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Area forecast discussion 
National Weather Service Jackson Kentucky
119 PM EDT Monday Aug 3 2015

issued at 1038 am EDT Monday Aug 3 2015

The showers and thunderstorms that moved into the northwest part
of the area this morning have dissipated. The outflow boundary
from this convection has made it almost all the way to the Tennessee
border per latest radar and visible satellite imagery. Not sure
how much further south this boundary will go. Since this boundary
may serve to focus convection later today have introduced a 20
percent chance for isolated thunderstorms in the Tennessee border
counties for late today.

A weak cold front extends from north of Lake Erie to northern
Indiana. This front will continue to move slowly southeast this
afternoon and should serve as the primary focus for convective
development late this afternoon. The latest hrrr and NAM both are
trending towards this afternoons convection developing later than
previous guidance...with development occurring in Ohio and not
moving into Kentucky until this evening. With this in mind have slowed
down the introduction of thunderstorm chances to very late in the
day in the north.

Update issued at 645 am EDT Monday Aug 3 2015

The cluster of storms just northwest of the County Warning Area is holding
together a bit better than modeled. It looks like it will brush
into at least Fleming County. Have updated the grids to reflect
this and also to match up the latest observation and trends for T/dew point
grids. Have uploaded these to the ndfd and web servers...along
with an updated zone forecast product issuance.


Short term...(today through tuesday)
issued at 345 am EDT Monday Aug 3 2015

07z surface analysis shows high pressure still holding on over
southern Kentucky ahead of a cold front off to the northwest.
Skies are clear through the County Warning Area and this has resulted in a good
night of radiational cooling. Accordingly...sheltered valleys are
running nearly 10 degrees cooler than the ridges with lower 60s
found in the hollows compared to the low 70s on the hill tops.
Dewpoints...meanwhile...are generally in the low to middle 6os across
eastern Kentucky with light to calm winds. No sign of any fog
yet...but do anticipate shallow and locally dense fog in the river
valleys toward dawn again this morning.

The models are in fairly good agreement aloft as they show the
southward shift in the broad and unseasonably deep low currently
over east central Canada. This movement will serve to tighten up
the middle level gradient through the Ohio Valley later today and
through the rest of the short term. East Kentucky will be in
northwest flow with ripples of minor waves passing through into
middle week. Of note...a more progressive wave will move east from
the plains and to the middle Mississippi Valley by late Tuesday with
some localized height rises for eastern Kentucky. The model spread
is a bit larger with this key feature as the NAM lags behind the
European model (ecmwf) and GFS in bringing the energy with this wave into Kentucky
later Tuesday night. Have favored the hrrr and nam12 early on in
the forecast given their higher resolution...before going with
more of a blend later as uncertainty with the details increase.

Sensible weather will feature ongoing storms to the north...out
ahead of the approaching cold front...likely fading as they close
in on the Ohio River. This will probably send a boundary or two
into northeastern Kentucky later this morning and it could become
a focus for convective development for our area through the
afternoon. This will take place in an environment increasingly
supportive of strong convection given afternoon convective available potential energy forecast to
be near 3k j/kg...some middle level drying...and steep lapse rates.
The winds aloft will be more favorable for organized
development...too...but the shear is not that impressive. Combined
with the approach of the main front...though this should be enough
for at least scattered strong thunderstorms to develop with strong
to severe ones possible over the northern portion of the County Warning Area late
in the day. With a relatively low wet bulb zero level and dcapes
approaching 1500 j/kg large hail and damaging wind gusts will be
the main threat. Storm Prediction Center has placed a large chunk of the northern
parts of our area in a slight risk for this concern. Will
highlight this in the weather story...via a web headline...and in the
severe weather potential statement. The threat of showers and storms will then continue through
the night as the front settles south across east Kentucky. The
clouds and boundary should be enough to keep temperatures warmer
and more uniform Monday night. Storm chances will continue on
Tuesday with the front over the area and the approach of the middle
level wave from the west.

Again started with the shortblend model for the T/TD/wind grids
through the day followed by the superblend for tonight and
Tuesday. Did make some adjustments to temperatures this morning a lesser extent...tonight with respect to ridge and
valley differences. As for probability of precipitation...went a bit higher than guidance
through the period given the front hanging over the area...despite
it being rather moisture starved.

Long term...(tuesday night through sunday)
issued at 358 am EDT Monday Aug 3 2015

The upper level pattern continues to feature a western Continental U.S. Ridge
and a trough over the east with an active jet stream. This brings
active weather to the region with a series off shortwaves riding
over the western ridge and into the Ohio Valley. Despite subtle shifts
in the western ridge with an active jet stream into the West
Coast...the GFS and Euro seem to agree on the longwave pattern.
Though besides the Wednesday and Thursday features...timing will
still pose quite a problem.

Details closer to the surface will feature a lingering surface
boundary being quasi/stationary draped across Kentucky and the lower Ohio
Valley as several shortwaves track east southeast from the Front
Range of The Rockies through the plains and into the Ohio Valley.
Heading further into the period...a good deal of upper level support
in the form of a strong jet streak enters the West Coast over
central California and through The Rockies weakening the ridge a bit. This
pushes a even better well defined wave through the plains and into
the Ohio Valley by Wednesday night. Models have been hinting at this
feature for quite a few runs that seem to bring a well defined
system through Kentucky Wednesday night through Thursday night. The super
blend agrees with this reasoning as well...advertising well above
likely probability of precipitation through the Wednesday and Thursday night period. This
period will prove to be the most interesting and active portion of
the extended.

As mentioned earlier...the GFS and Euro diverge in solutions
heading into the weekend with the upper level pattern flattening in
the GFS but remaining a bit more amplified in the Euro. Thus...a
stalled boundary over the Tennessee Valley and along the Appalachians and a
continued active but subtle upper level pattern will keep shower and
thunderstorm chances through the last few days of the extended but
lacking confidence...would be more inclined to stay with and just
below the solution of the super blend.


Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 119 PM EDT Monday Aug 3 2015

A weak cold front extending from north of Lake Erie to northern
Indiana will move slowly southeast today and tonight. This front
should trigger some thunderstorms over Ohio late today...and a
few thunderstorms may move into the northern part of the forecast
area this evening...but with minimal coverage. As such will carry
thunderstorms in the vicinity in the sym taf for a few hours this evening. Outside of this
an isolated thunderstorm cannot be ruled out late this afternoon
or evening across the remainder of the forecast area.
However...coverage is expected to be very minimal and will not
even warrant thunderstorms in the vicinity in tafs south of sym. A little more cloudiness
is expected tonight...but any ceilings should be VFR...and any
cloud will minimize fog development. Expect any fog to be similar
or less than last night...with most taf sites remaining VFR. The
exception will be sme where fog should result in IFR conditions
for a time again late tonight and early Tuesday.


Jkl watches/warnings/advisories...


short term...greif
long term...shallenberger

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