Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Jackson Kentucky
358 am EDT Monday Aug 3 2015
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
and...to 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
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 06z tafs through 06z Monday night)
issued at 230 am EDT Monday Aug 3 2015
A broad area of high pressure will remain in control of the
weather across east Kentucky through the rest of the night. This
will result in mostly clear skies and good radiational cooling
likely encouraging the development of fog in the river valleys.
The fog will become locally dense towards dawn. Accordingly...
brought some IFR or lower visible/ceiling into the sme and sjs sites with
MVFR visible possible at sym...similar to last night. All the fog
will lift shortly after 12z with VFR conditions through the rest
of the taf period for most sites. However...a cold front and
potential outflow boundaries will spread south into northeast
Kentucky later this afternoon and evening with a possibility of
strong thunderstorms...mainly north of Interstate 64. For
this...have left mention of thunderstorms in the vicinity in the sym taf after 21z. As for
winds...expect them to remain light and variable through most of
the taf period at all the taf sites...however later this
afternoon we could see some gusts of 10 to 15 kts out of the west-southwest
ahead of the front in the as it drops toward the far north.