Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 292039
AFDJKL

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

.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 18Z TAFS through 18Z Thursday afternoon)
ISSUED AT 215 PM EDT WED MAR 29 2017

Low level moisture continues to mix out with high pressure in
place. This is leading to improvement from MVFR to VFR from south
to north. SME and LOZ are already VFR and JKL, SJS, and SYM
should improve to VFR during the first 2 to 3 hours of the period.
VFR should then persist through the end of the period. An
increase in high and then mid level clouds should occur 0Z to 12Z,
with some low clouds and cumulus development possible late in the
period. A stray shower or even a thunderstorm cannot be
completely ruled out late, but better chances will come after the
period.

&&

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

&&

$$

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



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