Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
650 am EDT Wednesday Aug 24 2016
high pressure will persist over the region through midweek allowing
temperatures to climb back above normal. Another weak cold front will
approach the western Carolinas from the northwest toward the end of
the work week, with high pressure building back to our north over the
Near term /through tonight/...
as of 645 am EDT...more cloud cover out there than expected, as
stratocu continues to spread across the area. There is enough depth
to the instby and moisture for spotty shallow showers to pop across
the upstate and northeast Georgia. Pops were tweaked to match up with
this new trend. The latest hrrr has these cells becoming more robust
and numerous thru the late morning hours. Even though it seems to be
initializing all right, I did not bite on this just yet, keeping
pops slight chance.
As of 300 am EDT...a large upper 596 dam high at 500 mb will
continue to dominate the southeast, keeping a subsidence inversion
in place atop most of the County warning forecast area. As the high builds slightly, it will
push a plume of higher relative humidity in from the west. This axis will be across
the western third or so of the County warning forecast area this afternoon. So within that
moisture, a better chance of afternoon convection is expected. The
model consensus is for isolated to scattered showers with a few
embedded garden-variety tstms, as cape and shear will be marginal.
The 00z NAM was thrown out for this forecast, however, due to
grid-scale convective feedback over the southern NC mountains.
Steering flow will be out the north, and will try to carry this
activity into the upper Savannah valley, which should be modestly
unstable (sbcape around 1000 j/kg). So will go with consensus chance
pops in the mountains and the upper Savannah valley. Temps will be
similar to yesterday's readings.
Tonight...anyway showers and/or storms that do develop should
dissipate quickly with loss of heating this evening. However,
deep-layer moisture will linger atop the cwfa, which will keep skies
partly to mostly cloudy overnight. The 850 mb flow will shift from
southeasterly to northwesterly as the low-level high pres shifts westward slightly. So
instead of stratocu from upslope flow, perhaps more mid and high
cloudiness. Temps will bottom out at or slightly above normal.
Short term /Thursday through Friday night/...
as of 315 am EDT Wednesday...the short term fcst picks up at 12z
on Thursday with broad upper ridging in place over the southeast Continental U.S.
That will persist well into the medium range. At the sfc, high
pressure will move farther NE of our area on Thursday allowing The
Wedge to weaken and essentially dissolve. An area of pre-frontal
moisture will pass to our north later on Thursday and into Friday,
yet the model guidance has been trending a bit drier since yesterday
and keeps most of the deeper moisture out of the County warning forecast area. As the front
lifts farther NE late Friday and into Saturday, another high slides
over the Great Lakes in its wake. As for the sensible wx, the models
have been trending a bit drier overall but still have more moisture
over the higher terrain especially on Thursday. Temps start out around
climatology if not just above, and warm thru the period with highs on
Friday and Saturday about a category above normal.
Long term /Saturday through Tuesday/...
as of 3 am EDT Wednesday...the medium range fcst picks up at 12z
on Saturday with broad upper ridging in place over the southeast region.
The ridge is expected to flatten a bit on Sunday and Monday but
still maintain control over the overall synoptic pattern. It is
noteworthy that the 12z run of the European model (ecmwf) continues to develop a
tropical low over the Bahamas early in the period and move the
low due west across southern Florida and then north over the
northern Gulf Coast by day 7. The newer 00z run of the GFS also
tries to develop a low over the same area, but it remains considerably
weaker at this time. At the sfc, high pressure will be centered to
our north over the Great Lakes keeping things mostly dry for the
weekend and early next week. The big question comes just beyond
day 7 and the possibility of a tropical system moving into the
Gulf of Mexico and making landfall along the northern Gulf Coast.
It is still too early to have much confidence in any specific
model track, but it's looking like a tropical low will develop
during that timeframe. As for the sensible fcst, no major changes
were necessary with below climo pops for the period and temps about a
category above normal.
Aviation /12z Wednesday through Sunday/...
at kclt...pesky southeasterly upslope flow has produced patchy
MVFR-level stratocu across the Piedmont this morning. Forecast
soundings show that mixing heights will be slow to lift the stratus
to VFR levels...perhaps not until around midday. Overall, they
should remain patchy, but guidance hasn't been handling the low ceilings
well. Later today, more moisture around should support convection
across the mountains and the upper Savannah valley. In fact, shallow
showers have already been developing across portions of the upstate.
I will start the three upstate sites with vcsh this morning. Kavl
will feature thunderstorms in the vicinity this afternoon. Tonight, flow will turn more out
of the west or northwest. So it should be a little better fog setup
rather than stratus. However, will keep fog confined to mountain
valleys for now, with an IFR vsby at kavl late tonight. Winds will
be light thru the period, generally out of the south or southeast.
Outlook: a deep ridge will remain in place over the southeast thru
the weekend, keeping afternoon rain showers/thunderstorms and rain chances below normal each
day, and generally confined to the mountains. However, sfc moisture
will gradually return, and morning vsby restrictions will become
increasingly likely during this time.
11-17z 17-23z 23-05z 05-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 83% high 100% high 100% high 100%
kavl Med 61% high 90% high 100% high 100%
khky high 100% high 100% high 100% high 83%
kgmu Med 78% high 100% high 100% high 100%
kand high 89% Med 60% high 100% high 100%
The percentage reflects the number of guidance members agreeing
with the scheduled taf issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link: