Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 221332
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
932 AM EDT Sat Jul 22 2017

.UPDATE...
Issued at 932 AM EDT SAT JUL 22 2017

Made some slight configuration changes to the pop grids. Latest
GOES-16 satellite showing very little cloud cover over much of
eastern Kentucky. There is also no boundaries to note, so no
triggers for convection. Looks like the best chance at a shower
may be in southeast Kentucky along the higher terrain this
afternoon. We will have to monitor trends to see how convection
this evening may be impacted, but for the current update, plan to
remove thunder through the afternoon. Freezing levels near 15kft
and very little shear will keep updrafts from becoming all that
organized. Better shear is noted along and north of I-64, but it
looks like convection will probably stay out of these areas until
later this evening or tonight. Will be monintoring trends to the
north through the day to see if we will realize any convection
tonight.

UPDATE Issued at 750 AM EDT SAT JUL 22 2017

Did an update to the grids mainly to adjust the sky cover and
tweak the T/Td grids per the latest obs and trends. These have
been sent to the NDFD and web servers.

&&

.SHORT TERM...(Today through Sunday)
Issued at 415 AM EDT SAT JUL 22 2017

07z sfc analysis shows high pressure through the southeast portion
of the area with a wavy front lying just northeast of Kentucky.
Along this boundary a series of organized storms have been rolling
east southeast through the Ohio Valley. The latest of these
clusters are passing through the region with one to the northeast
dissipating over central West Virginia with another still strong
one progressing from northern Indiana across into northwest Ohio.
On satellite, the cold and high cloud tops are evident with the
Indiana storms (as the WV ones warm). These clouds are drifting
southeast into Kentucky but thinning with time. The clouds are
not preventing much of the radiative cooling, though high
dewpoints are limiting the low temperature potential so that ridge
to valley differences are running in the order of 5 degrees or
less generally from 70 to 75 degrees. The high dewpoints - in the
low 70s - will provide enough moisture to generate fog in the
river valleys again tonight, though signs of it - including from
GOES 16 nighttime microphysics composite channel - are so far
quite limited. Meanwhile winds are light and variable to calm.

The models are in fairly good agreement aloft with the longwave
pattern through the region during the short term portion of the
forecast. They all depict the southwestward retreat of the mid
level ridge that has brought the heat and humidity these past
several days as well as limiting the convection across the state.
This ridge retreats in the face of wave after wave coming through
in northwest flow just north of the JKL CWA. one of these waves is
passing through West Virginia early this morning while the next
will take a similar path across the area later today. A more
potent one then digs southeast through the Upper Midwest/Great
Lakes region and into the Ohio Valley for Sunday. This will bring
plenty of energy and significant height falls to the region to
close out the weekend. Given the model agreement will favor a
general blend with a significant lean toward the higher resolution
HRRR and NAM12 models for weather details and timing.

Sensible weather will feature another broiling day for much of
east Kentucky as temperatures climb again into the lower 90s with
dewpoints in the lower to mid 70s. This will mean heat indices
for the area topping out between 100 and 105 degrees - but not
quite enough for heat advisories in the JKL CWA. We will, however,
continue the SPS and HWO highlighting of the heat and humidity.
The storm threat will be more significant then yesterday as the
ingredients will be in place with good instability and flow aloft
to support any storms that manage to develop. That will be the
crux of the issue, though, as a small cap will be in place around
750mb and little in the way of triggers so long as outflow
boundaries stay off to the north and east of the area - as
suggested by the HRRR and NAM12. As such, have kept PoPs on the
low end of guidance, more toward the MET rather than MAV, allowing
the heat to be the main wx story. However, should storms to our
north manage to send a significant outflow boundary this way more
active weather could be in the offing with damaging winds the main
threat and a secondary concern for hail and excessive rains. Will
highlight this in the HWO and SPS. After another likely quiet
night for most of the area, barring any MCS activity making it
this far south, just as potent conditions, or greater, are
expected for Sunday though with more support from falling heights
and shortwave energy to better activate the heat and humidity
driven instability - along with the sfc boundary likely settling
through this part of the state. Fairly high PW air today and
Sunday will also threaten excessive rains though the storms should
be somewhat progressive given the winds aloft - training will be
a concern, nevertheless. Temperatures will likely be lower for
highs on Sunday given more clouds and storms around.

Used the CONSShort as a starting point for all the grids with some
adjustments to temperatures at night for ridge and valley
differences along with a beef up in them today for highs - using
persistence as a guide. Did tighten up the PoPs through the period
trying to time the evolution of MCS and frontal activity - but
uncertainty remains high throughout concerning the convectively
driven elements of the forecast - which will also affect
maximum temperatures and heat index values.

.LONG TERM...(Sunday night through Friday)
Issued at 345 AM EDT SAT JUL 22 2017

A shortwave trough is expected to work across the Great Lakes and
Ohio Valley to start the period while upper level ridges remain
in place centered east of Bermuda and over the Southern Plains to
Four Corners regions. At the surface, a cold front associated with
low pressure moving into the Northeast U.S. to start the period is
expected to be gradually working south and southeast into and across
the area early in the period, while high pressure is expected to
be centered over the Upper MS Valley.

The shortwave trough is expected to gradually rotate east and
northeast into the Northeast US and across the Appalachians from
Sunday night into Monday. Height rises should ensue from Monday
night into midweek as the axis of the ridge works toward and then
across the area, although as happened earlier this week, the
center of the mid and upper level ridge should remain to the west
of the area in the Plains states. Models diverge in the position
and magnitude of the ridge during the second half of the week
with the GFS stronger and further west with the center and axis of
this ridge by Thu into Fri. This leads to some differences in the
upper level flow becoming more northwesterly with weaker or
convectively driven shortwaves moving around the ridge through
the north- central U.S. and into the Great Lakes/Oh valley, with
some mesoscale convective systems possible initially north of
East KY through midweek. Also, downstream of the ridging, an upper
level low and stronger shortwave trough as well as associated sfc
low pressure system will move across the Canadian Prairies and
across James Bay and into northern Quebec by the end of the
period.

At the surface, high pressure should build into the area behind
the front Sunday night into Monday and usher in a drier and colder
airmass and some relief from the recent heat. This high, however,
should move east of the area by Wed into Wed night, with an
increase in humidity from Thursday into Friday. As has been the
case with extended models for the past few days, uncertainty
remains in timing of a cold front that will begin to shear out
during the second half of the week. This front should drop south
of the Great Lakes and to the OH River or even south across the
area before starting to stall. Behind the front, temperatures will
return to near normal with highs holding in the 80s and lows
dropping below 70 in all areas.

Scattered to numerous showers and thunderstorms are expected ahead
of the front early in the period, on Sunday evening. With the
ridge building in or in place across the area, chances for
convection should be generally diurnally driven and more or less
isolated in nature from Monday into Wednesday. Tuesday and
Wednesday should be end up dry in most locations, but with the
center of the ridge west of the area, the cap could be broken in a
few locations each day. More organized convection will be possible
during the second half of the week as the upper level flow becomes
more northwesterly and a stronger shortwave trough and cold front
approaches and moves into the Ohio Valley.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
ISSUED AT 755 AM EDT SAT JUL 22 2017

Mainly VFR conditions will prevail through the period, as we
remain under the waning influence of an upper level ridge for the
better part of another 24 hours. Isolated to scattered convection
will be possible for mainly northeast parts of eastern Kentucky
during the afternoon hours, but confidence - based on the latest
HRRR - is still too low to include VCTS at any site today or
tonight. Light and variable winds will increase to between 5 and
10 kts out of the west southwest for this afternoon then diminish
in the evening.

&&

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

$$

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


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