Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
954 PM EDT Tuesday may 5 2015

high pressure will remain across the region through the middle of
the week. A weak cold front is expected to drift down from the north
on Thursday. The remainder of the work week and weekend remains
fairly uncertain as a low pressure system develops off the southeast
coast and may move across the forecast area over the weekend and
early next week.


Near term /through Wednesday/...
0200 UTC update...probability of precipitation were adjusted per radar returns...maintaining
the best coverage over the northern NC mountains. Sky cover was
updated from infrared satellite imagery. Winds were updated with a
blend of the latest NAM and adjmav. Dew points were updated form a
model blend...supporting additional areas of fog in the northwest NC
Piedmont toward dawn.

As of 250 PM...the most recent visible satellite images indicated
broken cumulus across the mountains...with several areas of vertical
development across the ridges. The latest mesoscale cape analysis
indicated a broad area of 250 j/kg of SBCAPE...very similar to
yesterday afternoon. However...a significant inversion around 650
mbs has delayed convection...likely beginning over the next hour or
two. In addition...storm motions late this afternoon into this
evening will range from stationary to only 5 kts or so. I will use
schc to chance probability of precipitation across the mountains...with near zero probability of precipitation east.
Tonight...convective clouds should dissipate during the late evening
hours. Sky cover should become mostly clear late tonight with calm
winds. Patchy fog may develop during the pre dawn hours...especially
within the mountain valleys. Low temperatures are forecast to range from
the low 50s across the mountains to the middle 50s across NE Georgia and the
upstate. Wednesday...the center of a sub tropical low will lift
north along the Atlantic coast of Florida. As the low approaches...low level
flow is forecast to turn from the NE during the morning and from the
east during the afternoon. As a result...surface dewpoints are expected
to lower 2 to 3 degrees from today/S values. Forecast soundings
indicate the convective available potential energy will range lower than today...with sbcin
lingering through middle day. Models indicate that ridge top convection
may occur during the late afternoon...primarily over the Tennessee border
counties. Steering flow should result in a west drift...carrying any
convection toward east Tennessee. I will limit probability of precipitation to a schc along the Tennessee
border. High temperatures are forecast to range 1 to 2 degrees above
today/S highs.


Short term /Wednesday night through Saturday/...
as of 245 PM Tuesday...high pressure remains in control of the area
as we begin the short term...with an elongated ridge in place
aloft. we have been advertising the last couple of
days...should begin to see development of potential subtropical low
over the Bahamas...creating a weakness in the pattern aloft over the
southeast as the low approaches the South Atlantic bight. Synoptic
models starting to come into slightly better agreement with the
progression of the approaching low. In the meantime...expect
continued afternoon shower and thunderstorm activity mainly over the
mountains with highs just slightly above seasonal averages.

By Friday afternoon...synoptic models continue to vary on location
of the coastal low. NAM has it coming onshore near Savannah...GFS
farther north closer to Wilmington and European model (ecmwf)/sref still offshore by
this time. Have leaned slightly toward the GFS as a compromise
solution. Decent frontogenetical forcing pushes inland over the
Carolinas and sets up over the Lee of the Appalachians...and this is
combined with moisture transport vectors converging across the
Piedmont. Have trended toward a slight increase in probability of precipitation and of
course sky for eastern zones late in the short term...with an other
maximum over the mountains as has been the going trend...but expect
most of any impacts associated with this low to be in the extended.
Highs Friday will be highly dependent upon how far inland the low
pushes...if at all...with slightly cooler temperatures for portions
of the North Carolina Piedmont and warmer for western portions of
the upstate as well as northeast Georgia.


Long term /Saturday night through Tuesday/...
as of 315 PM Tuesday...uncertainty associated with the coastal low
continues to dominate the medium range/extended. As mentioned
above...GFS does bring the low onshore in northeast South Carolina
and subsequently weakens it...whereas the European model (ecmwf) continues to have it
spin offshore...moving inland over eastern North Carolina early
Monday. As might be expected...quantitative precipitation forecast fields with the GFS bring more
precipitation to our area than do those on the European model (ecmwf). Continued with
a middle-of-the-Road approach to probability of precipitation...with best chances across
eastern zones and over the mountains. Very diurnal trend to cape
values potentially enhanced by increasing low-level moisture
associated with the surface low...for example Monday afternoon and
evening GFS SBCAPE values increase to between 1500-2000 j/kg but
back down to 0 overnight. Other concern would be the increase in low-
level shear associated with the low. Not really much in the way of
deep layer shear to work with but with increase convective available potential energy...frictional low-level shear inherently associated with
tropical or subtropical systems...might have to keep an eye on
forecast progression for severe storm potential especially across
eastern zones...but definitely too early to pin much down just yet.

While the potential subtropical low continues to spin near or just
off the Carolina coast...a deep upper trough pushes off The Rockies
late in the weekend. While the amplitude of the trough decreases
toward the end of the period...GFS and European model (ecmwf) are actually in
remarkable agreement /for that far out anyway/ on the general
progression of the mass fields...taking a closed low across the
Great Lakes and sweeping the attendant cold front across the eastern
portion of the country. European model (ecmwf) is just a tad faster with bringing the
front into and through the southeast and thus taking the coastal low
back out to sea...but both definitely have a drying trend just
beyond the end of the period.


Aviation /02z Wednesday through Sunday/...
at kclt...VFR. Guidance does not support ceiling or visibility restrictions...
as the surface levels remain somewhat do the lower levels.
S winds will back to the east-northeast on Wednesday as a surface low moves
north from the Bahamas. Gusts pick up a bit Wednesday afternoon.

Elsewhere...VFR. Low VFR ceilings west of kavl this evening should
erode...but stand a good chance of returning with heating Wednesday
afternoon. Guidance favors fog in some parts of the mountains and
foothills...but not at kavl or khky. Southeast foothills winds will back NE
overnight as a surface low moves north across the Bahamas...while
kavl backs from S to north. On Wednesday afternoon winds veer to east-northeast at
SC sites...and southeast NC sites. Precipitation chances on Wednesday will
be best near the Tennessee border northwest of kavl.

Outlook...isolated to scattered diurnal mountain convection is
expected through moist flow persists. Low clouds and
or fog restrictions will be possible each morning where prior
afternoon precipitation occurred.

Confidence table...

01-07z 07-13z 13-19z 19-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 93% high 100% 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)



Gsp watches/warnings/advisories...


near term...jat/Ned
short term...tdp
long term...tdp

National Weather Service Glossary of Abbreviations