Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 221155 AAA

National Weather Service Jackson KY
755 AM EDT Sat Jul 22 2017

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

Challenging extended package this morning. Models are in good
agreement for DYS 3-4 but diverge significantly beyond that both
with the details of the mid/upper level pattern as well as with
sensible weather.

Mid/Upper level cut off low moving out of Texas Monday opens up into
a wave by Tuesday as it transits the MS/TN/OH valleys. This feature
will provide another soaking rain to the area with models spitting
out around an inch of rainfall on average, mainly from Monday night
through Tuesday.

Forecast gets complicated thereafter as a series of disturbances
rotate around a low pressure system residing along the Northern Tier
and southern Canada. Models appear to have difficulty with the
strength and timing of several lobes of energy rounding their parent
low. The GFS presents a quicker solution with weaker disturbances.
The ECMWF advertises stronger disturbances which dig a bit further
south resulting in an overall slower solution and correspondingly
stronger surface features. The Canadian generally provides partial
support for both solutions.

For sensible weather we can expect a seasonably warm start to the
extended but cool significantly through the period after Tuesday.
Rain will be tapering off Sunday night before redeveloping across
the region Monday night into Tuesday. There is very low confidence
with respect to precipitation types and amounts beyond that.
Depending on your model of choice we could see anything from rain to
snow from Wednesday night through Friday. At this time felt the best
approach was to stay close to the blend of model solutions and
guidance which suggest that a wintry mix of precipitation will be
possible through that time frame depending on the time of day/night.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)

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.




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