Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 271810 AAA

National Weather Service Jackson KY
210 PM EDT Sat May 27 2017

Issued at 210 PM EDT SAT MAY 27 2017

Scattered convection has initiated along a remnant outflow
boundary aligned from just north of KBWG to south of KEKQ.
Convection is also popping up closer to the nearly stalled
frontal boundary just north of the Ohio River. Have freshened up
the POP trends through the rest of this afternoon. Expect a
gradual uptick in activity, with a few cells pushing severe
limits. Updates have been sent.

UPDATE Issued at 1110 AM EDT SAT MAY 27 2017

The latest surface map reveals the quasi-stationary frontal zone
still north of the Ohio River this morning. Some weakening outflow
from previous convection that passed to our north during the pre-
dawn hours, allowed for some isolated showers across portions of
the area this morning. This boundary, when it was more evident
from the radar, was aligned from near a line from Inez to
Somerset. The latest satellite shows plenty of cloud cover back
off towards central Kentucky, with some partial clearing moving in
across western Kentucky out ahead of a weaker mid/upper level
short wave trough. A complex of storms was gradually weakening
across TN/KY, with only some widely scattered storms occurring
along the leftover outflow.

This short wave will move in across the area this afternoon, with
some thinning of the clouds and some destabilization expected.
As such, would expect some scattered to numerous development
between 18 to 00z, with a few cells potentially approaching
severe limits, as well as locally heavy rainers. This weak
forcing will be exiting after 00z. Still expect a lull in
convection, until late tonight into Sunday morning out ahead of
the larger complex of storms to approach from the west. Tried to
fine tune the POPs according to the latest mesoscale conditions,
as well as input from the higher resolution model guidance.
Updates have been sent.

UPDATE Issued at 652 AM EDT SAT MAY 27 2017

Isolated showers have worked south to near the Mountain Parkway
this morning. As expected, cap appears to be holding, so no real
impact from the showers this morning. Temperatures have come up
with the moisture advection, so have made some small tweaks to
capture current trends.


.SHORT TERM...(Today through Sunday)
Issued at 413 AM EDT SAT MAY 27 2017

An active weather pattern setting up to kick off this holiday
weekend. The lower levels are moistening up this morning thanks to
some southwest flow in the low levels advecting some moisture into
the region. This has already set off a few isolated showers in the
bluegrass region. This activity will likely expand through the
daybreak hours with cloud heights coming down as moisture streams
into the lower levels. We remain very capped this morning, so not
expecting any storms to develop just yet. This all changes as a
boundary just to our north settles south into northern Kentucky.
Instability will build to the south of this boundary with over
2000J/kg of MLCAPE and LI`s around -8C. This is pretty good
instability for our neck of the woods. Added convergence along the
frontal zone in the north could help to spark some showers/storms
as we head through the afternoon and early evening hours. With the
front stretched out east to west and the mean flow coming from
the west, will have to watch for potential some training storms.
PW values will reach nearly 1.6 inches by this afternoon putting
us very close to the 99th percentile for May. This all points
towards the potential for heavy rainfall and flash flooding.
Freezing level is also quite high supporting more warm rain
processes. Thus, we have issued a flash flood watch for most of
eastern Kentucky. There will also be a severe threat this
afternoon with the good instability mentioned above. Shear is not
the best in the northern portion of our area, but does get a bit
better to the south. Regardless, instability may be enough to make
up for the lack of shear and thus pose a damaging wind threat
with any storms that develop. The weaker shear would probably
limit the overall hail threat, although some small hail would be
possible. Finally with outflow boundaries possible, this could
help to escalate low level SRH and possibly lead to a low tornado
threat. However, this will be highly conditional on storm
evolution today, so will continue to downplay the overall tornado

Activity will likely diminish by late evening with a shortwave
ridge pushing across the Ohio river valley. This will provide a
lull overnight into early Sunday morning. The lull could be
interrupted by any upstream convection that may try to work into
the region from the west. This will be clearly determined by where
convection forms and the overall steering pattern. At this point,
CAMS all seem to support a more southerly track of any MCS
tonight, likely missing our area, assuming the mid level ridge is
strong enough.

Then the focus shifts to Sunday as instability builds once more.
00z NAM really doesn`t bring the better instability in until late
Sunday evening, suggesting we may see a later onset of stronger
storm potential on Sunday. However, shear is stronger on Sunday,
so whatever forms upstream and tracks into the area could have
some damaging winds with it late Sunday. Hail doesn`t appear to
be a great threat as storm initiation should take place upstream
with the cold front with stuff turning more QLCS in nature by the
time it reaches us. With the potential for additional rounds of
showers, have put the flash flood watch out through Sunday

.LONG TERM...(Sunday night through Friday)
Issued at 435 AM EDT SAT MAY 27 2017

Models remain in generally good agreement during the long term
period. the period should begin with a ridge across the Gulf of
Mexico with another ridge over the Western Conus. An upper level low
should be over western Ontario to begin the period with an
associated trough extending into the MS Valley to Arklatex region.
At the surface, a triple point area low low pressure should move
across the Southern Great Lakes and into Ontario and Quebec to begin
the period with the trailing cold front moving across the area
Sunday night into early on Monday.

Through midweek, the region will remain under the influence of the
upper low over southern Canada and the associated trough, the axis
of which should move east of the area around midweek while the axis
of the ridging in the western Conus moves to near the Continental
Divide at midweek as an trough nears the West Coast.  A shortwave
should rotate into the Great Lakes Tuesday into Tuesday night around
the upper level low. An associated surface low should move across the
Great Lakes with the trialing cold front moving across the area on
Tuesday into Tuesday night. The latest guidance from Wed night
through the end of the period has the trough lifting on out to the
north and east with general height rises across the OH Valley and
Southern to Central Appalachians though west to northwest flow
persists. The 0Z GFS and 0Z ECMWF are generally drier than their
previous operational runs from Wed night into Thursday with stronger
surface high pressure over the OH Valley. Pending disturbances in
northwest flow and available moisture, a few showers or even a stray
thunderstorm cannot be completely ruled out just yet during the Wed
night to Thursday period and on into the end of the period.

The most active portion of the period will be as the cold front
moves through the area on Sunday evening. MUCAPE and SBCAPE should be
in the 1000 j/kg to 2500 j/kg early in the period with bulk shear of
about 40KT and mid level lapse rates of 7C to 8C, damaging winds and
hail up to half dollar size would appear possible with any
thunderstorms along or in advance of the cold front on Sunday
evening as flow will generally be unidirectional. Locally heavy rain
will also be a concern until the boundary moves through and deeper
moisture departs.

The highest pops were carried early in the period with isolated to
scattered pops at times for the rest of the period with either the
secondary front or disturbances in west northwest to northwest flow.
Temperatures should average close to normals for late May.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)

MVFR stratocu will gradually mix out over the next hour or so.
Scattered to numerous showers and thunderstorms will affect the
area through 01z, before diminishing in coverage. Some of the
storms may approach severe limits with winds of 40 to 50 kts
potentially and temporary IFR or worse conditions. Given the
isolated nature of these, have not included a mention in the TAFs.
The next concern will be the potential of a larger complex
thunderstorms affecting eastern Kentucky generally between 06 and
12z. The models have trended a bit farther southwest with the
brunt of this complex. For now, have carried a period of showers
and thunderstorms, with rain showers likely lingering into the
mid- morning hours behind the main line of storms.


Flash Flood Watch through late Sunday night for KYZ050>052-



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