Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 230758
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
358 AM EDT Wed Aug 23 2017

.SHORT TERM...(Today through Thursday)
Issued at 358 AM EDT WED AUG 23 2017

A cold front is currently tracking eastward across Kentucky. Based
on latest IR imagery, there is a defined edge to the cloud cover
along and ahead of the frontal boundary verses the drying/clear
skies behind. Most pre-frontal precip has now moved out of the
CWA, with just a few lingering showers right along the VA border.
These showers should continue to exit over the next couple of
hours. The cold front should make it through the CWA and exit just
to our SE by dawn or just thereafter.

In the wake of the cold front, a large area of high pressure will
begin moving in from our northwest, encompassing much of the
north-central Conus. Meanwhile, aloft, a large upper level low
over eastern Quebec will result in longwave troughing across much
of the eastern Conus, including the Ohio Valley. This will produce
deep NW flow across the region, pulling in a cooler Canadian airmass.
Our position to the east of the surface high will promote
northerly flow at the surface, with NW flow aloft. All in all,
this will do two things: Promote dry (much less humid) conditions
across the region, as well as below normal temperatures. Highs
today will be in the upper 70s across much of the CWA with very
little humidity.

This pattern will remain in place through the rest of the short
term portion of the forecast. Radiational cooling and continued
weak northerly flow overnight will allow temps to drop into the
50s for lows, with lowest temps in the valleys. A near-surface
inversion will also set up overnight, which will trap lingering
moisture (generally in the valleys) and promote valley fog overnight.
Once fog lifts and dissipates Thursday, similar conditions to
today will be in place. However, the northerly flow will actually
intensify a bit, so temps on Thursday will only reach the mid and
upper 70s across the CWA.

.LONG TERM...(Thursday through Tuesday)
Issued at 450 PM EDT TUE AUG 22 2017

The models are in fairly good agreement aloft with the general
pattern through the extended, though there are some significant
difference with some of the smaller scale features. The core of
the main trough residing over the eastern half of the continent
will swing into the Ohio Valley on Thursday. The ECMWF actually
closes off this part of the trough as it crosses northern Michigan
while the other models are a tad weaker and more open with this
feature. The trough bottoms out Thursday night with lower heights
pushing south through Kentucky. The main trough will pull out this
weekend with heights rebounding locally in its wake. As it does
so, more energy packets will start to move across the area from
the west with a hint of weak northwest flow established by Sunday.
At this point, additional energy pours into the Upper Midwest
with the next trough developing to our northwest. In this low flow
environment at mid levels more energy will seep into Kentucky.
Additionally, we will have to watch the upper level core of
`Harvey` and how it eventually progresses onshore and creeps
northeast into the Lower Mississippi Valley on Tuesday - more
consolidated, but further south in the ECMWF and GFS than the CMC.
The remnants of this tropical system could increasingly be a
factor in the forecast for our area into the start of next week -
but that also adds in an amount of uncertainty for the latter part
of the forecast. Given the good agreement on the broad scale
initially and increasing uncertainty/lower confidence later the
blended solution looks like a reasonable starting point with minor
adjustments.

Sensible weather will feature very nice mid to late summer
weather for the bulk of the week as cool Canadian high pressure
settles into the region. This will make for dry and quiet weather
with temperatures below normal through the period. Each night will
likely see a moderate to large ridge to valley temperature split
as the boundary layer decouples. Patchy valley fog can be expected
to develop late at night and run through a couple of hours past
dawn. Return moisture does come back into the picture by Sunday
with diurnally driven air mass showers and through this evening
and thunderstorms possible. Monday and Tuesday`s weather is quite
questionable, though, as the remains of Harvey start to ooze this
way as well as low pressure inbound from the northwest along with
some mid level energy. As such, have brought better chances for
showers and storms into the forecast though with low confidence in
timing and amounts due to the tropical x-factor.

Made mainly terrain based adjustments to the lows each night from
the SuperBlend to account for an anticipated moderate to large
ridge/valley temperature split. Also, adjusted the PoPs to better
minimize any pcpn chances through the weekend and to be less
specific with the threat Monday and Tuesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
ISSUED AT 155 AM EDT WED AUG 23 2017

A cold front is continuing southeast, and will be crossing eastern
Kentucky through the overnight. Pre-frontal convection is now
confined to the far SE CWA along the VA border and will continue
exiting SE of the state over the next few hours. As we lose
instability, expect any remaining convection to be in the form of
rain. There is some residual low clouds and fog across some of
the TAF sites in the immediate aftermath of the precip, with
conditions ranging from VFR to VLIFR across eastern Kentucky.
However, strong dry air advection is in place, so do not expect
these cigs or fog to persist more than a couple of hours. That
being said, this will be something to monitor in case further
updates need to be made to keep it in for a longer duration.
Regardless, by daybreak tomorrow, high pressure will be in full
swing with VFR conditions in play through the remainder of the TAF
period. Winds will be light and variable, and a few fair weather
diurnal CU (between 4 and 6k) will be possible during the
afternoon hours.

A cold front is swinging south across eastern Kentucky tonight
with a period of showers and thunderstorms. The convection will
bring reductions in visibility and ceilings through midnight. As
the front passes later tonight, some lower cigs could develop late
bringing ceilings back into the MVFR category and possibly even
IFR late. Look for the low clouds to dissipate by midday on
Wednesday. VFR conditions then follow for quite some time. Winds
will be light and variable through the period, outside of any
storms this evening.

&&

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

$$

SHORT TERM...JMW
LONG TERM...GREIF
AVIATION...JMW



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