Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1040 PM EDT Sat Oct 3 2015
..historic and potentially life threatening rainfall continues...
an upper level low will drift slowly across the southeast United
States through the rest of the weekend. Tropical moisture will
produce widespread heavy to torrential rainfall across the region
through Sunday night. Dry high pressure will build in beginning
Near term /through Sunday/...
as of 1040 PM...radar confirming the guidance trends from earlier
with axis of heavy rain shifting west across the County warning forecast area. That
said... a large area of moderate to heavy rainfall is moving into
the upstate and NE Georgia...and will spread into the western NC mountains
overnight. Expect upslope precipitation along the NC foothills to
reignite as well as this area moves in. Therefore...no changes to
the Flood Watch given the impending heavy rain...and high water
levels where lower amounts are expected. Occasional strong wind
gusts continue across the area so Wind Advisory remains in place
as isolated trees and power lines continue to fall.
Otherwise...little change to going forecast.
As of 750 PM...latest synoptic and mesoscale guidance shows the axis
of heaver rainfall shifting slowly west and south overnight and
remaining to the south on Sunday. Before that occurs...yet another
round of widespread moderate to isolated heavy rainfall will move
across the area this evening. The next round around midnight will be
centered closer to the Savannah River valley. The 18z GFS keeps the
axis of heavy rainfall just east of the Savannah River...so have not
gone as far as the other models with the latest quantitative precipitation forecast values...but did
trend in that direction. That said...will keep the Flood Watch in
place as is due to the high water levels and continued run off
potential in the northern and eastern County warning forecast area where heavy rain is less likely.
Have additional quantitative precipitation forecast of 2 to 5 inches across the western upstate and NE
Georgia...1 to 2 across the NC mountains...and 0.5 to 1.5 inches across the NC
foothills and Piedmont and eastern upstate.
As of 245 PM EDT...rain continues. Should come as no surprise
that creeks/streams/rivers are all rising and more warnings are
being issued. Deep fetch of tropical moisture continues to stream
into the area...forced by the upper diffluence associated with the
deformation zone of the midlevel low...as well as right entrance
region synoptic lift from the strong upper jet streak. The midlevel
low will continue to spin and dig...with the upper jet pivoting
around from north-northwest/south-southeast orientation to northwest/southeast and more west-northwest/east-southeast with time.
The axis of the heavy precipitation will follow this pattern as
well. The good news is that as this happens...all guidance is
shifting the axis of heaviest rain south...more from an Augusta to
Myrtle Beach line across central South Carolina than upstate.
However...given these changes and taking into account run-to-run
continuity /or in this case the lack thereof/...am not willing to
pull back on qpfs too much just yet. Screaming 850mb jet will
continue as well...adding upslope flow/orographic lift to the
picture so along those east facing slopes of both the north and
South Carolina mountains...expect that light to moderate rain will
continue. Landslide threat remains...we have already had a handful
of debris flows in the mountains...and as the winds pick up tonight
with the saturated soils the problem may be exacerbated.
Speaking of the winds...Wind Advisory remains in place through
the near term /and have extended it further into the short
term/. Widespread 15-25mph winds with gusts up to 30-40 expected
this evening and overnight. While technically below advisory
criteria...with saturated soils we are getting scattered trees
coming down across the forecast area so it is prudent to leave
the advisory in place.
Below-normal temperatures continue but a bit of a warming trend
tonight and tomorrow versus last night and today.
Short term /Sunday night through Tuesday/...
at 230 PM EDT Saturday...on Monday morning a strong closed upper low
will be off the Georgia coast...while and upper ridge will be over the
plains...and another closed low will be over S California. By Tuesday
morning the upper low moves NE along the Gulf Stream...to a position
off the southern NC coast...while features upstream slowly progress
and deamplify slightly. On Tuesday night the eastern low fills...the
ridge upstream reaches the eastern plains...and the western upper
low reaches Arizona.
At the surface...Sunday evening features diminished isentropic
upglide...even though robust...moist...easterly upslope flow into
the NC Blue Ridge will be present. As winds back from east to NE on
Monday as the upper low moves up the Gulf Stream...upslope flow
largely terminates...while moisture becomes confined mainly to the
coastal plain. This drying trend over the Carolinas and Georgia continues
through Tuesday...although the models show some lingering moisture
over the southern Appalachians. Temperatures will run below normal
in cool NE flow.
Long term /Tuesday night through Saturday/...
at 230 PM EDT Saturday...on Wednesday morning a low amplitude upper
trough will be along the East Coast...a low amplitude upper ridge
will be over the plains...and a closed upper low will be over Arizona. By
Thursday upper low becomes zonal over the NE USA...with very low
amplitude riding over the southeast...while the upper low upstream reaches
nm. By Friday model solutions diverge...with the European model (ecmwf) closed low
retrogressing into northwest Mexico...and an upper trough amplifying over
the Appalachians...while the GFS low reaches Texas...with ridging
persisting downstream over the southeast. Little progression in these
patterns occurs by Saturday morning.
At the surface...on Wednesday morning high pressure will extend from
the great leaks to the Gulf states and eastern Seaboard. As the high
center progresses to Quebec on Thursday...the ridge becomes wedged
up against the east slopes of the Appalachians. Southerly moist
isentropic upglide over this ridge will be initially be over Florida and
the Gulf Coast...but will spread north over our area by Friday...
continuing into Saturday...while a slow moving cold front stalls and
weakens to our west. Temperatures will run near normal.
Aviation /03z Sunday through Thursday/...
at kclt and elsewhere...widespread rain showers/rain will continue overnight
with the heaviest rain shifting west and south across the area.
Expect IFR and LIFR ceilings to continue through the night and most of the
morning Sunday. Visibility this evening will be highly dependent on where
the heavier rainfall develops. However...expect MVFR visibility after
midnight with IFR kavl. Conditions will slowly improve through the day
Sunday as the heavier rainfall continues to shift south out of the
area. That said...at least MVFR ceilings continue through the end of the
period. The latest model guidance limits low level wind shear to the mountains...so after
coordination with the ztl cwsu...have removed at all locations
except kavl. That said...winds will be gusty through the column with an
increase in speed just above the low level inversion...but
directions will change little. Surface winds will be gusty NE through the
period with northerly wind at kavl.
Outlook...a moist easterly flow will diminish through the week...
bringing drying and an end to rain and restrictions.
02-08z 08-14z 14-20z 20-00z
kclt high 80% medium 77% high 82% high 100%
kgsp medium 72% medium 73% medium 79% high 97%
kavl medium 78% medium 78% high 93% high 91%
khky medium 63% medium 70% high 83% high 100%
kgmu medium 75% medium 76% medium 75% high 97%
kand medium 75% medium 69% high 90% 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)
have already seen widespread 2-4 inches across the area...with
additional totals of 1-5 inches expected through the near term.
These amounts take into account the guidance mentioned above and the
south and west shifting axis of heaviest precipitation. As mentioned
in the early morning discussion...the wet antecedent conditions will
contribute to life threatening and damaging flashing flooding and
river flooding as the heavier rain bands move across the region...
particularly in the western upstate and NE Georgia later tonight
where the bands may become stationary or pivot. The potential for
numerous slope failures and debris flows will increase sharply once
an additional period of heavy rain moves in over the southern
Georgia...Flash Flood Watch through Monday morning for gaz010-017-018-
Wind Advisory until 8 am EDT Monday for gaz010-017-018-026-028-
NC...Flash Flood Watch through Monday morning for ncz033-035>037-
Wind Advisory until 8 am EDT Monday for ncz033-035>037-048>053-
SC...Flash Flood Watch through Monday morning for scz001>014-019.
Wind Advisory until 8 am EDT Monday for scz001>014-019.