Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 250200

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1000 PM EDT Fri Mar 24 2017

High pressure will continue to move off the east coast tonight. A
low pressure system emerging from the Great Plains over the weekend
will bring moisture to the southeast Saturday and Sunday. Unsettled
weather will continue through Tuesday as another low pressure system
moves from the plains to the Ohio Valley, allowing Gulf of Mexico
moisture and some storms to persist over the southeastern states.


As of 1000 PM EDT Friday:  As the rather dynamic upper cyclone
rotates atop the southern plains, moisture continues to stream
in out ahead over the deep south.  With upper heights falling,
surface pressures are complying as well as the surface anticyclone
over the western Atlantic continues moving further out to sea.
As for sensible wx, current conditions across northeast GA and the
western Carolinas are highlighted by mid/high clouds riding the
mean flow aloft, while low stratocu is evident over northeast
GA and extreme northwest SC, per satdiff products and obs.
Guidance continues to favor expansion of low clouds through
early/mid morning, especially along/north of the I85 corridor.
Southerly winds have weakened substantially across the region
as decoupling begins to take affect.  As for the fcst itself,
all in all things look pretty good.  Did opt to make some minor
adjustments to t/td and sky to align with recent obs, with no
other changes needed/made at this time.

Otherwise, stout upper lvl ridging remains in place well into
Saturday allowing temps to climb higher. By the end of the period
00z Sunday, a large upper low will be approaching the region from
the west, but is not expected to have much impact on the fcst
area over the near-term period. At the sfc, high pressure will
continue to slide further offshore tonight and tomorrow while a
large low moves east across the southern plains and then begins to
lift NE and towards the Great Lakes late Saturday/early Sunday. No
significant pattern changes are anticipated over the period with
SLY low-lvl flow persisting over the region and temps expected
to remain above normal. The region of deep moisture associated
with the approaching sfc low is looking like it will take a bit
longer to reach the fcst area. Most of the model guidance currently
suggests that it won`t reach our western zones until just after 00z
Sunday. This should keep precip chances at bay until late Saturday.


As of 230 PM EDT Friday: The Gulf of Mexico will open up and say
aaahhh this weekend...with southerly flow transporting moisture over
the western Carolinas and northeast Georgia right on through the
period and into early next week. That should make for a relatively
unsettled weather regime. Upper forcing, in the form of a short
wave acquiring a negative tilt, will be on our doorstep Saturday
evening. This wave should translate northeast across the region and
act upon the plume of deep moisture to bring numerous to widespread
shower coverage... eventually. The trend in the guidance has been
to slow down the eastward progress, thus the I-77 corridor stays
drier a bit longer into Sunday. Precip chances ramp up from the
Blue Ridge Escarpment outward through the night and into Sunday
morning, with the best probability near the Blue Ridge owing to
the contribution of mechanical lift with the southerly flow at low
levels. After the first wave passes, chances may go down during
Sunday afternoon, but the timing is questionable. The GFS is a
bit slower and would suggest keeping a higher chance over the east
for longer. The main focus or problem for the short range is the
severe weather chances on Sunday. The model guidance provides an
environment with surface-based CAPE on the order of 500-1000 J/kg,
shear on the order of 30 kt, and SR helicity in the 150-200 m2/s2
range. At face value, this would make one go hmmm...but looking
at fcst soundings, the lapse rate is nothing to get excited about
and is thought right now to be a limiting factor. A trigger is also
in doubt in the wake of the main upper wave. Still, the hodographs
are impressive, so it would be most wise to not turn one`s back on
the possibility. The day 3 convective outlook features a marginal
risk, and I agree that it does in fact appear marginal, so we will
not mention this in the HWO just yet. From late Sunday onward,
we maintain the southerly moist flow, so a shower could develop
at just about any time. Will keep a precip chance across most of
the region, favoring the mountains and foothills, through Sunday
night and Monday. Temps will remain above normal through the period.


As of 220 PM Friday: Not much of a pattern change is anticipated
during the medium range, as a fairly amplified/split flow regime,
with a very active northern stream and somewhat less active southern
stream should persist through the period. The result will be a
continued tendency for upper lows to form across the south central
part of the Conus, only to get kicked out a day or two later by the
next strong wave bombarding the West Coast.

The period will begin Monday night with a dampening, but
nevertheless negatively tilted short wave trough approaching our
forecast area from the TN Valley. Although a decent amount of
buoyancy is expected to develop ahead of this feature Monday
afternoon, it appears that the diurnal cooling cycle may be
underway by the time the deeper forcing arrives Monday evening.
Based upon the latest guidance, shear parameters are not expected to
be especially impressive. Thus while the severe weather potential is
non-zero, it leaves much to be desired.

Cold front sweeps across the area Tue night and early Wed, resulting
in gradual drying. Otherwise, the Mon night through Thursday period
will be marked by well above normal temperatures.

By the end of the period, the global models depict surface high
pressure spilling east of the Appalachians, in the wake of a
substantial short wave trough pushing away from the New England
coast. This signals the potential for some cooler and drier air to
spill into at least part of the forecast area from the northeast.
Meanwhile, the global models are in good agreement that the next
southern Plains upper low will be kicked out across the Ohio Valley
by the end of the period, bringing another potential round of
showers and thunderstorms to our area. However, based upon the
expected surface pattern, there will be the potential for cold air
damming to impact part of the forecast area during this time.
Temperatures will therefore be forecast a little closer to (although
still above) climo by the end of the period.


At KCLT and Elsewhere: VFR conditions to prevail through the
first few hours of the taf cycle at all sites before developing
low stratus leads to restrictions.  As such, initialized all tafs
amidst low VFR stratocu/cu beneath mid/high clouds before western
Atlantic moisture advects in and settles across region.  With that,
guidance favors decoupling of the moistening BL leading to low
stratus development along the escarpment, spilling out across the
Fthills/Piedmont.  Therefore all sites feature lowering to MVFR
levels in the 09-10z time frame, with the exception of KAVL where
IFR is included.  Given the aforementioned decoupling, sly winds are
expected to weaken into the 3-4kts range overnight, increasing just
after daybreak into the 6-8kts range.  Cigs will prevail into the
late morning hours, eroding from the east leading to recovery at
KCLT first, generally around noon.  Beyond MVFR erosion, looking
for low VFR cu/stratocu to dominate through periods end.

Outlook: Increased precip chances return late Saturday thru Sunday
which will likely create flight restrictions across the area. An
unsettled and moist pattern will persist next week and maintain
the chances for low CIGS and VSBY.

Confidence Table...

            01-07Z        07-13Z        13-19Z        19-00Z
KCLT       High 100%     High  98%     High  82%     High 100%
KGSP       High 100%     High  81%     High  80%     High 100%
KAVL       High 100%     Med   72%     Low   54%     High 100%
KHKY       High 100%     Med   74%     Med   76%     High 100%
KGMU       High 100%     High  83%     Med   79%     High 100%
KAND       High 100%     High  82%     High  82%     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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