Area Forecast Discussion
Issued by NWS Jackson, KY

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559
FXUS63 KJKL 241512
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1012 AM EST Fri Feb 24 2017

.UPDATE...
Issued at 1012 AM EST FRI FEB 24 2017

Updated temperatures with the morning update. Temps are slow out
of the gate in the east, due to a bit of valley fog and rising
faster than expected in the western valleys where gradient winds
have already mixed out the morning inversion. Also seeing a brief
jump in winds to announce the breaking of that inversion. Expect
they will die back down a bit over the next hour or so before
increasing again later this afternoon. No updates to the zones at
this time.

UPDATE Issued at 657 AM EST FRI FEB 24 2017

Mostly clear skies with a few high clouds is the story this
morning for most. Surface analysis shows surface low is
strengthening across NE MO this morning as it continues NE.
Overall forecast grids remain in good shape this morning. In terms
of the severe weather, The HRRR is aligning close to the other
CAMs at this point in terms of overall trends. Overall looking at
the newest data think the current forecast is on track and will be
looked at in subsequent updates.

&&

.SHORT TERM...(Today through Saturday)
Issued at 340 AM EST FRI FEB 24 2017

Early morning surface analysis shows an area of surface low
pressure continues to track NE out of the Central Plains into
Missouri and a warm front is now well north across the Upper Ohio
Valley region. WV imagery this morning showing plenty of upper
level moisture streaming into the region and this is leading to
scattered high clouds across eastern KY. Given these have only
been high clouds and the valleys have decoupled leading to decent
temp splits from some of the cooler valley locales. A mix of high
clouds and possible some diurnally driven CU will remain the story
through the day. LLJ will continue to move east in response to
strengthening system to our west. This will begin to mix down as
we move into the afternoon hours with gust of 25 to 30 possible
across the Bluegrass and Lake Cumberland region. The other issue
will be downsloping flow across the far east could also mix down
drier air, with RH values in the mid 30s to lower 40s possible.
This will have to be monitored today given the gusty winds, but it
will help that the gustier winds will be west of the drier
downsloped air.

All the focus shifts to strengthening system to the west as we
move into the late evening and overnight period. Models remain in
good agreement with trailing cold front approaching from the west
through the day. Initially convection will fire out ahead of the
front and track across portions of the Midwest. Some of these will
be single cell type storms, but as the system progresses east
storm mode will become more multicell/QLCS. Given the strength of
the cold front, lapse rates will remain steep through the evening
and into the overnight. Also models, specifically CAMS, are in
good agreement with the trailing cold front catching up to the
initial convection possibly providing further lift as it moves
into the region. One issue will be how much instability remains in
place at that point given the time of day and year. However, if
we are able to maintain this, the speed shear environment will be
substantial through a deep layer and low level shear is rather
impressive with some directional shear possible. This seem to be
picked up well by the SHERBE (that deals with high shear low CAPE
environments) with values near 1 or higher. Overall storm mode
will remain more QLCS in nature and therefore the tornado threat
will be very isolated. Think the better threat for tornado will
exist further north across the Ohio Valley where more boundary
interactions will take place. The main threat across eastern KY
will be damaging winds given the storm mode and strong jet, and an
outside threat of marginal hail given the airmass cooling aloft
causing freezing levels to drop to near 10 KFT. In terms of grids,
did opt to slow the progression of POPs to better align with the
CAMs and also added enhanced wording for damaging winds given the
greater threat.

Behind this front will be progressively cooler temperatures from
west to east early Saturday into Saturday afternoon. That said,
higher terrain locations could hit the freezing mark by the end
of the period. Given some wrap around moisture is possible did
keep some sprinkles and perhaps flurries across the higher
terrain into the afternoon. Given the high temperature will occur
at midnight the temp curve will be quite unorthodox, and
therefore leaned away from blended diurnally driven curve.

.LONG TERM...(Saturday night through Thursday)
Issued at 415 AM EST FRI FEB 24 2017

The period will begin with the mid and upper level shortwave trough
and strong surface system that will affect the region during the
near term period departing to the north and east. A broad through
south of an upper level low in the vicinity of Hudson Bay is
expected to extend through the Central Conus at the start of the
period with rather zonal flow from the Plains east to the lower
Ohio Valley for Sat night and into the day on Sunday. This will
allow surface high pressure to build across the region on Saturday
night before departing rather quickly east by Sunday night. A few
sprinkles or flurries may linger into the start of the period
with some uncertainty particularly across the northeast and east
with timing of clearing on Sunday night. Also, the heart of the
colder airmass will move across the area on Saturday night setting
the stage for colder overnight lows compared to recent days. The
position of the high appears most favorable for clearing and
slightly colder overnight lows in the southwest part of the area.

With the high moving across the area, a considerable amount of
sunshine should lead to temperatures modeling into the upper 40s
to around 50 on Sunday while dewpoints remain in the upper teens.
Winds on Sunday will be light, but rh should fall to around 30
percent or below in some areas. Under the rather progressive
pattern, several shortwaves should move east to start the week
within this flow. One is expected to move through the northern
Plains and approaching the Great Lakes on Sunday before moving
into the St Lawrence Valley on Sunday night. Another more southern
wave should move across the Rockies and emerge into the Plains on
Sunday while a third shortwave is expected to move across the
Four Corners region and into the southern Plains by late Sunday or
Sunday night.

The two more southern shortwaves should move into the lower OH
and TN Valleys late Sunday night and into the day on Monday. An
area of low pressure should develop over the Southern Plains on
Sunday night in response to the more southern wave with an
inverted trough developing east northeast into the Ohio Valley.
Moisture returning with moistening from the top down could lead to
some light precipitation possibly rain or snow initially from
this very late Sunday night into the Bluegrass region and into
early on Monday. As the atmosphere continues to moderate on
southwest flow, any precipitation should quickly change to rain on
Monday. Models have generally decreased qpf with this since 24
hours ago, likely factoring the initially very dry low levels. By
the time the lower levels moisten it appears that the best forcing
will be departing.

Also over the weekend, another shortwave is expected to be
dropping south and southwest into the Pacific Coast. This trough
should rotate across the Western Conus through Monday night or
Tuesday before moving across the Plains and into the Eastern Conus
by Tuesday night or Wednesday. Models continue to vary with timing
of this evolution with the GFS generally faster than the ECMWF. The
period is expected to end with a trough across the eastern Conus.
As the trough moves across the western Conus and nears the
Plains, low pressure should organize over the Plains. This low
should track northeast into the Great Lakes on Tuesday night or
early on Wednesday. The associated warm front should lift across
the TN Valley by Tuesday night with the cold front entering the
Lower OH Valley late Tuesday night and then moving across the area
at midweek. Much of the early to midweek period should be
unsettled with chances for rain greatest Tuesday night into
Wednesday. Uncertainty remains during this period with timing, but
during the midweek period, showers and some thunderstorms appear
likely though confidence in the Tuesday night versus Wednesday 12
hour periods is below average. For now opted to go with likely
pops both periods or a little bit lower than the Superblend due to
the timing uncertainties.

After colder, but typical for late February lows on Saturday
night and near to slightly below normal highs on Sunday, above
normal temperatures should prevail through midweek. Behind the
cold front at midweek, temperatures should return to closer to
normal levels for early March.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
ISSUED AT 634 AM EST FRI FEB 24 2017

A mix of high and low clouds continue to stream across the
Bluegrass State this hour. However most of the lower clouds
remain west of eastern KY and will move in toward the afternoon
hours. That said, VFR conditions will remain the story through
the day before unsettle weather returns tonight. The VAD wind
profile was showing some fairly hefty winds aloft, however
reliability is low at this point. Do still think there could be
some LLWS just extent may not be that recorded by the radar. This
will subside as inversion mixes out through the afternoon. The
issue here will be a increase in winds with tightening gradient,
and this will lead gust of 25 to 30 mph will be the norm in the
Bluegrass and Lake Cumberland regions.

All focus will then shift to tonight, as a strong cold front
approaches and moves across the region. This will bring strong to
severe thunderstorms across the area with primary risk for
damaging winds. The risk for severe weather becomes greatest as
you move NW toward northern KY and portions of the Ohio Valley.
This line of thunderstorms will move east into SME by around 04Z
and are further east site of SJS by 07Z.

&&

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

&&

$$

UPDATE...RAY
SHORT TERM...DJ
LONG TERM...JP
AVIATION...DJ



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