Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 240805
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
405 AM EDT Thu Aug 24 2017

.SHORT TERM...(Today through Friday)
Issued at 338 AM EDT THU AUG 24 2017

Upper level pattern continues to support NW flow across the
region in a longwave troughing pattern. Meanwhile, a large area of
surface high pressure is currently spreading across much of the
Mid and Upper Mississippi Valleys and is continuing to push
eastward, encompassing much of the Ohio Valley through this
morning. At the surface, northerly flow is in place ahead of this
incoming surface high, with continued NW flow aloft. This will
promote cooler and drier conditions.

There has been some concern about fog development through early
this morning. Clearing skies, light winds, recent rains, and
incoming high pressure all support fog development. However, the
incoming flow is promoting much drier air across the region and
has been ongoing through the day yesterday, and with continued
northerly flow overnight still ahead of the center of high
pressure, this could prevent thicker/denser fog development. With
a lack of observations, the new GOES-16 fog channel did indeed
show a strong surge of fog development in the deepest river
valleys, where moisture is the most prevalent. As such, went with
areas of fog in the river valleys, combined with patchy dense fog
across all deep valleys of eastern Kentucky. That being said,
still expecting this fog to stay confined within the valleys
through daybreak, at which point it will begin to lift and
dissipate, possibly affecting some ridge tops in the process.

As for the remainder of the short term period...The building in of
this high pressure system will lead to calm and quiet weather.
Northerly flow will once again keep temperatures below normal
today, with highs in the mid 70s (and some upper 70s in the
southern CWA) both today and Friday. A few diurnal CU will be
present, but should once again dissipate as we head into the
evening and lose daytime heating both days as well. Expect a
similar set up tonight in the way of fog in the valleys. The only
exception will be that yet another day of strong drying will be in
the books, so moisture availability will be less than that of
tonight. As such, kept with patchy to areas in the deepest
valleys, but did not mention any dense fog.

.LONG TERM...(Friday night through Wednesday)
Issued at 405 AM EDT THU AUG 24 2017

High pressure will keep dry conditions firmly in control this
weekend across eastern Kentucky. Dry northwest flow will remain in
place aloft as a trough approaches Newfoundland, with shortwave
ridging stretching from Hudson Bay to the Ohio Valley. A shortwave
trough migrating into the western Great Lakes and upper Midwest
will follow on the heels of this ridge into Sunday, but local
downslope flow on the backside of departing surface ridging along
with subsidence aloft will keep precipitation out of eastern
Kentucky. Temperatures will warm a bit on Sunday underneath local
upper ridging, with highs in the upper 70s to lower 80s, following
a pleasant Saturday in the mid-upper 70s. Overnight lows will
subsequently bottom out closer to 60 Monday morning after being in
the 50s during the weekend.

Return flow ahead of the latter approaching upper trough should
spell an increase in diurnally driven showers and thunderstorms
Monday with additional chances through much of the week. Details
become murky through the week regarding synoptic forcing and
subsequent sensible weather. General agreement exists regarding a
digging trough/low across the western Great Lakes or Midwest,
while the evolution of Harvey`s circulation remains uncertain.
Trends continue to point to Harvey hugging the western Gulf Coast
through at least early week, before lifting northeast for mid-late
week. Simultaneously, another tropical wave may be riding north
along the Atlantic coast. A close enough passage of this system to
eastern Kentucky could counteract the increasing moisture from
Harvey out of the southwest as dry northerly winds would prevail
across the Commonwealth. Therefore, the threat for a long duration
of rainfall does exist next week, but low confidence in the
larger scale pattern precludes greater detail at this time.
Temperatures look to remain below normal, highs in the mid-upper
70s and lows in the upper 50s to lower 60s, with cyclonic flow and
the threat of enhanced rainfall/cloud cover.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
ISSUED AT 148 AM EDT THU AUG 24 2017

An area of surface high pressure will continue to build across
eastern KY tonight. This is leading to general NW flow, with
drier air continuing to spill southeast. The mostly clear skies
will combine with above elements for fog development in the
valleys, where moisture is more prominent. Some of this fog could
be locally dense at times into the early morning hours. While fog
should be confined to the valleys overnight, there is potential
around day break for some of this fog to lift into TAF sites.
Given the uncertainty on extent and timing, opted to just go MVFR
at TAF sites between 11 and 13Z to account for this possibility.
Will continue to monitor fog development throughout the night in
case further updates need to be made. Once we get into the
daytime hours today, any valley fog should quickly dissipate and
give way to mostly clear skies. Some fair weather diurnal CU
around 5k feet will set up in the afternoon, but should dissipate
again as we lose heating in the evening. Winds will remain light
and generally northerly flow throughout the period.

&&

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

$$

SHORT TERM...JMW
LONG TERM...GUSEMAN
AVIATION...JMW



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