Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 201040
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
640 AM EDT Thu Apr 20 2017

.UPDATE...
Issued at 640 AM EDT THU APR 20 2017

WSR-88D radar showing some isolated showers are moving east along
the TN/VA borders this morning. Otherwise a mix of clouds and even
some areas of dense fog are the story this morning. Given the
isolated showers added that to the grids and blended in the
latest obs and trends.

&&

.SHORT TERM...(Today through Friday)
Issued at 407 AM EDT THU APR 20 2017

Most spots have dried out this morning, however a fleeting shower
is affecting portions of Pike County. Otherwise surface high
pressure in the Carolinas is providing south to southwest flow
across the region. Despite a weakly forced environment, a subtle
wave and reasonable instability could be enough to kick off a
isolated storm this afternoon. The best chances will be near the
VA border where higher terrain could provide better lifting
mechanism. Would think these would weak given the lack of shear in
the lowest 6 KM to sustain better updrafts.

Tonight we will be watching a more robust upper level wave eject
out of the Northern Plains into the Great Lakes. At the surface,
a low pressure will move across the southern Great Lakes and
Northern Ohio Valley with attendant cold front extending into the
Central Plains. This front will slowly slide SE and bring better
chances of POPs into the region this evening into tonight. That
said, a line of thunderstorms along and ahead of this boundary
will move into the region. The timing is not the best instability
wise, but a better shearing environment will advect into the
region. Therefore, did keep thunder in the forecast through the
night.

Friday`s forecast is a bit more convoluted given the boundary
becomes quasi stationary across the region, as the better upper
level trough passes well to the north. The boundary will become
another focus for convection on Friday, but the question is how
much instability will be had given the potential cloud cover.
Therefore, while effective shear values will approach 30 knots and
convective temperatures of lower 70s will be reached the
conditional instability could be the killer. The best chances of
seeing a more robust storm will be in the far south and this is
where SPC has placed a marginal risk.

.LONG TERM...(Friday night through Wednesday)
Issued at 400 AM EDT THU APR 20 2017

The models are in decent agreement with the long wave pattern
aloft for the bulk of the extended. They all depict an amplifying
trough moving into the Ohio and Tennessee Valleys to start the
weekend with the GFS and ECMWF quite similar, but the Canadian is
lagging. This trough slows and further deepens overhead on Sunday
- most prominently in the latest ECMWF. However, it does appear to
remain progressive enough to bring a rising height regime back
into the area at the start of the new work week. While weak
ridging sets up over Kentucky on Tuesday the northern stream will
be active and may bring more troughing and energy into the Ohio
Valley by evening - particularly seen in the ECMWF as the GFS is
now out of step with its overseas cousin. Likewise, confidence
falls for Wednesday as the ECMWF quickly presses a trough through
Kentucky while the GFS holds on to ridging. With the agreement
early - particularly among the GFS and ECMWF - will favor a blend
having a good deal of confidence while further out as confidence
flags a blend splits the difference effectively.

Sensible weather will feature a wet and cool weekend as a nearly
closed low descends over the region and slowly passes through.
This will mean cloudy and wet conditions with a chance of thunder
- mainly for our southern counties on Saturday. As the sfc low
starts to pull away so will our thunder chances and better chances
for measurable rain. There is a potential for showers to linger
in the far east in Sunday night as the sfc low`s movement east is
hampered by high pressure off shore. In time though, it will pull
far enough away for drier conditions to prevail followed by more
sunshine during the days ahead and temperatures back above normal.
This will be the case through Tuesday before more uncertainty
crops up as to whether a low passing through the Great Lakes will
swing a cold front into Kentucky on Wednesday (ECMWF) or if sfc
high pressure holds on for another day at least (GFS). For now
have kept it dry, but confidence in this is lower than the rest of
the forecast.

Did make some adjustments to the temperatures each night with
minor/spot specific ones early and then larger adjustments for
Monday and Tuesday nights as ridge to valley splits should
develop. With the PoPs, did not make many changes as the blend
came in pretty good, but will caution that Wednesday may
eventually need some PoPs if future model runs trend toward the
ECMWF.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
ISSUED AT 120 AM EDT THU APR 20 2017

TAF sites are starting off the period VFR and should generally
remain that way through the TAF period. The caveat will be the
more valley sites could see fog, but right now will lean toward
LOZ for MVFR VIS potential and leave out at other sites. Outside
of TAF sites would think anywhere that saw convection and places
that saw showers would have the chance of seeing fog through mid
morning. While the convection this afternoon will be isolated in
a weakly forced environment will mention VCTS for all the TAF
sites. Winds will remain light out of the south to start the
period, but deeper mixing into a modest LLJ could provide gusts of
15 to 20 knots through the afternoon. The better gusts would
likely be in the Bluegrass and Lake Cumberland regions.

&&

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

$$

UPDATE...DJ
SHORT TERM...DJ
LONG TERM...GREIF
AVIATION...DJ


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