Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Jackson Kentucky
755 am EDT Thursday Sep 18 2014
issued at 745 am EDT Thursday Sep 18 2014
Radar trends indicate that the sprinkles and stray light showers have
been gradually diminishing since a little before sunrise across the
region. Some dense fog is still occurring on a couple of Ridgetop
locations...mainly in the Big Sandy region. Otherwise...middle level
clouds should continue to move across the region this morning...with
cumulus development possible in the afternoon. Hourly grinds have been
updated based on recent observations and trends. An updated zone forecast product is
Short term...(today through friday)
issued at 400 am EDT Thursday Sep 18 2014
A middle level disturbance continues to near the lower Ohio Valley region
this morning. This disturbance has brought some middle level moisture
and bands of middle level clouds to the commonwealth. These bands of
clouds do have some thin spots and holes in them. The most widespread
middle level clouds are near the Interstate 75 corridor. In fact...some
weak returns are working across the SW part of the County Warning Area nearer to the
Tennessee border. No measurable rain has been observed so far with this. In
the very near term we have opted to just include sprinkles across the
southern counties from McCreary east into Harlan County. The arrival
of the middle level clouds has at least temporarily led to improved visible
in the fog in most areas per recent observations other than ki35 and
kpbx and some of the Big Sandy region where the fog has been locally
dense. The middle clouds moving overhead should lead to improvements
there as well other than in breaks or holes in the clouds through
Behind the middle level disturbance to work across the region this
morning...the middle level moisture near 700 mb should also decrease but
the layer between 850 mb and 750 mb should still remain rather moist.
Some shallow diurnally driven cumulus will be possible in this layer from
middle day into the afternoon...as drier air is expected to move in...
especially aloft...behind the disturbance. 500 mb heights are
expected to rise as high pressure aloft builds into the lower Ohio
Valley from the central and Southern Plains regions. A subsidence
inversion between 800 mb and 700 mb should result and is expected to
remain through the end of the short term period.
At the surface...a large area of high pressure currently centered over
Ontario will build into the Ohio Valley and northeastern states through
early this evening. The high is expected to reach the New England
coast by the end of the period. Fog should develop tonight...especially
in valley locations under light winds and mostly clear to clear skies.
An inverted surface trough is expected to develop north into eastern Kentucky
late tonight and into the day on Friday. The inverted trough should
weaken and gradually shift west by the end of the period. This
inverted trough should draw enough moisture west across the
Appalachians for some mostly rather shallow cumulus development below the
subsidence inversion on Friday from midday into the evening. Some of
this could become deep enough for a few sprinkles or even a stray
shower or two near the Virginia border. For now...we have continued with a
dry forecast as MOS guidance generally has 10 percent or lower probability of precipitation.
However...several of the 0z models do have some light precipitation
near the Virginia border on Friday afternoon. The model blends also have slight
chance probability of precipitation in that area as well for Friday...so this scenario will
need to continue to be monitored.
Long term...(friday night through wednesday)
issued at 328 am EDT Thursday Sep 18 2014
A shortwave and inverted trough over the Appalachians should be
transitioning eastward and out of the region by 0z Saturday. More
zonal flow and high pressure at the surface will take hold for the
day Saturday. Meanwhile...the longwave pattern is planned to deepen
across south central Canada and the north central Continental U.S....eventually
making its way eastward into the Ohio River valley Sunday and
Monday. This will result in a cold front traversing Kentucky during the
day Sunday. The front should eventually pass through the jkl
forecast area...but models are still not locked in on similar
timing. A model blend would suggest Sunday afternoon/evening...and
will center the highest probability of precipitation at that time. As with the uncertain
start times...the exit of this front is also still somewhat
undetermined. Will lean more towards a blend of extended models at
this time until better coherency takes place in later model runs.
Southerly winds just ahead of this frontal passage...as well as
incoming cloud cover...will keep temperatures a little above seasonal for
Saturday through Sunday night. Kept with only mentioning slight
chance thunder for during the day Sunday. Just not picking up on a
lot of instability across the region...in addition to the fact that
the frontal boundary should be weakening/shearing out as it reaches
our area. 18z Sunday soundings seem to show the best instability
potential /though still marginal/...but winds are nearly
unidirectional from the west...so Don/T expect anything that
develops to stay sustained for any long period of time.
Behind this exiting trough will come strong upper level ridging and
surface subsidence. This will keep much of the central Continental U.S. Dry
through the remainder of the extended /Monday forward/. The depth of
this ridge will also work to pull strong northerly/Canadian air in
the low...mid...and upper levels across Kentucky. Temperatures will
be coldest on Tuesday...when some of the best/strongest northerly
air is tapped into. But overall...highs should only reach the upper
60s and low 70s for the remainder of the extended. With strong
radiational cooling expected overnight under mostly clear
skies...early morning lows will stay in the upper 40s/low 50s.
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 755 am EDT Thursday Sep 18 2014
A weakening disturbance into the lower Ohio Valley region at this time. This
feature has brought some middle level moisture and middle level clouds and
even some sprinkles and stray light showers to the region.
However...the stray light showers and sprinkles are diminishing and
what is left of these will not impact the taf sites. Otherwise...
the fog that developed before these bands of clouds moved across
the region has lifted onto the ridges in southeastern Kentucky. The fog
will initially be dense at sjs...but should gradually lift with VFR by
1430z. Some VFR or MVFR fog is anticipated at the remaining taf sites
initially with all locations improving into VFR around or shortly
after 14z. Any cumulus that develops and remaining middle level clouds will
remain in the VFR range through around 22z when they dissipate. Fog...
reducing visible down into the MVFR range...or possibly IFR range will
again be possible during the last 6 hours of the period.