Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
645 am EST Wednesday Dec 18 2013
high pressure will remain over the Carolinas through Thursday...then
move off the Carolina coast. Expect a cold front to approach our
region from the Mississippi and Ohio Valley Friday and Saturday...
crossing from the northwest Sunday and reaching the Carolina coast
Monday. Ahead of the front...temperatures will be much above normal.
Near term /through tonight/...
630 am update...few changes. Revised sustained winds slightly for
the rest of the morning. Gusts of 20-30 knots still possible this
morning though mesonet sites suggest gradient has already weakened
enough over our mountains to prevent gusts from getting into advisory range.
Boone and West Jefferson...steadily gusting 35-45 knots over the past
few hours...thus are not believed to be particularly representative of
gusts this afar south. Gradient will continue to weaken so kept
gusts as-is in the grids.
Today and tonight...deep trough moves off the middle Atlantic coast and
is replaced by short upper ridge with closed surface high. Deep relative humidity is
very low and upper flow largely free of disturbances...so virtually
clear skies are expected through the period.
Even with trough departing middle-upper height gradient is not shown to
relax appreciably until midday today. Winds in the higher terrain
are likely to keep gusting into early afternoon. The Piedmont is not
expected to see significant gusts. Surface high center in place to allow
calming late in the day which should generally persist tonight.
Highs near normal given that winds diminish toward peak heating. Relative humidity
values will be low in the Continental airmass...dropping below 25
percent across mainly the smokies and the upper Savannah valley. Fuel
moistures differ depending on the data source...but are likely
around 8 percent in NE Georgia. In light of that...will issue a fire
danger stmt for our Georgia counties which meet the 25 percent relative humidity criterion.
Low temperatures tonight should be near or slightly below normal with fairly
low dewpoints continuing...and winds becoming very light to calm.
Short term /Thursday through Friday night/...
as of 215 am Tuesday...a flat upper ridge moves east across the area Thursday
as surface high pressure slides offshore. The resulting mostly sunny
skies...southwesterly flow and increased thicknesses will lead to highs 5 to
10 degrees above normal.
The upper ridge axis moves offshore through the end of the period
allowing a deep layer SW flow to set up over the southeastern Continental U.S.. clouds
will increase through the period in this flow. A frontal system will
approach the area late Friday but stalls to the northwest Friday night. Low
level moisture increases across the western County warning forecast area leading to increasing
chances of scattered rain showers...mainly across the mountains and southern Escarpment late
Friday and Friday night. Highs Friday will be around 10 degrees above normal.
Lows Thursday night will be near normal mountains and 5 to 10 degrees
above normal elsewhere. Lows warm to 15 to 20 degrees above normal
Long term /Saturday through Tuesday/...
as of 230 am EST Wednesday...the medium range forecast picks up at 12z
on Saturday with a deep 500mb trough moving east across northern Mexico
with developing surface low over Texas. 20kt 925mb southerly flow
will produce upslope showers over the NE Georgia mountains to near Lake Lure
on Saturday. This flow will increase Saturday night as speed of
925mb flow GOES to 35 to 40kts. Shower activity should become more
widespread as the main lobe of precipitation approaches from the west. The
925mb flow will increasingly become parallel to the mountains as it will
be from the SW at 45 to 50kts by Sunday afternoon. The 850mb flow
Sunday PM will be from SW at 55 to 65kts and 700mb flow 65 to 70kts.
At the same time Sunday afternoon on the GFS...cape values increase
to 200 to 500. If the GFS timing is correct...this will be crossing
at the time of maximum diurnal heating for NE Georgia and NC SW mountains
the axis of instability would reach the I-77 corridor toward
midnight according to the latest run of the GFS. The GFS and European model (ecmwf)
are a bit closer in timing of the evolution of this system with the
GFS still a bit faster by only a few hours. Severe weather is
certainly possible Sunday afternoon and evening with convection
bringing the stronger wind just above the ground down to the
surface. Also...some parts of the southern mountains from Clayton to Brevard
could get over an inch of rain from this event.
The front moves to the Carolina coast Monday afternoon with drying
high pressure crossing the Tennessee Valley continuing into Tuesday.
Temperatures will be at least 15 degrees above normal Saturday and
Sunday and by Tuesday will be dipping a bit below normal. It appears
the cool temperatures will continue into late next week as the GFS
has a big trough over east and big ridge over West Coast. The European model (ecmwf)
is different but has general idea of a trough only with large cut
off low over SW part of nation.
Aviation /12z Wednesday through Sunday/...
at kclt...VFR with sky clear likely through the period...and too dry for fog
formation. Light northwest winds this morning. As high pressure center
approaches from the west...winds back to SW quadrant by middle afternoon.
However they should weaken as gradient diminishes over the course of
Elsewhere...virtually no cloud cover to speak of. Brisk gusts will
continue in the mountains through midday when they finally should diminish
with calm high pressure starting to build in. Fairly light northwest
breezes into early afternoon...with foothills/Piedmont winds backing to
SW as they begin to drop off this afternoon. Generally calm tonight with
dry conditions keeping visibility VFR.
Outlook...VFR Thursday with some restrictions possible Thursday
night...mainly over the Piedmont...as moisture returns to the
region. Moisture continues to increase through the weekend...with
restrictions becoming increasingly likely.
11-17z 17-23z 23-05z 05-11z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% 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 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)
a dry Continental airmass will accompany high pressure shifting over
the area today. Relative humidity values will drop below 25 percent for a few hours
early this afternoon across portions of far SW NC...far western SC...and
NE Georgia. Some fuel moisture readings in Georgia are around 8 percent.
Given the low relative humidity a Fire Danger Statement is posted for the counties
in Georgia where the relative humidity is below 25 percent. Winds will remain brisk in
the mountains...with strong gusts of 30-40 miles per hour possible through early
afternoon. The area of stronger winds is not expected to have
significant overlap with the area of low relative humidity. However...exercise
should be used if burning today.