Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 231555

National Weather Service Jackson KY
1155 AM EDT Mon Apr 23 2018

Issued at 1155 AM EDT MON APR 23 2018

Late morning update made to freshen up zones and remove any
morning wording. Still can not rule out a couple rumbles of
thunder across the south this afternoon as breaks in clouds allow
a bit more sun into the area and a vort lobe rotating around Mid-
South mid/upper level low steepens mid level lapse rates just a
tad. Depending on how much sun we see may need to bump up
afternoon highs just slightly. But for now temperatures are pretty
much on track for afternoon highs in the low to mid 60s.

UPDATE Issued at 1003 AM EDT MON APR 23 2018

Updated the forecast package to bring grids and zones in line
with most recent radar trends. This resulted in an increase top
PoPs. Otherwise the forecast generally remains on track. Will plan
to make a late morning update to freshen up the zone wording,
basically to remove morning wording. Will also want to take
another look at the potential of afternoon thunder as latest runs
begin coming in.

UPDATE Issued at 802 EDT MON APR 23 2018

The band of showers over the southern counties and northeast TN
seems to be holding together quite well despite southeast low
level flow. Cloud bases are on the high side, but rain is
measuring. With that in mind upper pops a bit over the southeast
counties as this lifts through to the 70 to 90 percent range.
Otherwise, some slight adjustments were made to hourly grids based
on observations trends with not other changes. Recent HRRR runs
continue to lift the initial band to the north and northeast with
potential redevelopment late in the day in the southwest with some
heating. A rumble or two near the TN border cannot be ruled out.


.SHORT TERM...(Today through Tuesday)
Issued at 445 AM EDT MON APR 23 2018

The axis of mid and upper level ridging will continue to depart to
the east and northeast of the area today with sfc high pressure
also moving further northeast of the area. This will allow the mid
and upper level low currently centered near the Memphis TN
vicinity to meander into middle TN this evening and then reach the
Southern Appalachians by the end of the period. Moisture has
already been increasing across the area, especially aloft.
However, the flow from around 800 mb to the surface is downslope
which is slowing down increases in dewpoints and rh at low levels.
Despite this, showers or light rain has been falling across the
southwest part of the area overnight from a mid cloud deck in a
band east of the center of the mid and upper level system.

Today, the model consensus is for a band or bands of showers
to continue moving north and east across the area through mid
afternoon. Toward midday activity will likely become more isolated
to scattered in nature over southwestern counties. At the same
time, temperatures aloft should begin to cool. Thus with daytime
heating especially in the southwest part of the area additional
activity is expected develop by late afternoon or early evening
while a drying trend over much of the northern and eastern
sections from mid afternoon into the evening. Enough instability
may be present in fact for some isolated thunder to extend into
the Lake Cumberland region and possibly as far east as Whitely and
Bell counties or approximately where the SPC general thunder
outlook extends.

Momentum transfer in bufkit from the NAM has gusts into the 20 to
25 knot range from late this morning into this afternoon for many
area with up to 30kt possible, especially behind this band where
some limited solar insolation may add to the mixing. Gusts on top
of Black and Pine Mountains, the Log Mountains of Bell County and
the Flatwoods area of Pike County may reach as high around 35 kt
from late morning into the afternoon. The southeast downslope
component of the flow should lead to a rain shadow downwind of
Pike Mountain across portions of the southeastern counties and
downward adjustments were made to the initialized superblend qpf
in those areas to account for this. The downslope flow should also
lead to locally warmer max T in valley locations such as Harlan
and Cumberland, Whitesburg and Cumberland northeast to Dorton and
Elkhorn City where upper 60s to near 70 are anticipated.

Locations closer to the center of the upper level system and less
influenced by downslope flow especially from I 75 west should
experience more persistent showers through the evening into the
over night hours while some southeastern locations may be rain
free for much of the night. A qpf trend which should begin during
today should continue into tonight with qpf twice or more times
greater in some of the southwest and western sections compared to
the southeast. Chances for showers should increase into central
portions of the area late Monday night as a band of showers is
expected per model consensus.

This band of showers should continue progressing north and
Northeast across the area into Tuesday afternoon. The coldest
temperatures over the next couple of days at 850 mb should be
during Tuesday morning into Tuesday afternoon. Steepening low
level lapse rates may lead to diurnal uptick in the rain rates
from the showers on Tuesday in the initial band with an isolated
thunderstorm or two also possible. Some redevelopment of showers
or even a thunderstorm or two may develop closer to the center of
the mid and upper level system on Tuesday afternoon.

Diurnal ranges will be much more limited than recent days through
the near term period with highs generally in the low to mid 60s
and lows tonight generally in the low to mid 60s. These readings
will be below normal for highs which are around 70 for this time
of year while lows will be a few degrees above normal as compared
to normal lows for this time of year in the mid and upper 40s.

.LONG TERM...(Tuesday night through Sunday)
Issued at 405 AM EDT MON APR 23 2018

A 500mb low will be collocated with a surface low pressure system
across Kentucky starting out at 0Z Wednesday. This will continue to
shift eastward out of the state throughout Tuesday night and into
the day Wednesday, with pops/rain chances expected to be on a
decreasing trend during this time. Upslope flow will keep some
chance pops in place through the day Wednesday, especially across
the SE, but additional QPF will be under a tenth of an inch through
0Z Thursday.

Models then lose considerable agreement by 0Z Thursday, with the NAM
and GFS planting a developing closed low over the south/central
Plains, and the ECMWF showing a strong closed low over the northern
Ohio Valley and southern Great Lakes region with a shortwave trough
over the central/southern Plains instead. The ECMWF shows this
closed low shifting SE across Ohio Wednesday night and into the day
Thursday, while both the GFS and ECMWF show the other shortwave/low
system tracking just south of Kentucky into the deep south during
this time. Surprisingly, which ever model pans out, neither actually
pulls precip from either system into the state, so will keep
Thursday dry in the forecast. Models then come into much better
agreement for another system which will affect the region to end out
the workweek.

A 500mb shortwave will follow deep troughing across the region,
deepening the trough as the axis moves through Kentucky Friday night
into Saturday morning. At the surface a cold front is expected to
shift from west to east across the state during the day Friday and
into Friday night, bringing with it a swath of chance pops.
Instability is still lacking, so will not include any mention of
thunder at this time. In fact soundings are actually quite dry, so
not expecting much QPF from this either. Even after tweaking QPF up
slightly from the Superblend, still only have well under a tenth of
an inch for the duration of the system.

During much of the extended period, temperatures will actually
remain relatively uniform, with highs in the low to mid 60s and lows
in the 40s. However, if clearing occurs overnight Friday night and
Saturday night (coupled with light winds), some sheltered valleys
may fall into the upper 30s. Ridging will slowly take hold
throughout the day Sunday and continue into Monday. This will allow
temperatures to climb into the low 70s both days.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)

An upper level low will meander across TN through the period and
reach near the northern portions of the Cumberland Plateau in TN
by the end of the period. For much of the period the bulk of the
region will maintain a downslope component of the flow with lower
levels slow to saturate. Thus mainly low and mid level clouds will
predominate through 3Z before the lower levels saturate in the
southwest as the center of the mid and upper level low nears. This
should lead to MVFR reaching near I 75 by the end of the period
with IFR possible near Lake Cumberland. Some brief MVFR vis is
possible in showers or any thunderstorms across western and
northwestern sections of the area through 0Z. Otherwise, vis may
fall as low as MVFR to IFR prevailing in the southwest after 6Z.
Winds should continue or increase to near or in excess of 10KT at
the TAF sites early in the period period with some gusts over
20kt. Winds should generally slacken around or after 0Z.





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