Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Jackson Kentucky
1111 am EDT Wednesday Jul 29 2015
issued at 1111 PM EDT Wednesday Jul 29 2015
The region is in the warm sector as an area of low pressure tracks
across northern Ontario. A warm and rather moist airmass remains
in place across the area. Dewpoints remain in the upper 60s to
middle 70s. However...the airmass is not quite as moist through the
entire column across the central and eastern part of the area
with precipitable water there around 1.6 inches. Precipitable water across the west is around 1.8
to 1.9 inches.
With daytime heating...low clouds should eventually lift into cumulus
across the southwest part of the area. The airmass should also
destabilize with ml convective available potential energy expected to reach the 1000 to 1500 j/kg
range across the west by middle afternoon. The moisture gradient
across central Kentucky into the western part of the County Warning Area may be a focus
for convective development later today. Also...the higher terrain
along the Virginia border that is getting plenty of sunshine at this time and
differential heating near where heavier rain fell on Tuesday may
also be possible locations for the first cell development. Shear
is rather weak today...and cells should more or less propagate via
outflow boundaries. More organized convection...and possible a
broken line of line segments of storms should near the region
around sunset. Isolated to scattered probability of precipitation.
Update issued at 805 am EDT Wednesday Jul 29 2015
Made only minor updates for early morning observation.
Short term...(today through thursday)
issued at 411 am EDT Wednesday Jul 29 2015
Isolated slow moving showers have persisted during the night in
the southeast part of the forecast area. Forecast soundings show
that some additional development can not be ruled out this
morning...but the probability at any given location is low.
Additional development of showers/thunderstorms is more probable
by this afternoon as heating occurs. Most of them should die out
again in the evening with loss of heating. A cold front will pass
late tonight and Thursday morning. A few showers may linger along
and ahead of the front in the southeast part of the area on
Thursday morning. However...drier air aloft and a Post frontal
inversion will put an end to any lingering showers...with most of
Thursday being dry.
Long term...(thursday night through tuesday)
issued at 354 am EDT Wednesday Jul 29 2015
We will be in a Post frontal regime to start off the extended
forecast at 0z Friday. Surface high pressure will continue to take
hold through the day Friday with slightly cooler nearly winds at the
surface. Meanwhile...a strong upper level low will become nearly
stationary across Hudson Bay...rotating around the Bay from Friday
through at least Monday. This will keep a longwave troughing pattern
across much of the Midwest and eastern Continental U.S....continuing the northwest
flow regime across the Ohio River valley. This northwest flow will provide
the perfect scenario for shortwaves to track southeast along the trough and
into the Ohio Valley as it rounds the base of the trough.
However...high pressure will have hold at the surface. The question
throughout the extended is whether or not this high pressure will be
strong enough to sway the impact of these shortwaves as they move
through eastern Kentucky.
The first of these shortwaves will move through Saturday into
Sunday. As of 0z Friday...the accompanying surface cold front is
posed to be located across the northern plains and upper Great Lakes
region...and will shift southward throughout the day Friday as the
shortwave nears. However...this boundary will be moisture deprived
and fairly weak. Latest models are still showing some disagreement
on if this boundary will be able to sustain precipital moisture as
it moves into the Ohio River valley. Just like yesterday...the GFS
is pointing towards a few scattered showers making it into the
County Warning Area...while the European model (ecmwf) is keeping US dry. Meanwhile...despite the
plan view GFS output...the GFS forecast soundings are only pointing
at a narrow window of middle level moisture with abundant dry air near
the surface. This is more characteristic of some scattered to broken cloud
cover rather than rainfall. As such...will continue to trend toward
the drier European model (ecmwf).
The next shortwave and surface boundary will move through Tuesday
into Wednesday as we finish out the 7 day forecast. Granted...this
is far out in the forecast...but models are in surprisingly good
agreement in the upper levels...so confidence is a bit higher. This
particular upper level shortwave will show significant digging as it
nears the Ohio Valley region...lending to a stronger surface
boundary. If this proves to be true...we may see our next better
chance for showers and thunderstorms during this time.
As for temperatures...
The Post frontal conditions and drier northwest flow will bring in slightly
cooler temperatures to start off Friday. However...strong surface
high pressure in place and clear skies will promote maximum daytime
radiation...so the temperature drop will not be that great...still
maintaining low to middle 80s. Perhaps more noticeable than the slight
change in temperatures will be the decrease in humidity...as light
winds at the surface and northwest flow aloft will promote lower relative humidity values.
As high pressure remains in place through much of the
extended...expect temperatures to gradually warm a degree or two each
day...staying in the middle 80s through the weekend and into next week.
Meanwhile...maximum radiational cooling during the overnight will
keep overnight lows in the low to middle 60s through the beginning of
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 805 am EDT Wednesday Jul 29 2015
Conditions were mostly IFR or worse at the start of the period.
Improvement will occur during the morning...with generally VFR
arriving early in the afternoon. However... the daytime warming
should also lead to additional scattered showers and thunderstorm
development this afternoon. They will bring localized IFR. The
showers and thunderstorms should then largely diminish in the
evening with loss of heating.