Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 200300
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1100 PM EDT Tue Sep 19 2017

.UPDATE...
Issued at 1100 PM EDT TUE SEP 19 2017

Made several updates throughout the evening to better capture the
radar trends for precip in the grids. Showers and thunderstorms
made their way into the northern half of the CWA, and are
continuing to fade out as we approach the 11pm hour. With the
continued rain through the late evening, and expected clearing as
we head into the overnight, conditions will be very conducive to
fog in these locations. Meanwhile, other sites are already
starting to drop to MVFR as we head into the overnight, with dense
fog expected in most of the river valleys if not in some of the
higher elevations as well. Based on this, as well as collaboration
with LMK, decided to go ahead and issue a dense fog advisory
through the morning commute. Also, updated the near term grids to
make sure they were on track with the current observations. All
grids have been published and sent to NDFD/web. A new forecast
package was sent out to reflect changes to pops. A new HWO will
also be sent out to reflect updates in thunder and fog.

UPDATE Issued at 647 PM EDT TUE SEP 19 2017

Radar trends showed significant diminishment of ongoing convection
across eastern Kentucky. However, all CAMS also pointed at
isolated to scattered redevelopment across the Bluegrass early
this evening, and then elsewhere across eastern Kentucky over the
next few hours. Ended up reducing pops across much of eastern KY
based on the latest radar trends, but as more convection began to
pop up across the Bluegrass as the CAMS had predicted, ended up
adding scattered pops back into portions of the northern CWA for
much of the evening. So far a few more isolated showers have
popped up, which is on par with isolated pops, but will continue
to monitor in case pops/weather needs to be altered further. Also,
given the expectation for fog overnight, went ahead and increased
coverage of fog outside of just the river valleys, and added dense
wording to the fog within the deeper/river valleys. Finally, made
sure the near term grids were on track with the current conditions
by loading in the latest observations. All change have been
published and sent to NDFD/web. A new forecast package has also
been sent out to reflect changes. This continues to carry a pre-
first period, so will have to update again once
showers/thunderstorms start diminishing late this evening.

&&

.SHORT TERM...(This evening through Wednesday night)
Issued at 435 PM EDT TUE SEP 19 2017

20z sfc analysis shows a bland sfc pattern in place over Kentucky
while a mid level disturbance is passing through the Ohio Valley.
This has activated the moist and humid air in place over the area
to generate showers and a few thunderstorms that continue to move
through eastern Kentucky this afternoon. The convection also
brought plenty of clouds to the area limiting the sunshine by
early afternoon. However, outside of any storm, temperatures had
little trouble climbing into the low and mid 80s while dewpoints
were in the mid 60s most places and winds light/variable.

The models are in good agreement aloft with the longwave pattern
through the short term portion of the forecast. However, some of
the smaller scale, but important, features are handled just a
little differently by each model. In general, though, they all
depict the mini trough moving through the region tonight into
Wednesday ahead of the ridge rebuilding from the southwest. The
energy associated with the troughing feature will temporarily
suppress heights and provide some impetus to any convection that
manages to develop on Wednesday. Given the model agreement have
favored a general blend updated with the latest guidance from the
CAMS and higher resolution models.

Sensible weather will feature convection dieing out this evening
with some subsidence overnight clearing the clouds - probably
leading to areas of dense fog developing in the river valleys, at
the least, by dawn. The fog will burn off in the morning on
Wednesday with isolated convection developing by afternoon -
propelled by the energy aloft. Similar conditions as tonight will
set up for Wednesday night with any convection fading out early in
the evening followed by some clearing and then areas of fog
developing - likely becoming locally dense in the river valleys
again.

The CONSShort and ShortBlend were used again as the starting
point for all the grids. Did make some adjustments to the
temperature ones each night based on terrain due to at least a
small ridge to valley split setting up. As for PoPs, did again
tweak them for the diurnal cycle peaking late in the afternoon and
then diminishing after sunset on Wednesday.

.LONG TERM...(Thursday through Tuesday)
Issued at 349 PM EDT TUE SEP 19 2017

Models continue to display good agreement with a highly amplified
long wave pattern developing across the CONUS into the weekend.
Deep troughing will establish itself throughout the West. In the
east, a seasonably strong ridge will build from the lower
Mississippi Valley into New England. By early next week, the
western trough and tropical activity in the western Atlantic
pinches off the ridge, forcing the center to lift into the Great
Lakes while the south end gradually dampens off.

Sensible weather for eastern Kentucky remains uneventful and warm
through most of the period. Thursday looks to be the only day
when a few showers may threaten the area, before ridging becomes
the dominant influence in our weather. Forecast soundings suggest
enough surface based instability to warrant the mention of some
thunder on Thursday. However, do question this after considering
the rapid height rises taking place across the region through the
day. For now, went ahead and included a slight chance of thunder,
which actually helps to match up better with neighboring offices.

After Thursday, the start of autumn looks to be dry and warm.
Temperatures will run around 5 degrees above normal but will
gradually drop closer to normal as heights slowly fall off with
time. Highs will run in the mid 80s through the weekend, dropping
off to the lower 80s by the end of the extended. Daily lows will
start out rather warm as well, generally in the mid 60s but will
drop closer into the lower 60s with time. Drier air settling into
the area will allow for some cooler 50s in the valleys through the
latter portion of the forecast.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
ISSUED AT 755 PM EDT TUE SEP 19 2017

Showers and thunderstorms are redeveloping across portions of the
Bluegrass and will likely impact the northern most TAF sites over
the next few hours. A few more isolated showers/thunderstorms
cannot be ruled out over the next couple of hours as well, mainly
for JKL/SYM/and SJS. If a site is impacted by one of these
storms, expect a brief period of MVFR or lower CIGS and VIS. After
the convection diminishes late this evening and into the
overnight, skies may clear off again, leading to another night of
likely valley fog. Indications are that the fog tonight could be a
little worse than it was this morning with dewpoints running a
tad higher. Kept with TEMPO groups to account for fog impacts at
TAF sites, generally between 8 and 12Z. Forecast MVFR conditions,
however, it is possible that rain that occurred today could
result in periods of worst conditions. Will continue to update as
needed. For will burn off by 14z with VFR conditions returning.
Winds, outside of any storm, will be light and variable through
the period.

&&

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

$$

UPDATE...JMW
SHORT TERM...GREIF
LONG TERM...GEOGERIAN/RAY
AVIATION...GREIF/JMW



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