Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 251928
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
328 PM EDT Sun Sep 25 2016

.SHORT TERM...(This evening through Monday night)
Issued at 327 PM EDT SUN SEP 25 2016

An upper level ridge remains in place across the Ohio River Valley
today, but should start to break down overnight as an upper level
wave and associated troughing move eastward into the upper
Mississippi River Valley. The associated wave will strengthen into
an upper level low across the Upper Great Lakes by tomorrow
afternoon, with the deepening axis of the trough swinging across
Kentucky throughout the day tomorrow as well. Models remain in good
agreement through Monday night, with the closed low showing little
forward momentum, but continuing to rotate and gain intensity
throughout the night. This will keep lowering heights across the
Ohio River Valley as well. This low will tap into the northern
stream of the jet stream, with a strong pull of Canadian air
expected to propagate southward and into the Ohio Valley.

At the surface, a low pressure system is currently in place across
southwest Ontario, coinciding with the upper level wave. This too
will continue to push eastward and just north of Lake Superior
overnight. A cold front is extending southward from this low,
presently draped across WI, western Iowa, Missouri, and points to
the southwest. A weak warm front is also extending from the low,
currently draped across KY. This warm front is causing little impact
to the ongoing weather pattern, however, with light winds and the
upper level ridge still in place, abundant sunshine has boosted
temps back into the mid and upper 80s once more this afternoon.
Temperatures will quickly start to cool down this evening and into
the overnight, with the cold front expected to make it into western
KY by around midnight. Winds across eastern KY will begin picking up
in speed slightly as they shift to a more S to SW direction during
the overnight as well. This will begin pulling in some moisture in
the form of clouds into the region, and then eventually
precipitation just after daybreak tomorrow (Monday).

Best precipitation chances will occur along the actual frontal
boundary as it traverses eastern KY during the day tomorrow. Decent
CAPE values according to the latest NAM12 soundings would promote
some thunder potential with this system. While soundings do not
favor hail potential, especially given the warm airmass in place
leading into this event, some gusty winds and frequent lightning
cannot be ruled out. That being said, once the cold front pushes
through a location, winds will quickly shift to the Northeast and
allow much cooler and drier air to begin filtering in. This, in
addition to loss of daytime heating tomorrow evening, should quickly
cut off best thunder potential across the CWA after 0Z. This is
reflected in the quick loss of instability in the forecast soundings
as well. Some rain showers may continue throughout the night in the
far southeast due to upslope flow.

It is during this time Monday night that we really start to tap into
that strong pull of drier and colder Canadian air. This will result
in clearing skies, aiding in the fall of overnight temps. While
overnight lows are expected to be in the mid 60s tonight, overnight
lows on Monday night will likely drop into the mid and upper 40s for
much of the region. This pull of colder and drier air will continue
into the mid and long term portion of the forecast as well as the
upper level features show slow progression from this point
forward.

.LONG TERM...(Tuesday through Sunday)
Issued at 327 PM EDT SUN SEP 25 2016

After agreement on a cool and dry start to the period, models
continue showing disparity for the remainder of the period. A cold
front will be well to our southeast on Tuesday, with dry air in
place behind it for the local area. The upper level system which
initially drives the cold front through will then become our main
player. This large upper low will be moving south over the western
Great Lakes Tuesday and Tuesday night, and into the Ohio Valley
Wednesday. Although models do not agree on the evolution of this
system, there are trends observed. The ECMWF is fairly consistent,
but has shown a bit of a drift even further to the west as compared
to 24 hours ago. Meanwhile, the GFS and its ensemble have made the
larger shift, with a move toward the ECMWF solution. Instead of
being progressive with the system, the GFS now also holds back a
significant upper low (although, not as far west as the ECMWF). That
being the case, will take the forecast more toward the ECMWf. This
results in below normal temps and a potential for showers during the
later part of the work week as the large, nearly cut-off upper low
wobbles near KY.

By the weekend, both the ECMWF and GFS have the low weakening
considerably and lifting out to the NE-- albeit with different paths
and timing. There is no return of low level warm air advection yet,
but mid/upper levels warm with the jump in geopotential heights.
This should lead to stabilization and an end to showers, and
decreasing clouds.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
ISSUED AT 139 PM EDT SUN SEP 25 2016

High pressure will remain in control of our weather through
Monday morning resulting in VFR conditions for most areas with
light and variable winds. A few diurnal CU could be possible across
the forecast area through this afternoon, with bases mainly
between 4.5 and 6k ft. These clouds will then quickly dissipate
with the loss of daytime heating this evening. Patchy valley fog
will be around again towards dawn Monday, possibly affecting LOZ
and SME with some MVFR BR for a couple of hours. A cold front
will begin nearing eastern KY by Monday afternoon. This could
bring with it increasing chances of showers and thunderstorms for
the remainder of the afternoon. Best potential should be after
18Z, but did try to trend the TAFs towards these changing
conditions, while still remaining VFR.

&&

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

$$

SHORT TERM...JMW
LONG TERM...HAL
AVIATION...JMW


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