Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
958 PM EDT Tuesday Sep 2 2014
a weak cold front will push through the region tonight followed by
Bermuda high pressure Wednesday into Friday. Another cold front will
sweep through the region over the weekend providing an additional
focus for thunderstorms on Saturday and Sunday. Building high
pressure will highlight the weather pattern for the start of the
next work week leading to cooler temperatures and more stable
Near term /through Wednesday/...
as of 945 PM EDT...outflows from the midlands and Piedmont
convection have moved west to trigger additional thunderstorms in the residual
evening instability across the forecast area. This has been
occurring in tandem with a convective line moving southeast into the western
mountains ahead of an approaching upper shortwave and a weak surface front.
It remains quite possible that these outflow systems will meet in
the middle near the Blue Ridge to trigger one last line of robust
thunderstorms late this evening/early overnight. However...surface based
instability is slowly waning on the laps analysis...and infrared imagery
and lightning strike rates are subsiding with the activity arriving
from the northwest. The overall trend should thus be a gradual diminution
of coverage and intensity of the thunderstorms. However...will still need to
carry at least isolated to low end scattered probability of precipitation across the area overnight
given the upper shortwave moving ovhd. Expect continued mild mins in
the soupy airmass with debris clouds around all night...and another
round of Mountain Valley fog.
On Wednesday...the models agree with stalling the remnants of the
upper wave over the Piedmont to our south and east...yet still
develop convection in the afternoon because of ample moisture at low
levels. This seems like a decent bet over the mountains and along the
eastern border close to the wave remnants...but there is uncertainty
in between these two areas. The guidance paints a slight chance pop
across the entire area while the ongoing forecast has a chance. Will
keep the chance everywhere based on the latest sref for the time
being. Temperatures should be a few degrees cooler based on increased cloud
Short term /Wednesday night through Friday/...
as of 230 PM Tuesday...on Thursday...middle level ridge will remain
centered across the Carolinas and western Atlantic. Short range
models indicate that moisture will lift north across the region
during the daylight hours on Thursday. Moisture is likely increasing
south of weak warm front lifting north across the Carolinas.
Forecast soundings indicate that precipitable water will increase to 1.75
inches...with convective available potential energy around 1500-2000 j/kg. This environment should
primarily support air mass thunderstorms and rain...forming over the mountains during the
early afternoon...expanding east through the remainder of the
afternoon. Pulse severe thunderstorms and rain appear possible across the County Warning Area. On
Friday...conditions will change little from Thursday. Forecast
soundings indicate the precipitable water will increase to 1.8 inches with broad
low level southeast flow. Weak upslope flow will favor the higher elevations for
thunderstorms and rain. Both days...I will forecast 30 probability of precipitation across the Piedmont to
40-50 across the mountains Thursday and Friday...high temperatures are
forecast to range near normal.
Long term /Friday night through Tuesday/...
as of 200 PM EDT Tuesday...latest global models continue to agree
that an upper trough will progress eastward across the Great Lakes
region Sat and into the eastern Seaboard by sun. This will push a
trailing cold front into the southern Appalachians by late Sat.
With increasing Atlantic moisture advection in southeasterly flow along with
diurnal buoyancy...probability of precipitation ramp up into the solid chance range over the
Piedmont and into the likely range over the NC mountains Sat afternoon.
The front will then considerably slows down its Seward progression
as it reaches the Tennessee/NC border line. Both the GFS and the European model (ecmwf)
indicate that the frontal boundary will become stationary over the
Piedmont by early sun before it eventually pushes south of the area
by Monday. This will yield widespread convection ongoing Sunday into
early Monday and thus have mentioned solid to likely probability of precipitation across much
of the region during this period. Given the moist profile (pw values
near 2")...good low level convergence and moderate instability...heavy
rainfall looks to be the main concern att. Monday and Tuesday...probability of precipitation ramp
down as the front remains south of the area and a wedge of surface
high builds in from the NE. Temperatures will stay near normal
Friday/Sat before cooling down considerably by Monday.
Aviation /02z Wednesday through Sunday/...
at kclt...isolated thunderstorms continue to dance around the kclt vicinity on
residual evening outflows...but have missed the airfield thus far.
Additional foothill/Piedmont coverage to the west will have to be
monitored late this evening...and outflows from the mountain convection
could approach overnight. None of this poses any imminent
threat...so the taf will remain free of convection for the moment.
Will feature just debris ceilings overnight with no restrictions. Winds
may toggle briefly northwest on any outflows...but expect mainly west-southwest to south-southwest
surface winds with mixing on Wednesday...building cumulus...continued high
level ceilings...and then scattered late day thunderstorm coverage once again.
Elsewhere...thunderstorms have backbuilt toward kand late this evening on
the strength of Piedmont outflows...but convective chances at the
other foothill sites are still fairly remote through midnight. A
more solid line of rain showers and thunderstorms and rain over the western mountains is expected
to weaken as it moves east to the Blue Ridge...but confidence is
high enough to warrant a tempo for thunderstorms and rain at kavl. Will mention
nothing more than vcsh at khky for now...but scattered thunderstorms may survive
the trip to threaten the foothill sites into the early morning
hours. Otherwise...debris clouds should inhibit low clouds and fog
at most locations...but brief restrictions will be possible anywhere
rain falls tonight. Kavl remains hard to pin down...but soundings
and MOS have good consensus on IFR ceilings and visibility toward
daybreak...so will go ahead and include that despite inconsistent
performance the past few mornings. Winds may go northwest throughout as any
boundaries slip through...but prevailing SW winds are expected at
the foothill sites on Wednesday. Will feature prob30 throughout for
afternoon/evening thunderstorms triggering in the unstable airmass again on Wednesday.
Outlook...generally VFR conditions are expected through the rest of
the week...but low cloud and fog restrictions are likely early each
morning in the mountain valleys. Diurnally driven thunderstorms and rain are expected
each afternoon and early evening...with the most coverage over the
mountains. Thunderstorm coverage may increase this weekend as a cold
02-08z 08-14z 14-20z 20-02z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 94% high 100% high 100%
kavl high 81% medium 65% high 81% high 100%
khky high 94% high 83% high 100% high 100%
kgmu high 100% high 91% 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)