Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 260307 AAB

National Weather Service Jackson KY
1107 PM EDT Fri May 25 2018

Issued at 1102 PM EDT FRI MAY 25 2018

Pops have been updated based on recent radar, satellite, and
convective allowing model trends. This led to a bit higher pops in
parts of Pike County and McCreary County over the next hour or so
with a lull for a few hours overnight. However, several of the
convective allowing models including the HRRR have showers and
possibly a thunderstorm moving north into the TN and VA border
counties prior to dawn as the upper level trough approaches. Some
slight chance pops were added during that time frame in the south.
Also, coverage of fog was adjusted to add more fog in Pike County
down into Letcher County where heavier rain fell.

UPDATE Issued at 840 PM EDT FRI MAY 25 2018

A few showers and even a thunderstorm near the TN and KY border
linger as sunset approaches. This convection is largely diurnally
driven and should wane over the next 1 to 2 hours. The overnight
hours should generally be dry. With the upper trough working in,
some of the convective allowing models indicate the potential for
an isolated shower toward dawn in the southwest part of the area.
We will continue to monitor trends on this. Otherwise, outside of
showers, clouds will decrease and locations that picked up
heavier rainfall from showers and thunderstorms today are expected
to experience more widespread fog and it may become dense. Some
of these locations include Whitley County near the I 75 corridor
and portions of the Big Sandy region and portions of Letcher
county. More widespread coverage of fog has been added overnight
in these areas. Otherwise, fog development is anticipated in the


.SHORT TERM...(This evening through Saturday night)
Issued at 405 PM EDT FRI MAY 25 2018

The afternoon surface analysis shows a weak boundary across the
the Tennessee Valley and a surface high pressure off the North
Carolina Coast. This high pressure is continuing pump more
moisture into the region this afternoon, with dewpoints climbing
into the mid 60s this afternoon. The latest radar data shows some
showers and perhaps a isolated thunderstorm developing across the
region. Interestingly, the lack of steering flow is leading to
arcing outflow boundaries this afternoon and this could make
convection more difficult to nail down through the early evening.
Given this will keep thunder in isolated to scattered wording
this afternoon and early evening. The CAMs have the mess of
convective outflows and thunderstorm complexes across western
Kentucky weakening this evening. Given this and the waning
convection through the evening, and did lower POPs overall given
the lack of steering flow from the west and loss of daytime
heating. There could be some fog tonight in the river valleys or
perhaps more so where rain has fallen this afternoon. This will
have to be more fine tuned this evening. Temperature splits remain
possible tonight, but they should be less overall given a little
more cloud cover to contend with.

An upper level trough across the Midwest will slowly spill weak
height falls into the Ohio Valley Saturday. This in combination
with strong low level lapse rates, modest mid level lapse rates,
increased moisture and MUCAPE around 2000 J/kg should yield more
numerous coverage in convection mainly in the afternoon on
Saturday. Given the lack of larger scale forcing or surface
features to work with this will be harder to nail down exact
locations. The main threats will be heavy rain and frequent
lightning given the lack of shear. By later in the afternoon,
this maybe driven by convective outflows to a extent, but that
will have to be evaluated as we get closer to that time. This
convection may linger into the evening and perhaps some showers
linger into the overnight. Did add some fog again for Saturday
night, but again this is hard to nail down given the uncertainty
on exactly where rain will fall. The temperatures splits will
likely be more limited Saturday night as well given the additional
cloud cover.

.LONG TERM...(Sunday through Friday)
Issued at 405 PM EDT FRI MAY 25 2018

A large but weak upper trough axis will extend from the Great Lakes
to the Gulf of Mexico at the start of the period, with a
strengthening tropical system (Alberto) over the northern Gulf. The
northern extent of the trough will be progressive, while the flow
around the tropical system becomes more closed as it deepens. This
would shear the deep moisture regime originally over our area at
start the period, shunting it southeastward, and allowing drier air
aloft to advance southeastward over eastern KY. This would tend to
keep precip more sparse, especially over the northwest part of the
area. Meanwhile, the developing col in the geopotential heights over
our area would lead to weak flow and rather featureless mid/upper
levels, resulting in the movement and coverage of any precip
becoming more limited from Sunday into Monday.

As Alberto moves inland and weakens beginning Tuesday, we will come
more under its influence and deep moisture should make a comeback.
Along with this would come an increase in precip coverage. Aside
from this generality, there is still a lot of uncertainty. The mid
week forecast depends largely on the development and demise of
Alberto, and models do not agree on the timing and track of the
system in its waning stages. It`s still possible for it to track
north, south, or over our area. Whatever the case, it should be to
our east by Friday. However, without any real change in air mass,
there still could be diurnal convection which pops up on Friday.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)

Much of the first 6 hours of the period should feature VFR,
outside of the few locations experiencing some initial showers
and or an isolated thunderstorm with brief MVFR or IFR
reductions. Otherwise, valley fog with mainly MVFR reductions and
spotty IFR is expected to develop by 6Z to 8Z and generally not
affect the TAF sites with the exception of SJS that received rain
this evening. Reductions to at least MVFR by 3Z are anticipated
there and some IFR fog is anticipated from about 6Z to 12Z.
Convection is expected on Saturday, possibly before 15Z with
chances lingering through the end of the period. Confidence in
timing of onset and thunder on station at any one location was to
low for more than prevailing VCTS during the second half of the
period. Winds will average light and variable outside of any




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