Area Forecast Discussion
Issued by NWS Jackson, KY

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809
FXUS63 KJKL 271053
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
653 AM EDT Sat May 27 2017

.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
threat.

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
evening.

.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 12Z TAFS through 12Z Sunday morning)
ISSUED AT 652 AM EDT SAT MAY 27 2017

A small window of MVFR conditions will be seen this morning. A
few showers will also accompany the lower cigs. Cloud heights
will return to VFR by early afternoon. The threat for showers and
storms will develop as we head through the afternoon and evening
hours today, but still some question on how much coverage we will
see with this activity. Thus, have gone mainly VCTS in the TAFS.
Strong winds will be seen around the thunderstorms that do develop
today. Activity will wind down by late evening with a lull
expected overnight Saturday night. There is a chance that a
complex of showers and storms upstream may move into eastern
Kentucky after midnight tonight, but confidence is low on how the
system upstream will track.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 2 PM EDT this afternoon through late
Sunday night for KYZ050>052-058>060-068-069-079-080-083>088-104-
106>120.

&&

$$

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



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