Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 250741
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
341 AM EDT Fri Sep 25 2020

.SYNOPSIS...
Widespread rain is expected today as the remnants of tropical system
Beta move across the area. A mainly dry weekend is expected before
unsettled weather returns with a cold front next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM: The center of post-tropical storm Beta will track
roughly along a wedge/warm front extending e-w just north of I-20
attm today. As the low approaches, the wedge front is expected
to lift north into the southeastern portions of the FA by late
morning. This will bring a moderately unstable warm sector air mass
into the area for this aftn. The front may get all the way up to
the I-85 corridor by late aftn with instability of 1000-1500 J/kg of
sbCAPE with bulk shear of 35-45 kt. Enhanced storm relative helicity
is also possible near the front, with sfc winds out of the SE, and
a SWLY 850 mb flow around 25 kts. Strong DPVA around the 500 mb
level will be passing the area right around peak heating, so any
convection that gets going with have some good upper support. The
latest CAMs show simulated reflectivity increasing along a cold
front and atop an eroding in-situ wedge, as it crosses the eastern
third of the forecast area. The 00z HREF even shows a few weak
updraft helicity swaths across the I-77 corridor late aftn thru
late evening. So the new Day 1 SPC convective outlook with a 2%
tornado area looks well placed. This is in addition to a 5% severe
wind threat. As for excessive rainfall, the heavy rain threat
will shift east this morning with the best overall forcing. The
expected next round of convection will have a decent storm motion
with it. That said, a marginal risk will linger thru the day,
especially across the eastern half of the forecast area. High
temps will range from the mid 60s in the northern NC foothills
(where the wedge will last longest), to the upper 70s in the
southern SC Piedmont and Upper Savannah Valley.

Tonight, precip should taper off to just an isolated shower
chances, as the remnants of Beta shift east. A residual wedge of
high pressure will nose back in under drying aloft. This may result
in areas of fog and stratus developing overnight. But confidence
in dense fog is still low. Low temps will be mainly in the mid
50s to lower 60s in the mountains and mid 60s in the Piedmont,
which is 8-10 deg above normal.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 130 AM Fri: Although the offshore Bermuda High will remain a
prominent feature for the short term, shortwave upper ridging will
bring calm/dry weather back to the CWA for Saturday. Subsidence
should inhibit deep convection and weak low-level winds suggest
little to no mechanical lift or moisture return at that time. Maxes
will be a hair above normal.  Midlevel height falls will occur
Saturday night into Sunday as the ridge shifts east and the cutoff
low initially over the lower MS Valley drifts north, opening up as it
is picked up by the deep trough digging into the central CONUS. This
should result in less subsident profiles by Sunday afternoon as the
open wave moves into the Carolinas; in conjunction with better
defined southerly low-level flow and some degree of moisture flux, we
will reintroduce PoPs across the majority of the CWA, with the best
chances along the Blue Ridge Escarpment. Max temps will trend
slightly further upward. The axis of the shortwave likely will not
clear the area until late evening at the earliest, and continued
low-level moistening may keep isolated activity going thru daybreak
Monday, so the small PoP will persist overnight. Min temps will run
well above normal both nights.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 300 AM Fri: A deep trough will swing across the Mississippi
Valley in the early part of the medium range. There continues to
be considerable spread between the major global models, and their
ensemble members, in timing the eastward progression of this trough
and its attendant cold front. Confidence is below average for the
period, overall.

The GFS has been clearly trending toward a solution suggested by
the ECMWF and Canadian GDPS, in which a wave/low separates from the
base of the trough near the Gulf Coast, delaying the cold fropa for
our CWA and potentially drawing out the transition to a cooler and
drier continental airmass. The new 25/00z ECMWF has doubled down
on that concept, so to speak, now cutting off the low deeper and
further west than previously shown, suggesting we experience a cold
front stalling and reversing before the low blows through.  It does
look a little more likely than before that Monday and Monday night
will be active, in the moist southerly flow ahead of the trough,
but beyond that the solutions remain divergent. Seeing the new
ECMWF and how the midweek drying could be delayed from our earlier
expectations even if the cutoff low is not as slow as the EC now
suggests, we have raised PoPs a bit Tue night and Wed, and bumped
up temps as well, to hint at that possibility. For now we will
continue to advertise dry and more fall-like conditions Thursday.

On almost all the available guidance, model depictions of CAPE and
0-3km shear ahead of the front are sufficient that at any prefrontal
line of storms may pose at least an isolated risk of damaging wind;
locally heavy rain certainly can`t be ruled out due to PWAT values
possibly exceeding 1.5 inches.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: All sites expected to go down to IFR or
LIFR thru daybreak 11-12z this morning, as widespread light to
moderate rain crosses the area, tapering off to scattered showers
by midday 15-16z. A weak wedge will make for slow improvment on
cigs, but is expected to erode from the s/se in the aftn, bringing
winds from NE to SE and lifting cigs to MVFR or even VFR. There
will be a chance for some TSRA, but confidence is still low, with
guidance generally developing activity just south and east of the
TAF sites. Highest chances will be KCLT and to a lesser extent,
KGSP/KGMU. Will continue the PROB30 for those sites. Winds become
light and the low-levels will be very moist, so decent confidence
on lowering cigs and possibly vsby after sunset tonight.

Outlook: Brief drying is expected over the weekend before a cold
front brings another round of unsettled weather early in the new
work week.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  86%     High  85%     Med   68%     High  84%
KGSP       High  92%     High  90%     High  92%     High  88%
KAVL       High  91%     High  84%     High  90%     High  92%
KHKY       High  86%     High  88%     High  85%     High 100%
KGMU       High  93%     High  85%     High  91%     High  93%
KAND       High  95%     High  84%     Med   77%     Med   78%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...ARK
NEAR TERM...ARK
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...ARK


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