Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
751 PM EDT Sat Sep 20 2014

Synopsis...
a weak ridge of high pressure will linger atop the region through
tonight. Temperatures will warm on Sunday before a cold front
crosses the Carolinas later that night. Cool and dry high pressure
will then spread across the region and remain in place through at
least middle week.

&&

Near term /through Sunday/...
as of 730 PM...forecast still in really good shape. Ridgetop convection
never made it past the cumulus stage...so the evening looks dry. The high
cloud shield on the western periphery of the coastal low will continue
to spread northwest and stream overhead across the Piedmont of the
Carolinas. Think this should thin out by the early morning hours and
not affect min temperatures...yet. No changes.

Remainder of previous discussion...

Guidance suggests a good night for at least patchy dense Mountain Valley
fog... while most areas outside of the mountains generally too dry. Skies
should be mostly clear with lows near to slightly below normal.

An upper trough digs toward the area Sunday pushing a front into the
mountains by the end of the period. Little in the way of precipitation is
expected until very late in the period...then only across the mountains
where moisture...forcing and instability will be better. Low level
flow remains westerly keeping areas outside of the mountains drier with a low
level warm nose keeping the atmos generally capped with only weak
elevated instability at best. Bring chance pop into the Tennessee border
counties by late afternoon...with slight chance across the rest of the
mountains...and a dry forecast elsewhere. Cannot rule out an isolated severe
wind gust across the mountains given the increasing shear and dry middle
levels...but chance is low. Highs will be 5 to 10 degrees above normal
as thicknesses increase significantly ahead of the front along with
the increasing westerly low level flow.

&&

Short term /Sunday night through Tuesday/...
as of 145 PM EDT Saturday...the short term forecast period
initializes Sunday evening amidst an upper trough ejecting to the east
across the Great Lakes region with a weak surface low sliding
through southern Ontario. A cold front will be draped from this
parent surface low southwest along the Appalachian Mountains.
Closer to home...a weak upper/surface low off the Carolina coastline
will migrate northward with no significant impacts expected. There
remains some uncertainty regarding the amount of ongoing convection
at forecast initialization. The usually more aggressive/unstable
NAM indicates nearly 1000j/kg SBCAPE across the warm sector with
convection spread out across the high terrain and NC
Piedmont...likely tied to the prefrontal surface trough as a focusing
mechanism. The GFS/European model (ecmwf) favor a more stable solution with little
to no prefrontal trough focused activity. Although shear will be
increasing...the best upper jet dynamics will pass by to the
north...therefore decided to lean toward the more subtle solutions
of the GFS/ECMWF. The front will sweep through the region during
the overnight hours with overall convective coverage expected to
dissipate with loss of heating. The front looks to clear the
forecast area by not later than noon on Monday before any diurnal heating
induced instability can play a factor.

Behind the front on Monday afternoon...upper trough axis and
associated vorticity maxima will slide through the southern/central
Apps. Moderate to strong northwest flow through the profile will
lead to the possibility of a few upslope induced showers along The
Spine of the Apps. Thus the forecast will feature chance level probability of precipitation
across the high terrain Sunday night tapering to slight chance probability of precipitation
across the remainder of the forecast area through Monday morning to
account for passage of the front and weak northwest upslope flow
showers along the Tennessee line. The above mentioned upper features will
slide east late in the day making way for rising heights and
Canadian high pressure at the surface that will last through the
period. This will yield more fall like conditions with temperatures
generally 8-10 degrees below normal amongst drier northerly flow.

&&

Long term /Tuesday night through Saturday/...
as of 2 PM Saturday...the medium range forecast picks up at 00z on
Wednesday with heights steadily rising as a broad and steep upper
ridge spreads across the region. This impressive ridge is expected
to remain in place through new day 7 over the eastern half of the Continental U.S.
While another steep upper trough digs down over the West Coast by
the end of the medium range.

At the surface...strong Canadian high pressure will be centered to our
north with cool and dry air over the forecast area. Over the next couple
of days...the long range guidance suggests that this high will
gradually drift eastward with some deeper layer moisture encroaching
upon the County warning forecast area from the south and east. The models differ with
regards to how far northwest this deeper moisture will expand. The European model (ecmwf)
continues to show greater coverage over the County warning forecast area on Friday and most of
Sat while the newer run of the GFS keeps the moisture plume more to
the south on Friday and then expands it farther north on Sat.
Interestingly...the latest 12z run of the CMC model keeps the deeper
moisture even farther south than the GFS through its forecast ending time
of 12z Friday. Based on the mixed model spread towards the end of the
forecast period...I did not make any major changes to the sensible forecast.
I gradually increased probability of precipitation to slight chance during the latter part
of Thursday and keep them there for Friday. I did include an area of solid
chance values over the far southern zones for Sat afternoon into Sat
evening...otherwise most other areas still only carry slight chance.
Temperatures were changed little with values remaining a few degrees below
climatology for late sept.

&&

Aviation /00z Sunday through Thursday/...
at kclt and all but kavl...VFR. Cirrus exhaust on the western periphery
of the coastal low will keep high clouds over the Piedmont. Kept
wind generally light north through Sunday morning. Convective cumulus returns
with heating Sunday...but coverage looks to be scattered. On Sunday
afternoon...the wind should veer around to SW in response to the
approaching cold front. The NAM develops some precipitation east of the mountains
late in the day but at this point the coverage seems isolated at
best...as the strong west flow at 850 mb is not conducive.

At kavl...all parameters show a good night for valley fog. Have
continued the plan of IFR/LIFR around daybreak to indicate
this...but vlifr is still a possibility. Fog should burn off quickly
after daybreak with cumulus developing again with heating. The
approaching front may push an area of showers into the mountains from the
west late in the day. Included a vcsh starting at the time a broken VFR
ceiling is expected to form...but showers have a better chance of
happening more toward 22z. Wind should be northwest on Sunday.

Outlook..a cold front will bring a chance for restrictions mainly
over the mountains Sunday night and Monday morning. High pressure builds
back in early next week with VFR conditions.

Confidence table...

00-06z 06-12z 12-18z 18-24z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% medium 70% medium 71% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% 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)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...jpt
near term...PM/rwh
short term...cdg
long term...jpt
aviation...PM

National Weather Service Glossary of Abbreviations