Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 172147

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
547 PM EDT Thu May 17 2018

The threat for numerous showers and a few thunderstorms will linger
for the rest of the week as a slow moving low pressure system
impacts our region. Locally heavy rainfall remains possible each
day. High temperatures will be near normal and low temperatures
about ten degrees above normal.


AS of 545 PM EDT: Though temperatures warmed up a degree or two
warmer than previously anticipated, mainly along and south of the
I-85 corridor, overall, temperatures have maxed out for the
day, with current temperatures in the upper 70s/lower 80s.

Otherwise, the near term forecast remains well on track as radar
imagery is depicting well the expected increased shower activity
with a few isolated thunderstorms across portions of the FA. The
whole area is uncapped (as it`s been most of the day), and
convection should have little trouble continuing to blossom through
the peak of diurnal heating and sundown, waning slowly later in the
evening. Guidance has come down as far as QPF estimates for this
afternoon, though with PWATs observed close to daily records this
morning at GSO, any cell that is able to form could contain heavy
rainfall with rates up to 1-2" per hour. Flow is also generally
unidirectional from the SE, so anchoring or training of cells is
also possible and would increase the flash flood threat locally.
Considering the wettest antecedent conditions exist over the Blue
Ridge Escarpment and that is where convection is initially
concentrating this afternoon, the flash flood watch still looks like
it is well-placed. Will let future shifts overnight determine the
necessity of any extensions (it currently runs through 12Z Friday)
once convection this afternoon and evening play out. There is a weak
surface low attempting to scoot northward from the Gulf coast at the
moment, but the current track would take it closer to Columbia`s
area, keeping any large shield of organized heavy precip to the east
of our forecast area.

Tomorrow will have the best forcing of any day this period, with an
upper low beginning to swing towards the northeast up the spine of
the Appalachians. This will place the CWFA in an area of favorable
QG ascent tomorrow afternoon, along with the ever-present southerly
upslope flow induced by the surface Bermuda high. Moderate
destabilization (currently progged at 1500-2000 J/kg) is likely
again tomorrow afternoon, and convection will likely again be
widespread, perhaps even more widespread than today. 850mb flow will
also be slightly more stout tomorrow inducing marginally better
shear, and despite the continued very wet profiles, isolated strong
storms with gusty winds and small hail appear more likely tomorrow
than they have any day this week. Max temperatures will be near
climo, with min temps still about 10 degrees above average due to
the persistent moisture across the CWFA.


As of 230 PM Thursday: Deep southerly flow remains over the area at
the start of the period with an upper low over the midwest, and its
associated trough over the OH and TN valleys, and a subtropical
anticyclone to our east. However, this flow weakens and turns more
southwesterly Saturday as the low opens up and the trough lifts and
begins moving east to the north of the area. The trough moves east
of the area Sunday with weak short wave ridging moving into the OH
valley, but the center of the anticyclone remains in place, although
maybe suppressed slightly to the south. This keeps the deep
southwesterly flow in place. At the surface, the southerly flow is
also replaced with southwesterly flow as ridging associated with the
Bermuda high builds west across the area. This results in somewhat
of a pattern change. There will still be quite a bit of low level
moisture across the area, but the deep tropical fetch will diminish.
Instability will increase each day with steepening lapse rates, but
PW values fall with drier mid level air. This brings an end to the
widespread heavy rainfall threat, but isolated heavy rainfall
remains possible. The increased instability and drier mid levels
leads to an uptick in surface delta theta-e values, increasing the
potential for strong or isolated severe storms, but DCAPE values
remain modest keeping the threat low. Despite these trends, likely
PoP is still needed for Saturday but only for the mountains on
Sunday. The PoP also takes on more of a diurnal trend with these
changes as well. Highs will be around normal Saturday rising to
around 5 degrees above normal for Sunday. Lows will be 5 to 10
degrees above normal each morning.


As of 245 pm Thursday: The upper pattern for the early part of next
week looks somewhat non-descript as we are stuck in-between an
Atlantic sub-tropical ridge, a weak upper low over the northeast
Gulf of Mexico, and upper ridging over the OH Valley region. Weak
sfc high pressure will be trapped over the region for Monday
and Tuesday, resulting in slightly above normal temps and mainly
diurnal convection. The pattern slowly evolves toward the middle of
the week as a nrn stream short wave moves across the Gt Lakes/OH
Valley/Mid-Atlantic, which may push a sfc boundary down from the
north toward our region. The WPC guidance suggests the boundary
will remain to our north, and going with that scenario would
mean a continuation of mainly diurnal convection given adequate
moisture. The op GFS would drive the boundary down into/across the
western Carolinas during mid-week, which would mean a higher precip
prob and less diurnal variation. The fcst generally goes for the
middle ground, which ends up looking an awful lot like every other
day recently. However, the fcst looks much more interesting out
beyond the end of the medium range. By now some of you are aware of
what the operational GFS and the 00Z ECMWF have cooked up late next
week over the ern Gulf/FL. For the time being, readers are cautioned
to manage their expectations in spite of the general model agreement
on an early-season low pressure system of tropical origin, as a
great deal can and will happen between now and a week from Saturday.


At KCLT and elsewhere: VFR prevailing at TAF sites this afternoon as
this morning`s low cloud decks have generally become scattered.
Convection has blossomed across the area as expected, with much of
the activity focused along the Blue Ridge and NC Piedmont. All sites
have the potential to see brief isolated thunderstorms this
afternoon and evenings which could temporarily reduce cigs/vsby to
MVFR or IFR levels. Overall, however, restrictions through the
evening should be temporary and were mostly handled via tempo.
Guidance is again hitting lowering cigs and vsby hard tomorrow
morning likely due to the very moist airmass and influence from
climo, though if the past 2 mornings are any indication as to how
tomorrow will shape up, widespread IFR cigs and vsby are likely too
pessimistic. Confidence on when any one site will see restrictions
is rather low, though brief drops to at least MVFR cigs with some
IFR potential at CLT and the other terminals is feature in the TAFs.
Convection will again increase late tomorrow morning through the
afternoon with SHRA and TS possible at all sites.

Outlook: Unsettled weather will continue through the end of the
work week, with isolated to scattered SHRA/TSRA at all hours, and
periodic flight restrictions expected. Over the weekend,
precipitation is expected to return to a more typical
afternoon/evening occurrence.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High  94%     High  98%     High  90%     High 100%
KGSP       High  94%     Med   72%     High  89%     High  92%
KAVL       High  89%     High  96%     Med   68%     Med   77%
KHKY       High  98%     Med   76%     High  83%     High  87%
KGMU       High  96%     High  90%     High  81%     High  90%
KAND       High  98%     High  88%     High  92%     High  86%

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:


GA...Flash Flood Watch through Friday morning for GAZ010.
NC...Flash Flood Watch through Friday morning for NCZ033-049-050-
SC...Flash Flood Watch through Friday morning for SCZ001>003.


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