FXUS62 KGSP 161746

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
146 PM EDT Thu Aug 16 2018

An upper level trough of low pressure will develop west of the
Appalachians today and persist well into the weekend as a stalled
surface front lingers north of the region. Moisture will increase
ahead of this system to produce a lengthy period of unsettled
weather from Friday onward. A weak cold front will approach slowly
from the northwest early next week and likely arrive in the region
on Wednesday.


As of 140 PM: Regional radars are just beginning to show shower and
thunderstorm development over the lower piedmont this afternoon in
the vicinity of a lingering surface trough and associated weak wind
convergence. A gradient of sbCAPE exists in this area, with values
around 2000 J/kg along the southeast fringe of our piedmont zones.
Anticipate isolated to scattered coverage continuing in these areas
through early evening before dissipating quickly. Meanwhile, the
mountains remain fairly quiet but with an uptick in scattered
coverage expected through early evening as weak vorticity lobes
arrive from the west aloft.

For the larger picture, a low pressure system deepening over the
Midwest late today will carve out height falls and an associated
trough from the lower OH Valley to the mid MS River Valley overnight
through Friday. A series of mid-level vorticity lobes will eject
eastward from this developing trough and cross the southern
Appalachians and surrounding foothill/piedmont areas tonight through
Friday, with the best upper support likely over the mountains Friday
afternoon. Increasing southwesterly flow at low levels will enhance
terrain triggering in the southwest mountains as well. Instability
should be decent on Friday afternoon with good insolation, but mid-
level lapse rates will remain a fairly modest 6 deg C/km. Deep layer
flow picks up a bit, but with bulk shear values in the lowest 6 km
of the atmosphere remaining less than 20 kt. Cannot rule out a few
stronger storms, especially over the mountains Friday afternoon, but
most should be garden-variety. Overnight min temps should be 1 to 2
categories above climo, with Friday afternoon max temps about one
category above.


As of 145 AM Thursday: An active pattern will continue across the FA
thru the short term period. A broad ulvl trof will deepen and slowly
cross the ern CONUS with the mean axis remaining west of the mtns.
This will leave the area in broad lift with continuous pockets of
small-scale dPVA traversing thru the flow. In the low levels...sw/ly
moisture adv will continue and the column will remain quite moist.
The deep moisture will help keep instability elevated until late
afternoon Fri when sbCAPE increases over 1500 J/kg....altho shear
parameters will be low. The wrn zone will be under gun for stronger
storms aided by mech and dyno lift...so will continue the mention
of possible svr activity in the HWO inline with the latest SPC Day 2
products. Saturday has similar looking profiles with relatively more
deep layered shear so will anticipate isol/sct stg/svr convec within
the better large scale lift or wrn half of the FA. Hydro could also
be an issue during this time with continued activity over the same
localized areas esp across the far wrn zones. Max temps will be arnd
or little cooler than climo due to the high cloud cover and
precipitating storms.


As of 200 AM Thursday: Little change was made to the going mid-range
fcst as the guidance continues to show a broad and moist ulvl trof
dominating the pattern early on. Soundings begin highly saturated
Sun and this could be a problem hydro-wise as the overall steering
flow weakens during the day and the previous days will likely have
generated a broad area of precip coverage. The mid-levels dry out a
little Mon into Tue as a h5 ridge traverse the area...but enuf llvl
moist flux will keep an active pattern in the works. Surface based
CAPE and bulk shear are not impressive...so will expect a pulse tstm
mode with a few stg/svr storms possible. Basically a nominal
summertime sensible weather fcst is in store to begin the work week
with max temps remaining a little below normal.


At KCLT and elsewhere: Afternoon convection in the lower piedmont is
still expected to stay southeast of KCLT, while scattered ridge top
convection across the mountains will likely provide little more than
vicinity coverage around KAVL. Expect VFR conditions at the TAF
sites through the period, but KAVL could see another round of brief
mountain valley restrictions around daybreak if the encroaching high
clouds remain thin enough. Low-level flow will remain S to SW, with
the gradient picking up on Friday and some low-end gusts possible
near, or just beyond, the TAF period. Convection chances will ramp
up more strongly in the 18Z to 00Z period Friday afternoon,
especially over the mountains.

Outlook: Increasing coverage of diurnal showers and thunderstorms is
expected through the weekend and into early next week. Restrictions
will be limited mainly to areas in heavy showers and low clouds/fog
across the mountain valleys each morning.

Confidence Table...

17-23Z 23-05Z 05-11Z 11-12Z
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% Med 64%
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:





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