Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
153 am EDT Tuesday Sep 1 2015

high pressure will linger across the region early this week. Weak
disturbances may ripple across the lower and middle appalachian
region by middle to late week. A cool and moist area of high pressure
is expected to settle across the western Carolinas and northeast
Georgia this weekend.


Near term /through today/...
as of 140 am EDT...infrared satellite imagery shows patchy stratocumulus
fanned out under a middle level subsidence inversion...but this cloud
layer should not become widespread enough to have much impact on
daybreak fog development. Still anticipate the best fog/low cloud
chances in the mountain river valleys...but model profiles also
feature some patchy stratus in the Piedmont as well.

Otherwise...upper heights will generally build over the southeast today
before falling a bit from the north tonight as a wave gets picked up
over the Ohio Valley and carried eastward. Surface high pressure will
remain centered along the Appalachians to the north of the forecast
area. Water vapor imagery continues to show persistent tropical
moisture extending from central Florida to the extreme southeast coastline...but
moisture will be much sparser in profiles over our forecast area.
Given the warm temperatures aloft and weak instability in model
profiles...expect a repeat of mainly isolated to scattered weak
mountain Ridgetop convection this afternoon and early evening. Maximum
temperatures should return to 90 degrees for a good chunk of the
foothills/Piedmont...with mild mins expected tonight.


Short term /tonight through Thursday/...
as of 330 PM Monday...eastern Continental U.S. Will be under the influence of a
mean ridge which will give the area temperatures around 5 degrees
above normal. A well-defined small shortwave currently over
Indiana...which is clearly seen in WV imagery...will provide some
enhanced chances for thunder storms...especially over higher
terrain...through Thursday. Due to the isolation of the Indiana
shortwave from the mean flow...predictions of its movement may be
somewhat inaccurate...however...models show some weakening of the
ridge by Wednesday with the wave being slightly entrained into what
becomes broad westerly flow aloft...which moves the lobe across the
County Warning Area on Wednesday. Instability is forecast to be higher over the
mountains...but still somewhat limited with the best model
instability in the NAM at around 1500 j/kg of cape...and hardly any
low-level windshear. With a little forcing from the
shortwave...some isolated severe wind gusts may be possible in the
afternoon on Wednesday.


Long term /Thursday night through Monday/...
as of 230pm Monday...the extended period begins on Thursday night
with an amplifying longwave trough in the middle-levels over the
Pacific northwest building a downstream ridge that extends from the
Southern Plains to the Great Lakes and well into Canada. An East
Coast trough axis extends into the southeast and lower Mississippi
River valley where a residual closed-low circulation remains trapped
underneath the ridge. Heading into the weekend...the parent closed-
low centered over Labrador shifts into the Atlantic as persistent
ridging builds over New England...leaving behind a weak trough or
cyclonic circulation over the southeast U.S. That lingers across the
area through the end of the period on Tuesday morning.
Therefore...the flow aloft through the period will be highly
dependent on the exact positioning of the residual trough...but
should generally be quite weak and transition from northerly to
southeasterly by Tuesday.

At the surface...a large area of high pressure centered over Quebec
but extending well southwest into Texas will gradually build south
into New England underneath a building ridge aloft. The high
pressure system will anchor across New England through Sunday before
shifting offshore heading into early next week. The pattern favors
the surface ridge axis nosing southeast into the forecast area...resembling
a cold-air damming scenario. The resultant northeasterly flow over
the weekend will veer easterly as the parent high shifts offshore by
Monday...but in either case the pattern favors upslope flow and weak
isentropic upglide over the western Carolinas through the end of the

In terms of sensible weather...the above pattern favors chance probability of precipitation
for showers and general thunderstorms through the period for a
majority of the forecast area...with the highest probability of precipitation favoring the
mountains. High temperatures should be a few degrees above normal
on Friday but will drop to a couple of degrees below normal for
Saturday through Monday. With increased overnight cloud cover
anticipated this weekend...expect low temperatures to be slightly
above normal.


Aviation /06z Tuesday through Saturday/...
at kclt...stratocumulus trapped under middle level subsidence inversion
has generally thinned out to the west of the airfield. Less overnight
clouds may increase the chance of daybreak low stratus and fog as
hinted in the model profiles. However...the MOS is quite reserved on
morning restrictions so will not mention for now. VFR cumulus will
redevelop with heating on Wednesday. Light northwest winds early will turn around
the dial toward srly through the day.

Elsewhere...patchy stratocumulus under the middle level inversion will
persist aloft. The best chance of morning low stratus and fog will
be in the mountain river valleys...affecting kavl. VFR cumulus
should develop with heating today. Profiles do not exhibit much
instability...but instability and coverage will be best near kavl
where a prob30 will be advertised. Expect light winds
throughout...with more north to NE flow in western NC and more srly flow at
the upstate taf sites.

Outlook...a plume of Gulf and Atlantic moisture will continue to
wrap up along the southeast coast through the week. Restrictions are
possible during periods of any scattered showers or thunderstorms.
In addition...low clouds and fog may develop each night...especially
in the mountain valleys.

Confidence table...

06-12z 12-18z 18-24z 00-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 93% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 97% high 100% 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: (must be lower case)



Gsp watches/warnings/advisories...


near term...hg
short term...wjm

National Weather Service Glossary of Abbreviations