Area Forecast Discussion
Issued by NWS Jackson, KY

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802
FXUS63 KJKL 292349
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
749 PM EDT Wed Mar 29 2017

.UPDATE...
Issued at 748 PM EDT WED MAR 29 2017

Good subsidence passing by to the north has cleared out a good bit
of the cloud cover over eastern Kentucky. Have updated to decrease
cloud cover over the area through early this evening. Out west,
widespread cloud cover ahead of the next system to impact the area
is moving east and will likely overspread the area by 06Z or
shortly after. This will play a role in fog development in the
valleys as well as the the temperature forecast. Adjusted the
temps for the current trends through the evening. The grids have
been updated and sent to the NDFD and web servers.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 428 PM EDT WED MAR 29 2017

As of mid to late afternoon, an upper level ridge extended from
the Caribbean across Florida and into the Southeast, OH Valley,
and on to the southern Great Lakes. Meanwhile, an upper level low
was moving across the Plains and was centered over south central
KS as this time. At the surface, low pressure was centered over
southeastern KS with a cold front south into East TX and a warm
front extending east near the AR and MO border then southeast into
GA. Meanwhile, surface high pressure was centered over northern
Ontario with ridging south into the OH Valley and Appalachians.
Persistent low level clouds have gradually mixed out through the
day with just some cumulus remaining across most of the area
although some of the low stratocu lingers northeast of the area
over HTS.

The axis of the mid and upper level ridge will move east of the
area this evening and tonight with the axis of the surface ridge
also moving east of the area. At the same time, the upper level
low will meander into eastern KS late tonight and then to the mid
MS Valley by Thursday evening. The Surface low is expected to
reach IL on Thursday evening with the warm front lifting into the
area late tonight and across the area on Thursday. This should set
the stage for some isolated to perhaps scattered convection on
Thursday, mainly afternoon and early evening. As the surface low
approaches the southern Great Lakes at the end of the period and
cold front tracks across the area late in the period and the upper
level low moves into IL and IN, more organized convection is
anticipated.

With high pressure in place, and a few hours of decent net
radiational for the first half of the night before increasing
clouds associated isentropic lift ahead of the warm front and
approaching upper level low and trough, eastern valley locations
should experience temperatures dropping off through the 50s and
into the 40s within a couple hours of sunset. Some valley fog
should develop at least in the river and larger valleys and
could become dense in spots, but confidence in this is low since
mid and high clouds are expected to increase late and the pressure
gradient will also be increasing. Trends for this possibility
will continue to be monitored. With the increasing pressure
gradient and winds aloft overnight, winds should increase a bit
late tonight in the more open terrain locations of the far west
or northwest as well as the higher elevations generally 2500 feet
and above where gusts of 25 to 30 mph are possible late.

The pressure gradient will increase across the entire area with
mid and upper level moisture increasing a bit more quickly than
at the lower levels. Winds will have a downslope component for
much of the day especially in the southeast and this should limit
chances for isolated to scattered convection there. Warm advection
will also be occurring especially as the region gets into the
warm sector from midday into the afternoon and temperatures could
approach 80 in the normally warmer locations. This may be most
likely for valley locations immediately downwind of Pine and Black
Mountains. Also due to this downsloping have undercut the blend
forecast dewpoints a bit for the southeastern counties during the
day on Thursday.

The best forcing as well as instability with CAPES possibly
nearing 1000 J/KG in the southwest and 0 to 6km shear reaching the
45 and 55KT range will be favorable for at least some strong to
possibly severe convection during Thursday evening into Thursday
night. Some isolated to scattered convection will be possible in
the evening, but a line or lines of thunderstorms in advance of
the cold front and 500 mb trough axis is expected to move into the
area. This convection will be arriving as the boundary later
begins to stabilize, but shear and CAPE should be substantial
enough for the threat of damaging winds and hail possibly
exceeding 1 inch in diameter and maybe a stray tornado,
particularly western locations closer to the sfc and upper low
track and slightly more favorable arrival time after diurnal
heating and expected higher dewpoints. Chances for severe are
obviously much more favorable to the west and especially southwest
of the area closer to the I 65 corridor and west. Obviously there
remains uncertainty in the degree of instability when this line
or line segments would arrive due to timing differences. In
addition to the chance for wind and hail, locally heavy rain
cannot be completely ruled out. These threats continue to be
highlighted in the HWO and Weatherstory, etc.

.LONG TERM...(Friday through Wednesday)
Issued at 400 PM EDT WED MAR 29 2017

Models appear to be in good agreement with mid/upper level features
until the very end of the extended window. Overall pattern is quite
progressive as well. Cutoff low will be pulling off to the east at a
rapid pace Friday. Shortwave ridging rides across the region through
the weekend. A second cutoff low then transits the Tennessee Valley
for the start of the new week before sliding out into the Atlantic
by late Tuesday. Final wave of low pressure pushes into the
Mississipi Valley and lower Ohio Valley region by the end of the
forecast period. At the surface, a frontal boundary will be all but
through eastern Kentucky by Friday morning. Wrap around moisture
from the exiting system will allow for some lingering instability
showers Friday which will taper off to very light rain and/or
drizzle by Friday night. We do lose dendritic support which would
suggest drizzle through this time frame. But boundary layer moisture
is so deep that any lingering precipitation may effectively seem
more like very light rain. Thus left any mention of drizzle out for
this cycle. The remainder of the weekend looks dry with seasonably
warm temperatures.

Rain overspreads the area again from the west-southwest late Sunday
night and Monday as the second storm system of the extended period
pushes through the Commonwealth. Some marginal, mainly elevated
instability will keep the threat of thunder in the forecast Monday
into Monday night. Decent southeast winds associated with this
second system may eat away at potential rainfall totals given
downsloping effects. Consequently model QPF may be a bit overdone
across our southeast. Once again wrap around moisture from this
system may keep a chance of showers going into mid week, at least
until another system manages to work its way into the Ohio Valley at
the very end of the period, or possibly just beyond bringing
another round of rain showers and thunderstorms to the area.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
ISSUED AT 748 PM EDT WED MAR 29 2017

With clear skies now and lasting into the first 6 hours of the TAF
period tonight, some fog development is possible in the deeper
valleys. However, ample cloud cover moving east ahead of the next
system should be enough to cut off any fog development later
tonight thus, went with a VFR forecast for the TAF sites. As the
next system approaches with the low level jet ramping up above the
inversion, have put in some low level wind shear after 06Z tonight
to account for this. By 14Z tomorrow, those southerly winds will
mix to the surface with some gusts reach 20 knots ahead of the
approaching cold front. Significant convection should hold off
until after 00Z but a few stray thunderstorms are possible so have
put VCTS in the TAFs after 18Z.

&&

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

$$

UPDATE...SHALLENBERGER
SHORT TERM...JP
LONG TERM...KAS/RAY
AVIATION...SHALLENBERGER



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