Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KGSP 200734

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
334 AM EDT Thu Apr 20 2017

More seasonable conditions return today in the form of a warm and
moist southerly flow of air, with showers and storms breaking out in
the afternoon. A cold front will make a run at the region on Friday,
leading to still higher chances of showers and storms. The front
is expected to stall over the area this weekend, with unsettled
weather continuing as a result.


As of 300 AM Thu: Sfc high pressure will drift further off the
East Coast this morning, with upper ridge axis just east of the
Appalachians, bringing diffluent upper flow. For these reasons, the
shallow CAD wedge remaining over the area should break down after
daybreak. But until then, the main concern will be patchy dense
fog arising from moisture trapped within it. Vsby obs have proven
somewhat variable so far, with low stratus seemingly hindering
radiation from the fog layer that would be necessary for sites to
"lock in." Nonetheless, obs of 1/2SM or less are contiguous enough
in the eastern Piedmont that a Dense Fog Advisory will be issued
there, with the morning package. Certainly, however, there will
be isolated pockets of very low vsby elsewhere in the CWFA, so a
patchy dense fog mention has been made in the fcst grids.

Otherwise, today we will see southwesterly return flow restore
springlike conditions around the area. A modest amount of buoyancy
should develop by midday over the mtns, leading to scattered
t-storms forming there and advecting generally eastward. Revised
PoPs reflect this trend, but with hi-res models not depicting
much activity near/south of I-85, so chances have been reduced
there. Shear and dry air are lacking so the main concern with the
t-storms looks to be locally heavy rainfall. Max temps should be
two or three categories above normal. Tonight, skies should be
generally mostly clear in the Piedmont; however, as low pressure
moves through the Great Lakes, the system`s warm conveyor belt is
expected to bring increasing midlevel moisture over East TN and
the NC mountains, so clouds are retained there. PoPs also increase
toward dawn but remain below slight-chance until after 12z.


As of 250 AM EDT Thursday: A very active short term in store for the
area, and it will begin with the upper ridge axis off the East
Coast, with WSW flow aloft over us. A fairly stacked and weakening
midlevel low over the Great Lakes and Ontario will have its
attendant front draped down the Ohio Valley toward the Southern
Plains, where it will be in the process of merging with a developing
warm front from a storm system pushing off the Rockies (more on that
later). The cold front will sag down the western slopes of the
Appalachians on Friday as temperatures in the low-level WAA regime
across the Southeast increase in to the mid-80s, only kept in check
by increasing clouds from the incoming moisture. sbCAPE values will
increase as well, with even the GFS sporting over 1000 J/kg across
the NC Piedmont. Convection most likely nearer the proximity of the
front across the mountains, but at least possible just about
everywhere. The front itself is progged to weaken, but enough
dynamics still with it that deep-layer shear values of over 30kt
forecast to exist over the mountains during peak heating. Generally
still expect convection to remain pulse-like, but some multicell
clusters may be possible as well with the limited shear. New SPC
Day2 outlook has introduced Marginal across this area and that seems
well-warranted, with isolated strong to marginally severe storms
certainly not out of the question. Lapse rates not impressively
supportive of hail but should see some weak CAA at 500mb, so
although the primary threat would be wind, some small to marginally
severe hail possible as well.

The front stalls over the area Friday night, and as the Plains
system continues to wind up, downstream ridging will increase,
lifting the front back to the north generally more along the NC/VA
line. This continues the northerly trend we have seen in new
guidance the past couple of days, and will help mitigate the heavy
rain threat with the warm front remaining to our north on Saturday.
Nevertheless, upper diffluence (jet level dynamics) plus increasing
DPVA, and a corresponding increase in deep-layer shear all with the
warm sector solidly over our area will lead to another round of
convection developing Saturday afternoon. CAPEs should increase
quite a bit during the afternoon, and disturbingly both the NAM and
GFS have 1500-2000 J/kg across portions of the western NC Piedmont.
Add to this the proximity of the warm front, along which weak
mesolows east of the slopes will be possible resulting in enhanced
low-level vorticity, and result is not a bit surprising that
suddenly we are seeing SigTor values >1 along and just north of I-
40. The gradient in CAPE will be pretty strong with the warm front
somewhere in the area, and certainly some convective bust potential
depending on where the front ends up, but concern is growing for
potential for severe convective activity on Saturday (in addition to
Sunday). New SPC Day3 outlook has introduced 5% probs across the
area and with the amount of sbCAPE, low-level helicity, at this
point any manner of severe cannot be ruled out, though main threats
would be wind and hail.

So speaking of Sunday, the surface low will works its way generally
up the TN Valley Saturday and Saturday night, and should cross the
Appalachians sometime early Sunday. ECMWF a little faster than the
GFS, NAM a little slower still. What is problematic (well, one
portion of the problem) is a surface high ridging east along the
Great lakes that, under confluent flow over New England, should be
enticed to ridge down the Eastern Seaboard, pushing the warm front
back south toward SC as a wedge front (interesting to watch the
evolution of forecast soundings at KHKY with CAPE/shear on Saturday
transitioning to low-level inversion/wedge by Sunday morning). The
approaching surface low has little wiggle room in this case. Deep-
layer shear becomes more unidirectional on Sunday and once again
instability will increase in the warm sector - wherever the warm
sector may be, depending on the location of the warm/wedge front.
With the front shifting south, best convective potential will be
closer to the NC/SC state line, with some margin of error of the
eventual actual location. Instability will be the limiting factor on
Sunday with isentropic lift ahead of the synoptic lift resulting in
widespread cloud cover and maybe even some drizzly conditions across
the northern NC Piedmont. Timing of the actual cold frontal passage
will be tricky as well, with rapid stabilization ahead of it.
Another round of severe potential expected Sunday, with again all
severe modes possible. Temperatures will be correspondingly

The front pushes through Sunday night, taking the ruckus with it,
with a cooler/drier airmass beginning to push in by the end of the


As of 330 AM EDT Thursday: Lucky for us, the extended is pretty
quiet in the wake of the weekend mess. The upper low dives south
over our area on Monday and off the SC coast Monday night, with high
pressure in its wake. The parent high for this ridge is way up west
of Hudson Bay. The high pressure will remain in control with fairly
seasonal temperatures in place initially, but with a gradual warmup
through the end of the period as the high modifies. The 00z ECMWF
does want to bring a front through Wednesday night (the GFS is about
24h slower), but even on the faster ECWMF the southern edge of the
front dries up over our area. Have depicted a slight increase in
cloud cover but otherwise no sensible weather impacts.


At KCLT and elsewhere: Cold air damming wedge will remain over the
region early this morning, though it is in its final stages and
restrictions will not be as widespread as they typically would be in
CAD. Already, bands of MVFR to IFR stratus and patchy FG has formed,
and vsbys generally are expected to worsen toward daybreak. It
remains to be seen how much fog will be able to form beneath the
existing low stratus; rapid changes have been noted so far as fog
forms but settles out. The consensus of guidance suggests both
will be a problem by daybreak if not in the next hr or two. A lack
of higher cloud decks, plus the continued weakening of the wedge,
should erode the cigs by mid morning. Diurnal destabilization is
expected to be more or less climatological today, with the mtns
seeing isold-sct TSRA break out beginning around midday. Hence
TEMPO at KAVL. These will advect east into the NC Piedmont and
may also affect KHKY in the aftn. Winds generally SW.

Outlook: Diurnally driven TSRA are expected again Friday. Another
backdoor cold front is expected to settle over the fcst area this
weekend, with restrictions and precipitation likely.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       Med   61%     High  86%     High 100%     High 100%
KGSP       Low   44%     Med   78%     High 100%     High 100%
KAVL       Med   74%     High  92%     High 100%     High 100%
KHKY       Low   41%     High  93%     High 100%     High 100%
KGMU       Low   35%     Med   76%     High 100%     High 100%
KAND       Low   52%     Med   63%     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:


NC...Dense Fog Advisory until 10 AM EDT this morning for NCZ036-037-
SC...Dense Fog Advisory until 10 AM EDT this morning for SCZ008-009-


NEAR TERM...Wimberley
AVIATION...Wimberley is the U.S. government's official web portal to all federal, state and local government web resources and services.