Area Forecast Discussion
Issued by NWS Greer, SC

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161
FXUS62 KGSP 261512
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1112 AM EDT Sat May 26 2018

.SYNOPSIS...
Very moist conditions will persist over the region through the
weekend as Alberto develops across the Gulf of Mexico. Tropical
moisture will likely affect our area through much of next week,
bringing the potential for continued heavy rainfall and associated
flooding to the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1050 am: One small area of moderate to heavy showers
(responsible for producing locally heavy rain across the Upstate
earlier this morning) is dissipating across the northwest NC
Piedmont, while another area has been gradually building into the
northern NC Piedmont. These will be the two areas that see the
highest pop over the next 1-3 hours, while the remainder of the
forecast area will only see 20-30 pops during that time. In terms of
the redevelopment potential later today, instability is limited by
morning cloud cover and poor mid-level lapse rates in semi-tropical
environment. Meanwhile, PWATs per the 12Z regional RAOBs are around
1.5": certainly juicy, but not a slam dunk to yield widespread
showers and storms this afternoon in light of modest-at-best
instability and forcing. In fact, guidance is in relatively good
agreement in building in weak subsidence and lower PWAT air from the
south in association with a mid-level short wave ridge. As such,
will continue to highlight the high terrain with likely pops this
afternoon and evening, with mainly 40-50 east of the Blue Ridge.
Halfway decent mid-level flow will allow cells and clusters of
convection to move toward the NE at around 15 kts, so excessive
rainfall concerns will remain localized and by and large limited to
anchoring along terrain features, cell training, and/or locations
with poor antecedent conditions due to heavy rain that has fallen in
the past 24-ish hours.

850 mb flow will stay mainly southwesterly through the evening
before starting to back to light southeast in southern sections
early Sunday morning. We should see a repeat of this morning, with
very mild minimum temperatures and showers continuing, especially in
and near the mountains, but with enough steering flow to continue to
keep any hydro issues quite isolated. This will continue to saturate
soils, however, and likely set the stage for later hydro issues.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 300 AM EDT Saturday: With sfc high pressure shifting into the
OH Valley and a (slightly weakening) subtropical ridge persisting
with it`s westward extension into the Carolinas, focus remains on
Subtropical Storm Alberto and it`s affect it`ll have on our FA over
the next several days. Unfortunately, discrepancy continues with the
latest 00Z runs of the GFS and ECMWF. The GFS places the center of
Alberto just south of the AL coastline Sunday morning, while the
ECMWF is just about 24 hours slower with the progression, bringing
it just south of the AL coastline Monday morning and not moving
onshore until Monday night. The timing of the heaviest rainfall
across our FA will absolutely rely on the Alberto`s progression,
thus do note the general timing mentioned below is not set in stone.

Noticeable mid-level dry air in place which allowed for a brief lull
in shower/thunderstorm activity coming into the short term forecast
period will (per the GFS) quickly be dismissed into Sunday afternoon
as a surge of deep tropical moisture makes it`s way northward
through portions of the Southeast and into the FA (not until Monday
morning per the ECMWF), re-saturating the atmospheric column. As has
been discussed the past few days, model guidance continues to
support PWATs ranging between 1.5 to 2.0 inches late Sunday into
Monday (again, depending on which model you reference), which is 2+
standard deviations above climo. With the tropical airmass in place
as Alberto meanders across MS/AL through Monday night/early Tuesday,
and weak instability becomes available, expect isolated to scattered
showers and thunderstorms to become widespread in nature beginning
Sunday evening. The potential for heavy rainfall and localized flash
flooding remains the primary threat attm with the rainfall
rates of most interest (through both the short term and medium
range forecast period). Taking into account the recent rainfall
over the past week which caused flooding/landslide issues, this
will be especially a concern for areas across the NC mountains
and the Escarpment, overnight Sunday into Monday per the GFS (or
moreso Monday, based on ECMWF). To give an idea of the rainfall
we`ve seen across the NC mountains this past week, for the
month of May, the all-time monthly total precipitation record
for Asheville which was 9.18 inches (set in 2009) has already
been broken - with 9.69 inches already recorded for May 2018,
and we still a few days to go yet in the month. Even so, with
rainfall being the primary concern, will note that while winds
attm look to be minimal, any brief gusts in thunderstorms along
with the already saturated soils - it won`t take much to bring
down weak trees. While not issued with this update, a flash
flood watch remains a possibility. High temperatures will remain
just below normal, with overnight lows hovering around normal.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 315 AM EDT Saturday: Given both the temporal and spatial
discrepancy of Alberto between the 00Z runs of the GFS and ECMWF, a
general consensus will be used for this forecast period. With
Alberto gradually weakening as it moves up through the TN valley
around midweek, latest guidance continues to suggest the opening of
it`s associated 500 mb low as it eventually merges with the upper
trough in place across the upper midwest into the latter part of the
week. With the persistent SSW flow/moisture supply, heavy
downpours/localized flooding will remain a concern, especially
across the upslope areas, as long as showers and thunderstorms
continue to develop and move across the FA. Even as the remnant sfc
low tracks west of the FA, from the TN Valley to the OH Valley and
then into the northeast into the weekend, guidance continues to
depict an upper longwave trough north of the area with weak
disturbances traveling within the feature and nearing the FA. Thus
keeping an overall unsettled weather pattern through the end of
the week, with coverage gradually transitioning from widespread
to isolated/scattered in nature by Friday. High temperatures
will continue to teeter around normal, climbing to above normal
at the end of the week, with overnight lows remaining fairly
mild.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Showers departing the Upstate this
morning will likely work over the region from KHKY to KCLT through
14Z or 15Z. Then, expect mainly scattered showers and occasional
TSRA this afternoon, especially over the mountains, with profiles
remaining very moist across the region and upslope triggering
persisting. Surface winds will remain south to southwesterly through
the period. IFR cigs this morning will gradually improve through
MVFR through late morning, with some lower end VFR likely for much
of the afternoon hours. Lowering restriction ceilings will develop
once again this evening.

Outlook: The moist and unsettled pattern will continue through the
period. Tropical moisture will increase by late weekend into
Memorial Day, further increasing the frequency and intensity of SHRA
and associated restrictions. Morning stratus/fog are possible each
day, especially following any heavy rainfall the previous day.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Med   75%     High 100%     High 100%     Med   77%
KGSP       Med   73%     High 100%     High 100%     Med   69%
KAVL       Med   75%     High 100%     Med   77%     Med   66%
KHKY       High  81%     High 100%     High 100%     Med   74%
KGMU       Med   75%     High 100%     High 100%     Med   65%
KAND       High  85%     High 100%     High 100%     Med   69%

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...HG
NEAR TERM...HG/JDL
SHORT TERM...SGL
LONG TERM...SGL
AVIATION...HG



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