Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211445

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1045 AM EDT Sat Apr 21 2018

High pressure over the eastern United States will keep conditions
dry across the southeast for much of the weekend. However, moisture
will return late Sunday through Monday as strong low pressure
approaches from the lower Mississippi River Valley. Rainfall,
possibly heavy at times, will likely develop by Monday and linger
into Tuesday. Unsettled weather could continue through late week.


As of 1030 AM EDT Saturday: Forecast is generally on track, with
very slight tweaks on temperatures to account for a quicker warm up
than anticipated at a few sites. Otherwise, today will feature
mainly clear skies and light winds. In fact, today will be the only
overall rain-free day the area will likely see for the next seven
days. As dry sfc high pressure continues to influence the weather
pattern, areas across the Carolinas and northeast GA can expect
plenty of sunshine with afternoon high temperatures slightly below
normal, in the upper 60s/lower 70s, and cooler across the mountains.
As sfc high pressure shifts east of the area, do anticipate the
gradual infiltration of thin cirrus across the FA from west to east
ahead of the next approaching system. At the surface, flow veering
to ESE/SE by this afternoon will begin to gradually moisten the low
levels. With the continuation of increasing clouds tonight,
overnight low temperatures will be warmer than this morning, but
still slightly below normal, with mid to upper 40s across the
Upstate and NW Piedmont, into the lower 40s across the mountains.
Frost/freeze concerns are not anticipated.


As of 330 AM Saturday: The early part of next week is looking quite
wet across the southeast. The numerical models now feature good
agreement through the short-term period, with a deep closed low
pressure system moving east across the lower/middle MS River Valley
Sunday through Sunday night. Meanwhile, the strengthening downstream
ridge will likely develop an Omega or Rex-type blocking pattern
which may contribute to very slow evolution eastward Monday into
Tuesday. Ahead of the system, the leading edge of the better
moisture and thicker clouds will start to push in on Sunday
afternoon but any precipitation in the most southeasterly upslope-
preferred areas will likely hold off until later in the day.
Increasing clouds will keep Sunday afternoon temperatures at least a
category or two cooler than Saturday.

Strengthening southeasterly winds at lower levels will then begin to
produce mountain upslope precipitation through Sunday night as
surface low pressure starts to approach from the MS River Valley.
Meanwhile, a strong Atlantic moisture fetch will develop Sunday
night through Monday and persist into Monday night. The deepest
layer Q-vector convergence will likely cross our forecast area
slowly, mainly Monday through Monday night. The expectation is that
surface high pressure persisting to the northeast will keep some
measure of hybrid cold air damming going through the short term
period, and this will keep the diurnal temperature range quite small
and cause most of the forecast area to remain too stable for deep
convection. Still, the robust southeasterly jet, precipitable water
values running 1 to 2 standard deviations above climo, and slow
movement of the system appear destined to produce at least some
hydrology concerns throughout the early part of the week. The main
uncertainty for our flooding concerns will be whether or not
developing upstream convection along the GA/SC coasts will rob some
of the moisture flux into our region. Surface low pressure will
likely transition through the region to the coast on Tuesday. The
heavy rainfall threat could linger as late at Tuesday night, but
with greater uncertainty. An HWO mention of heavy rain and possible
flooding will be needed mainly for Monday with this forecast


As of 340 AM Saturday: The passing Appalachian low pressure system
will finally phase and lift northeast by midweek, but with another
low pressure system/trough digging through the midwest Wednesday
night into Thursday. Steeper lapse rates will be possible ahead of
this system, but will keep any precipitation mention as showers
instead of thunderstorms for now. The GFS is at least 10 dm lower in
the 500 mb height field as this trough reaches the Appalachians on
Thursday, so confidence on PoPs is fairly low at this point.
However, there should be some brief resurgence in lift and moisture
in the consensus solution. Another deep system will sharpen up to
the west on Friday. Instability could be a bit better ahead of the
associated cold front. All told, unsettled weather should continue
Wednesday through Friday with minimum temperatures slightly above
climo and maximum temperatures slightly below climo.


At KCLT and elsewhere: Still expect VFR to prevail through the valid
TAF period, with increasing high clouds expected overnight into
Sunday, but any cigs remaining VFR. Dry sfc high pressure off the
Mid-Atlantic coast continues to influence the weather pattern for
today as light and mainly NE (though at times calm) winds hang on
through the balance of the morning hours, gradually veering to
ESE/SE and increasing to 5 to 8 kts by early afternoon at all TAF
sites as the sfc high shifts east of the area. Anticipate winds will
become light and variable around ~00Z-02Z and continuing through the
period. Skies will remain nearly clear through this afternoon, with
a gradual increase of thin cirrus from the west ahead of the next
approaching system this evening, and increasing mid to high-level
clouds beginning overnight. Expect any precip to hold off until just
past the valid TAF period (not reaching KCLT until after 06Z per
latest trends).

Outlook: Approaching low pressure system will bring increasing
clouds and precip late Sunday through at least Tuesday, with flight
restrictions anticipated. Otherwise, expect VFR.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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:




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