front later this morning. These clouds
will thicken and lower in time as the boundary lifts northeast
through eastern Kentucky. Along this front a few showers will be
possible just after midnight - increasing in coverage through the
night and into Saturday morning. Winds generally will be light
and variable through the TAF period.




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