Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211816

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
216 PM 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 200 PM EDT Saturday: Today will likely be the last time we`ll
see the sun across much of the FA in quite a while. Dry surface high
pressure has shifted off the Mid-Atlantic coast, with weak surface
southeasterly flow at the surface and only high cirrus spilling into
the area. Gradual low-level moistening will occur overnight and
tomorrow as a healthy Atlantic/Gulf moisture tap sets up. An upper
low and surface low pressure system will emerge from the Deep South
tomorrow, beginning to approach the area by the late afternoon. At
the same time, a wedge of surface high pressure will slide down the
spine of the southern Appalachians, setting the stage for what will
likely become a prolonged cold-air damming event across the area in
the short term period. Skies will become progressively cloudier
tomorrow as deeper moisture enters the Carolinas, though held off on
spreading pops into the forecast area during this near-term period
except for our north GA counties and far southwestern Upstate SC.
Lows overnight tonight will be warmer than this morning (though
still below average) due to cloud cover spreading in overnight, with
highs tomorrow expected to be about a category below average for the
same reason.


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: VFR conditions will prevail almost all the
way through the current valid TAF period. High clouds will continue
to increase tonight as an upper low/surface low pressure system
closes in on our area, but with dry surface high pressure off the
Mid-Atlantic coast still in control, any cigs through tomorrow
afternoon should remain VFR. Flow will remain generally
southeasterly at all TAF sites through the overnight period, with
winds becoming quite light. Increasing moisture ahead of the system
tomorrow will introduce more mid-level clouds to the area, and TAFs
hint at gradually lowering cigs of a progressively more overcast
nature through the day. KAND is the only site with precip before the
end of the valid TAF period (beginning ~15Z), with MVFR vsby and
cigs expected with any showers that develop.

Outlook: Expect VFR conditions for approximately the next 24 hours
before an approaching low pressure system brings increasing clouds
and precip late Sunday through at least Tuesday. Some flight
restrictions are anticipated through early next week as this system
moves through the southeast.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
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  98%

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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