Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200252

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1052 PM EDT Sat May 19 2018

A slow moving low pressure system will move away from our
forecast area Sunday, however the atmosphere will remain very moist
for the next few days and keep a rather high chance of showers and
storms each afternoon and evening.


As of 1050 PM EDT: Only minor tweaks needed for this update as the
overall near term forecast remains on track. A Flash Flood Watch
remains in effect for portions of the area through midnight.

Latest radar imagery continues to depict isolated showers and
thunderstorms moving across the FA this evening, and while they do
not look overly impressive structure-wise, their packing quite a
punch, bringing heavy downpours to areas across the NC mountains
which continue to have flooding concerns. Overnight, am still
expecting  the coverage of shower/thunderstorm activity to become
minimal, though latest trends seem to be possibly lining up with
recent HRRR run, showing increasing activity towards the I-77
corridor before activity wanes. Aside from this activity, expect
lower cloud cover increasing overnight with low temperatures dipping
into the 60s.

Otherwise, an upper trough will continue to lift north and deamplify
as its axis crosses the CWFA this evening. This is a result of the
large subtropical ridge over the North Atlantic ridging westward
into the Southeast U.S. for Sunday. The low-level and mid-level flow
will briefly turn out of the NW in the wake of the trough, then
become very light. Not expecting much change in air mass, but
guidance generally agrees on slightly drier air and less convective
coverage than last couple days. Very weak shear and SBCAPE of 1500-
2000 J/kg should support pulse convection with a non-zero severe
threat tomorrow. Highs will climb into the 70s mountains and mid 80s
piedmont on Sunday under partly to mostly cloudy skies.


As of 215 PM EDT Saturday: Subtropical anticyclone remains remains
anchored off the southeast coast through the period, but it does
weaken slightly and slowly drift southward. A weak upper low starts
the period along the Florida panhandle then opens and slowly drifts
northward into the southern Appalachians through the period. This
keeps a south to southwest low level flow across the area. The
guidance shows instability each day, but weak overall as some low to
mid level warming keeps CAPE values on the low side. The guidance
has also trended away from the drying mid levels keeping surface
delta theta-e and DCAPE values in check. The pattern still looks to
be mainly diurnal in nature, but severe chances have diminished.
Heavy rainfall is possible with any thunderstorm and FFG values will
be low so the flood threat will continue, but hopefully not as
widespread as in previous days. Highs a few degrees above normal
Monday fall to near normal for Tuesday. Lows around 10 degrees above
normal drop a couple of degrees.


As of 235 PM EDT Saturday: Upper ridging builds in from the west
through the period and merges with the subtropical anticyclone off
the southeastern coast. At the surface, high pressure remains over
the area but does become suppressed as a front drops toward the area
and stalls. This keeps a warm and moist airmass over the area
through the period. However, with the upper ridging in place,
instability is relatively weak due to warm mid level temps. Still,
expect mainly diurnal convection through the period. Severe chances
look to be on the low side with the weak instability, shear, and
little in the way of DCAPE. PW values are relatively high, so
isolated heavy rainfall is possible. The wet antecedent conditions
will keep a flood threat in place through the period. Lows remain
nearly steady around 10 degrees above normal while highs remain
nearly steady around 5 to 10 degrees above normal.

The wild card for this period is the potential tropical or sub-
tropical system that develops over or near the SE CONUS by the end
of the period. Guidance has been insistent that something will form
and be near the SE CONUS or our area by Saturday. Of course, it is
way too early to know if this will directly impact our CWFA.
However, given the rainfall our area has recently received, this
type of system could cause significant problems if it directly
impacted our weather. For now, worry more about the near and short
term convective threats, but keep an eye on the forecast for next


At KCLT and elsewhere: Mainly low-end VFR cigs prevail this evening,
as isolated showers and thunderstorms continue to move across the
area, which could still potentially create a brief period of
MVFR cigs/visibilities to any of the terminals over the next few
hours, especially in heavier downpours. Overnight, as flow
veers to the west, expect stratus to develop, bringing the
potential for IFR cigs and patchy MVFR fog as well. After
daybreak, expect fog to quickly dissipate with a gradual
improvement of cigs to VFR. Isolated showers and thunderstorms
will be possible once again, though coverage is expected to be
less than the trend over past few days. Aside from varying wind
direction with any convection or outflow boundaries, expect SSW
5 to 10 kts through the valid TAF period.

Outlook: An unsettled weather pattern will continue into early next
week with isolated to scattered SHRA/TSRA and possible flight
restrictions each day.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       High  87%     High  81%     Med   70%     High 100%
KGSP       High  89%     Low   51%     High  94%     High  85%
KAVL       Med   67%     High  85%     High  96%     High 100%
KHKY       High  89%     Med   66%     High  88%     High 100%
KGMU       High  96%     Med   66%     High  94%     High  85%
KAND       High  98%     Med   66%     High  90%     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...Flash Flood Watch until midnight EDT tonight for NCZ033-049-
SC...Flash Flood Watch until midnight EDT tonight for SCZ002-003.


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