Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 231002
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
502 AM EST Thu Feb 23 2017

.SHORT TERM...(Today through Friday)
Issued at 400 AM EST THU FEB 23 2017

Morning surface analysis showing surface high pressure parked off
the Carolina coast line. This is bringing some return flow to the
region with winds out of the SE to SW for most. The valley sites
have been able to decouple and the has lead to areas of dense fog
especially in locations that saw rain yesterday and/or nearer bodies
of water. Overall think the SPS will handle the situation, as we
could see improvements as high clouds move into the region. WV
loop does show a weak upper level wave moving east out of Missouri
this morning with weak mid to upper level QV convergence seen in
portions of southern Indiana and Northern Kentucky. This combined
with mid level moisture being transported from the Baja is
inducing a line of lower clouds to our NW, and this lines up well
with some of the short term guidance data. That said did keep
some isolated showers mainly north of the Mountain Parkway, but
think the instability is overdone for this afternoon in much of
the short term guidance. That said, did opt to remove thunder for
the afternoon and this also aligns with the SPC thinking at this
time. Clouds and potential for precip will keep temps in the low
70s for most which remains 20 degrees above normal for his time of
year.

All the focus will shift to the west, as deepening surface low and
upper level system takes shape across the Central Plains by
tonight. This dynamic system will begin to induce a LLJ out ahead
of the main system, but the valleys will still have a chance to
decouple. This could lead to some slight temp splits and perhaps
some patchy fog. However do think as low level inversion mixes out
the winds will begin to kick up early in the day Friday. Winds of
25 to 30 mph will be common in the afternoon across portions of
the Bluegrass and Lake Cumberland regions. Right now model
soundings suggest the driest air is not quite taped into, but this
will have to be looked at for brief window of fire weather
concerns Friday afternoon. Also this could lead to even higher
afternoon temperatures with many locales looking at upper 70s and
some even closing in on 80 degrees. These kind of temperatures
would shatter the current record high temperatures for Friday of
71 at JKL and 70 at LOZ. Based on model trends think the Friday
afternoon period will remain dry for most and therefore did slow
POPs from what the model blend produced.

.LONG TERM...(Friday night through Wednesday)
Issued at 445 AM EST THU FEB 23 2017

Early in the period, a ridge is expected to extend north into the
Carolinas while an upper level low is expected to be centered over
the MS Valley region with an associated shortwave trough extending
south through the MS Valley. From the Plains west to the Western
Conus, a broad trough is anticipated. At the surface, an
occluding area of low pressure will be nearing the western Great
Lakes to start the period, while a cold front should extend south
into the lower OH Valley and TN Valley from a triple point low
over the Great Lakes region. A prefrontal trough could also be
moving across the Commonwealth at that point.

The cold front is expected to move across the area Friday night
into early on Saturday as the sfc low tracks to the northern Great
Lakes and into Ontario. Convection, a line or perhaps one or more
broken lines of showers and thunderstorms should precede the front
into the region late Friday evening or Friday night. This dynamic
system will have a decent amount of moisture to work with a PW may
approach 1 inch. Timing of the front is a limiting factor as
instability should be limited to modest at best, but winds aloft
and shear will be on the strong side with 700 mb winds near 60
KT. The shear is generally unidirectional as well so strong to
damaging winds will be possible as any thunderstorms should be
able to mix down higher momentum from aloft. SPC has placed the
entire region in a slight risk of severe thunderstorms for Day 2
with the exception of the far southeast which is in a marginal
risk. The HWO has been updated to highlight the threat although
there is uncertainty as to how quickly convection might arrive
into the region or perhaps develop and how far east the strong to
severe threat would last as locations further east may decouple in
the evening and these areas would also be farther front the best
forcing. Most if not all locations should experience measurable
rainfall on the order of two tenths of an inch, but with
convection, amounts should be variable.

Meanwhile, the coldest air will lag behind closer to the axis of
the 500 mb trough, and a secondary cold front or trough should
usher in the coldest of the air from about midday Saturday through
Saturday evening. Although a dry slot should have brought a
decreasing chance for showers by dawn on Saturday, the chance of
showers will persist as cold air advection and solar insolation
steepens low level lapse rates. This will lead to a continued
chance for showers during the day on Saturday, but as drier air
moves into the area with high pressure building in at the surface
and aloft, this threat should diminish by around sunset on
Saturday. Highs on Saturday should occur in the morning or by
midday, with falling temperatures by mid to late afternoon. 850
mb temperatures should bottom out around -8C to -10C near 6Z or
about 1 am on Sunday. Surface high pressure will build into the
area on Saturday night and with some clearing and the colder
airmass in place, overnight lows will fall into the 20s. These
readings are not atypical for later part of February, but are
colder than what has been experienced recently.

By late Saturday night and Sunday the flow aloft will be generally
westerly and the airmass will moderate on Sunday. Near normal
highs are expected on Sunday. Uncertainty then grows by Sunday
night through the middle of next week. The 0Z GFS is a bit quicker
dropping a shortwave south along the west coast of the CONUS over
the weekend and then across the Four Corners region and into the
Plains by Sunday night. The GFS runs have generally been more
robust with this shortwave as well and have a stronger and more
northern surface low track. Meanwhile, the ECMWF is generally
slower and weaker. For now have followed the Superblend for timing
of pops and temperatures to start the week. If precipitation were
to arrive as quickly as the GFS and Superblend pops would suggest
on Monday, there could be some precipitation type issues at
onset, especially if eastern valley locations can decouple and
drop below freezing on Sunday night. The temperature profile is
marginal at best for rain versus snow on forecast soundings then
as well. At this time, we have continued with a chance of rain or
snow showers for late on Sunday night into very early on Monday
with the uncertainty. Further warm air advection should change any
linger precipitation to rain on Monday morning.

The region will remain very mild through early next week with the
axis of the 500 mb trough approaching by midweek. Models vary
from run to run and model to model, but the general consensus at
this time is that an area of low pressure should track northwest
of the area around or just before midweek with a cold front
approaching in advance of the mid and upper level trough that
should approach the MS Valley near the end of the period. This
pattern will lead to continued unsettled weather with the best
chances for precipitation near the end of the period. Given the
mild pattern, thunder cannot be ruled out Tuesday night into
Wednesday as well and models do have some limited instability
during that period as well.


&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
ISSUED AT 1230 PM EST WED FEB 22 2017

The TAF period will be a complicated one with several dependent
variables to deal with. The period is beginning with some lower
stratus streaming across the Bluegrass and some high clouds mixed
in at times across the remainder of the region. Valley locations
that have cleared and decoupled are seeing widespread to areas of
dense fog. This will as usual be more dense in areas that saw
more rainfall yesterday and/or areas nearer bodies of water. The
TAF sites are currently not seeing any issues with most of the
ridge sites remaining mixed this hour. The only question at some
of the elevated sites will be does any fog actually advect in
toward dawn and right now will leave that thought out given low
confidence.

The next issue will be cloud cover overnight and just how much
are we going to see. The guidance is certainly struggling
initiation wise including much of the short term guidance, and
this is making for a challenging sky cover forecast. Right now
sticking close to previous forecast and given the HRRR which is
the best guidance right now has clouds developing toward dawn.
That said it does look like this will be in the low MVFR range at
this point. Winds will remain light through the period.

&&

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

$$

SHORT TERM...DJ
LONG TERM...JP
AVIATION...DJ


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