Area Forecast Discussion
Issued by NWS Jackson, KY

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346
FXUS63 KJKL 300545
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
145 AM EDT Thu Mar 30 2017

.UPDATE...
Issued at 145 AM EDT THU MAR 30 2017

Latest HRRR and RAP run continues to show some light shower
activity developing across our northern counties around daybreak
associated with the mid level isentropic ascent. In fact, the last
several runs have been fairly consistent on showing this activity.
Given the consistency, have opted to include a few showers across
the northern counties, mainly along and north of I-64 around
daybreak. After this early day activity, looks like most of
Thursday is shaping up to be dry as we await the arrival of
showers/storms Thursday night. Thus, have backed off rain chances
through much of the day on Thursday. In fact, we could get quite
warm as we will have southeast winds through much of the day.
This could push highs to around 80. Will likely be updating these
high temperatures to reflect the warmer readings shortly. Looking
at the latest guidance suggest dewpoints may not get to or exceed
50 ahead of any showers/storms tonight. If this is the case,
instability may be lacking quite a bit and this may limit overall
severe threat. Certainly will be watching these trends going
forward.

UPDATE Issued at 1152 PM EDT WED MAR 29 2017

Updated the grids to input the latest observations into the
forecast. Adjusted for the current hourly temperature trend as
well. With the exception of the north and the far eastern valleys,
some locations will be reaching their low temps in the next few
hours before some temps begin to rise at the onset of southerly
flow. The grids were freshened up and sent to NDFD and the web
servers.

UPDATE Issued at 1034 PM EDT WED MAR 29 2017

Current conditions across the area feature mainly clear skies
continuing to hang on over eastern Kentucky. Cloud cover still
attempting to enter the area is still out in central Kentucky.
However, the dry air in place is still present as some locations
still have a 10 degree dew point depression. Thus, with the dry
air, winds still up a bit, and cloud cover due to reach the area
after 06Z, have decided to remove fog from the forecast. Also
lessened the ridge to valley temperature difference a bit. Due to
the removal of the fog, a new zfp has been sent out.

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 06Z TAFS through 06Z Thursday night)
ISSUED AT 145 AM EDT THU MAR 30 2017

VFR conditions will prevail through the TAF forecast period. Some
mid/high level clouds will be drifting across the area through
Thursday evening. Showers and storm chances will increase Thursday
evening, but the bulk of the activity will likely hold off until
the very tail end of the forecast period, closer to 06z.

&&

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

$$

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



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