Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Jackson Kentucky
817 PM EDT Tuesday Jul 22 2014
issued at 817 PM EDT Tuesday Jul 22 2014
Convection is on the decline with the loss of heating. Will hang onto
isolated showers and a rumble of thunder for another hour or two...
before a lull is then expected into the overnight. As weaker
southwest flow engages in th low levels towards dawn...there may be a
small increase in convection once again...which the current forecast
already has well in hand. Updates have been sent.
Short term...(this evening through Wednesday night)
issued at 420 PM EDT Tuesday Jul 22 2014
As of late afternoon...an upper level low is meandering along the
Gulf Coast while the eastern extent of an upper level ridge centered
near The Four Corners region extends into the lower Ohio Valley region.
A shortwave is rotating across the northern Great Lakes and Ontario
with another upstream entering northwest Ontario.
The shortwaves will combine to help form an eastern Continental U.S. Trough
through the period while the ridge amplifies across The Rockies. A
surface low associated with the developing trough will track from just
north of the northern Great Lakes and into the Maritimes through Wednesday
night with the developing trough and northwest flow pushing the
trailing cold front into the Ohio Valley late tonight into the day on
Wednesday. This front will gradually cross the region late on Wednesday afternoon
into Wednesday evening.
In the meantime...isolated showers and possibly a stray thunderstorm
will remain possible mainly until near sunset and daytime heating is
lost. There will be some minor height falls overnight and with a
moist air mass in place...a stray shower cannot be ruled out
overnight...but this would be few and far between otherwise...the
consecutively driven cumulus should dissipate overnight and patchy River
Valley fog is again anticipated.
More organized convection is anticipated as the front approaches and
crosses the region and the upper level trough develops over the
eastern Continental U.S.. this front and upper level forcing from the developing
trough will interact with a moist prefontal air mass to bring more
organized convection to the region from Wednesday afternoon into Wednesday night.
Morning heating will be Delaware pendant on how much cloud cover there is in
the am and this will ultimately determine instability. Moderate
instability is anticipated with temperatures in the middle 80s to around 90 and
dewpoints in the upper 60s to around 70. The low level gradient will
be rather weak and so will the winds while the upper level winds will
not be all that strong so shear will be weak. Precipitable waters is prognosticated to climb
1.5 to 1.8 inch range from midday Wednesday into Wednesday night...so storms
should be able to produce locally heavy rain. A stronger storm or two
might also be able to produce strong wind gusts. Once the front
crosses the region...instability will diminish and the threat for
thunder should also diminish late Wednesday night.
Temperatures will continue to be above normal through the period...but a
colder and drier air mass will already be starting to advect into the
area late in the period.
Long term...(wednesday night through monday)
issued at 348 am EDT Tuesday Jul 22 2014
The models are in only fair agreement aloft through the extended
portion of the forecast. While they all depict the abnormal pattern
of a strong Four Corners Region Ridge and deep troughing in the east
continuing through the end of the week and into the start of the
next week...they differ on the magnitude and timing of the evolving
eastern trough...particularly over the weekend and into Monday.
Specifically...falling heights over Kentucky will be the rule into
Thursday night as the retrograding low to the south pulls away and
the northern stream sends a series of shortwaves through the area in
northwest flow. This eastern trough pulls away to start the weekend
with heights climbing temporarily before they fall again in the face
of a Node of the large northeast trough gearing up for a dive into
the upper Midwest and Ohio Valley. The 00z GFS is most aggressive
with this evolution while the 12 European model (ecmwf) keeps its core wave further
to the northwest and lags the GFS development. However...the latest
CMC and European model (ecmwf) are trending more toward the GFS. Regardless...
northwest flow will dominate the local pattern as heights fall into
Monday morning. Eventually...the European model (ecmwf) catches up...with the deeper
and more western trough idea of the GFS but some significant
differences remain with the pattern over the Ohio Valley to close
out the week. The better agreement earlier in the extended supports
a blended solution while the larger spread later on lowers
confidence so that the ensembles and a slight lean toward the GFS
are preferred solutions.
Sensible weather will feature an approaching cold front serving as a
focusing mechanism for showers and thunderstorms Wednesday night
into Thursday. Cooler and drier air will filter into this part of
the state later in the day Thursday putting an end to the convective
chances from northwest to southeast. Dry and relatively cool weather
follows into the start of the weekend thanks to a brief visit of
high pressure. However...the cold front that brought the drier
weather will start to surge north over the western parts of the
region and bring warmth...moisture...and better convective chances
back into the area on Saturday. A series of short waves at middle
levels...the developing trough...and the surface frontal structure over
the Ohio Valley keep shower and thunderstorm chances going into
Sunday and Monday.
The CR grid Load provided a decent start to the forecast for the
extended...though did adjust the probability of precipitation to add a tinge of diurnal to
the timing of the convection. Also...tweaked the low temperature
grids a tad to reflect ridge and valley differences Wednesday and
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 817 PM EDT Tuesday Jul 22 2014
A rogue shower will affect sme from the east through 0030z before
dissipating...otherwise convection will be on the wane through the
overnight with IFR to MVFR fog once again to contend with between 06
and 12z. Expect that the deeper valleys will see the LIFR or worse
conditions...with better conditions in the broader valleys and
ridgetops. Sme could be in for IFR or worse fog depending on the
rainfall amounts that occur there. Isolated convection will threaten
once again by around 12z...with better chances occurring in the
afternoon with the approach of a cold front from the northwest. Light
south to southwest winds will shift to the west and northwest