Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171108
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
708 AM EDT Thu Aug 17 2017

.SYNOPSIS...
High pressure will gradually erode over the next few days, though
warmer than normal conditions will remain. Moist and unsettled
weather will return late week and into the weekend. Some drying
is favored by Sunday with a return to moist conditions to start
the new work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 655 AM EDT: Isolated showers east of the I-77 corridor early
this morning have largely dissipated. Patchy fog and low clouds have
developed in the mountain valleys and over the foothills, but this
has been quite variable and will remain so through mid morning. Some
slightly deeper moisture has started arriving in the western NC
mountains, but this is not contributing to any returns on radar thus
far.

The convection-allowing models today still have mountain showers and
thunderstorms developing from 18Z to 20Z this afternoon in the
region of better low level wind field convergence over the high
terrain. This activity should move east off the Blue Ridge around
20Z to 21Z. A secondary axis of low level convergence should impact
piedmont sections east of the forecast area, so piedmont coverage
will depend on the mountain activity surviving eastward. 850 to 500
mb lapse rates appear a slight tick higher this afternoon than
yesterday, with sbCAPE values at 2000+ J/kg, so an isolated severe
storm cannot be ruled out. Precipitable water values remain above
climo, but storms should move along steadily from west to east to
keep any excessive rain/flooding problems isolated in nature. Heat
will be an issue this afternoon with many piedmont temperatures in
the low to mid 90s, with dewpoints in the lower 70s. Have posted an
SPS today for piedmont Heat Index values in excess of 100 degrees at
times.

The shortwave ridge over the region late today will move east
tonight, with heights falling once again from the west overnight.
Precipitation chances will increase along the spine of the southern
Appalachians during the early morning hours as deeper moisture
begins to push back in. Expect continued mild and muggy min temps
early Friday, with mountain valley fog and low stratus redeveloping.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 245 AM EDT Thursday:  The short term fcst period kicks off on
Friday morning amidst a departing shortwave ridge in lee of the
southern Appalachians while a region of height falls associated with
a longwave trof approaches from the west.  At the surface, a cold
front tied to a surface low will extended southward beneath said
upper trof into the OH/TN Valleys as a prefrontal lee trof extends
across northeast GA and the western Carolinas.  Pattern evolution
through the short term will feature the intrusion/passage of the
cold front on Friday afternoon/evening with modest cape in excess of
1.5/2.0k j/kg with shear generally maximized along/ahead of the
front over western NC in the 25kts range at 0-6km.  With that, the
fcst will feature low/mid chance pops along/south of I85 where shear
will be weakest, while favoring mid/high chances with a few areas of
likely pops across the NC high terrain eastward.  Given the
abundance of dry air evident in the profiles, wouldn`t be shocked if
a few storms became severe with damaging winds and large hail being
the primary threats.

Moving into Saturday, the cold front will sweep through remainder of
the fcst area overnight into the mid morning hours as cooler/drier
high pressure spills in from the northwest.  Fortunately given the
drier nature of the airmass present over northeast GA and the
western Carolinas behind the front, conditions look rather stable
with a dry fcst to prevail across the vast majority of the region
with the only mentionable pops for Saturday afternoon being
along/south of I85 in closer proximity to any residual frontal
convection.  Temperatures on Friday will top out a few degrees above
normal with heat indices reaching near the century mark across the
far southern zones.  By Saturday temperatures will top out near
normal with dewpoints being a few degrees cooler, thus keeping any
excessive heat concerns at bay.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 300 AM EDT Thursday:  The medium range fcst period initializes
on Sunday morning amidst the passage of a dry upper trof, with
shortwave ridging building back across the southeast states thanks
to a strengthening H5 anticyclone along the Gulf coast.  At the
surface dry high pressure will be in control once again across
northeast GA and the western Carolinas effectively keeping the fcst
dry to round out the weekend amidst slightly above normal
temperatures.

As for Monday (Eclipse day), conditions do look to be improving for
our region as guidance still favors the resurgence of a stationary
front, however at a slower rate than previous model runs on both the
GFS and ECMWF.  With that, the front slowly surges northward out of
the Low Country and Midlands, yet now not reaching the I85 corridor
until mid/late afternoon, most likely after totality.  In addition,
low level RH fields have improved as well with values ranging in the
45-55% percent range as opposed to nearly 10% higher in days past.
All in all, think viewing conditions are looking better.  That said,
by the late afternoon hours the front is expected to slide into the
region increasing chances for showers and thunderstorms along/south
of I85.  Thus pops on Monday will be mostly dry, with the exception
of slight chances over the higher peaks given ridgetop convection,
as well adjacent to the approaching front south of I85 with some
chance pops highlighted as well.  Tuesday looks to return to more of
a diurnal convective trend as moisture streams into the region from
the south as the front lifts well northward, while another cold
front approaches the region from the west on Wednesday.  Pops both
days will be at or slightly above climo levels regionwide.
Temperatures through midweek look hold at near normal levels.

&&

.AVIATION /11Z THURSDAY THROUGH MONDAY/...
At KCLT: Still cannot rule out some low cloud and fog restrictions
for an hour past sunrise given the boundary layer moisture, but very
little has developed in the piedmont thus far. Anticipate slowly
building cumulus today, with late day chances of convection arriving
from the west after 19Z as thunderstorms build east off the Blue
Ridge. Light winds will become steady SW through the day. Profiles
and MOS have little to no fog/stratus for tonight.

Elsewhere: Restrictions have been quite variable at KAVL and KHKY
this morning, and will likely continue to vary from LIFR to IFR
through 14Z. No morning restrictions are expected at the Upstate TAF
sites. Otherwise, cumulus will build today with mountain showers and
thunderstorms developing 18Z to 20Z, with TEMPO TSRA most likely
from KAVL to KHKY, and PROB30 after 19Z for the Upstate SC TAFs.
Expect light winds to toggle southwest through the day, but remain
less than 10 kt. Any late day thunderstorms should move east this
evening with just lingering debris clouds around. Another round of
mountain valley fog and low stratus are quite possible at KAVL
overnight.

Outlook: The transition to a more typical late summer pattern
continues Friday through early next week. Expect scattered afternoon
convection and morning low stratus and patchy fog each day,
especially in the mountain valleys and in locations seeing
appreciable rainfall.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Low   30%     High 100%     High 100%     High  83%
KHKY       Med   72%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...CDG
NEAR TERM...HG
SHORT TERM...CDG
LONG TERM...CDG
AVIATION...HG



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