Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210605

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
205 AM EDT Tue Mar 21 2017

A cold front will approach the forecast area Tuesday into Wednesday,
increasing chances for rain and thunderstorms. A few of the
thunderstorms could be strong tomorrow evening. Cooler air will
return behind the front to end the work week before another frontal
system approaches the area next weekend.


530 UTC update...I started updating PoP/sky/WX grids for the 06z
TAFs, trying to refine timing of convection with latest guidance. In
the very near term, some new convection has started to develop along
the front over central KY. Some of this activity may reach the
northern NC mountains by around 12z. So will leave mention of
thunder along TN line. Otherwise, forecast is on track.

The 1000-850mb thicknesses continue to increase overnight, which
indicates warmer min temps tonight (vice last night) and max temps
tomorrow: mins in the upper 40s to around 50, and maxes as warm as
the lower 80s in the Upper Savannah Valley and Lakelands. The
increased thicknesses also reflect significant warming throughout
the low levels. A key difference between models is how much of a cap
develops and, if so, how soon it erodes as the cold front sinks down
into the area and/or as a shortwave enters from our west. Sfc-based
instability may be significant, though significant capping may also
be present for much of the daytime hours. Accordingly, PoPs will
increase through the afternoon with the greatest values after 00z.
However, the chance of thunder and/or severe weather looks greatest
during the late afternoon when CAPE is maximized, and we begin to
get the dynamic lift associated with the shortwave. Given modest
deep shear plus the front serving as a focusing mechanism, SPC has
now included a portion of the western CWFA in a Slight Risk area for
Day 2, and has extended the Marginal Risk area east through the
remainder of the CWFA, for elevated wind/hail risks.


As of 2pm EDT Monday:

General pattern of northwesterly flow aloft with a weak embedded
shortwave and cold FROPA will continue risk for thunderstorms
Tuesday evening, with surface instability and threat risk declining
after dark.  Cold front is expected to be past the mountains Tuesday
evening, with deep convection shifting to Piedmont areas in the
evening.  There is some risk of on-going severe thunderstorms
Tuesday evening per SPC Day2 outlook, as CAPE is forecast of over
1000 j/kg.  While CAPE is healthier than usual for this time of
year, deep layer shear is weaker as upper and lower winds have
little directional shear, suggesting microbursts and marginally
severe hail as the main threats.  Total precip. expected from the
round of thunderstorms from Tuesday/Wednesday is around 3 tenths of
an inch on average, though due to the convective, showery nature of
the precipitation, some areas will get a lot more and some will get

Following FROPA Tuesday evening, boundary layer drys and cools, with
Wednesday highs being 10 to 20 degrees cooler than Tuesday, and
Thursday lows being back into the 40s in Piedmont areas, with a
light freeze in the mountains and frost possible in the foothills
and adjacent areas.  The upper pattern shifts on Thursday from
northwesterly to advancing upper ridge with southerly surface flow
and warming/moistening trend beginning Thursday evening.


As of 125 PM EDT Monday: Confidence is moderately high through the
period with reasonably good model/ensemble agreement. A 500 mb ridge
axis will greatly amplify from the MS River Valley to the southern
Appalachians Thursday night through Friday in response to a cutoff
low developing from the four corners region to the southern Plains.
Mid and high level moisture will likely ride over the developing
ridge axis, but stay generally north of the region through Friday.
However, as the surface to 850 mb ridge slips offshore on Friday, a
low level return flow of better RH and very weak upglide will
develop. Temperatures will be hard to pin down since this should
lead to increased clouds, but precipitation looks much more
questionable. Will lean toward the cooler MOS with limited late day
PoP in southwest sections for light rain, but any in-situ cold air
damming should be fairly weak.

The ridge axis will move offshore on Saturday as the closed system
lifts into the Midwest. Southerly flow will increase into the base
of the southern Appalachians on Saturday, with upslope showers
likely breaking out. Any lingering surface stable layer east of the
mountains will retreat quickly northward. A narrow zone of deeper
pre-frontal moisture should then push into the southwest mountains
sometime late Saturday and slowly cross the piedmont through Sunday.
A trailing band of showers should accompany the dissipating frontal
zone Sunday night, but with PoP likely more confined to the
mountains as the low level flow veers more westerly. Flat ridging
should return from the west by Monday along with much drier
profiles. Temperatures should rebound to a couple categories above


At KCLT and elsewhere: a cold front will push thru the area from the
north this afternoon and evening. Scattered to numerous showers and
thunderstorms are expected to develop along and south of the front
and track SE thru the area, with a few storms possibly producing
strong winds and hail. The best chances at the TAF sites will be in
the 21-03z time frame. Showers and possibly a few storms will linger
thru late evening. Winds will be out of the SW ahead of the front,
then switch to north as it passes this evening. The exception is
KAVL, where all the guidance keeps winds out of the north thru the

Outlook: Low level moisture may not completely dry out on Wednesday
(especially in the southern part of the area), before starting to
lift back north and deepen by Thursday, keeping chances of showers
and low cigs into the weekend.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
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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