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FXUS62 KGSP 292358

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
758 PM EDT Wed Mar 29 2017

Dry high pressure will build into the area from the north through
tonight. Moisture will gradually increase through the day tomorrow,
with abundant moisture in place by Friday as a strong low pressure
system moves east from the Mississippi River Valley. Drier weather
will return for the weekend, with unsettled weather returning
early next week.


As of 750 PM EDT: Fair weather, comfortable temps, light wind, and 
dry air make for a nice evening. Enjoy it will not last. The 
only tweak to the fcst was to modify the temp trend based on 
observations, which were running a bit warmer.

Change is on the way. The upper ridge axis over the area tonight 
slides east to the coast Thursday. High pressure remains ridged into 
the area from the NE tonight into Thursday. Low level moisture 
increases over the area tonight with deep layer moisture increasing 
on Thursday as upper heights begin to fall and flow becomes SW ahead 
of an upper low moving into the Mid-South. The low level flow 
becomes steadily more upslope in nature tonight and increases in 
magnitude Thursday. Isentropic lift increases through time as well. 
All this leads to increasing clouds tonight and a chance of rain and 
drizzle developing along the Blue Ridge after midnight. This precip 
spreads out across the CWFA through the day Thursday as deep 
moisture moves in and stronger forcing develops.

With the surface ridge nosed in from the NE and clouds and precip 
developing, a cold air damming wedge develops. The guidance differs 
a little on where the southern extent of the wedge and resultant 
TMB. The NAM, ECMWF, and SREF all showing instability developing 
over NE GA, much of the Upstate, and the NC Mountains indicating a 
farther north boundary than the GFS. The GFS shows little to no 
instability keeping the boundary farther south. Have gone with a 
blend to come up with thunder chances Thursday afternoon. This keeps 
TSRA limited to west of the French Broad valley in NC, and generally 
west of I-26 across the Upstate and into NE GA, though only small 
chances at best for northern Greenville and Spartanburg Counties. 

Lows tonight will be quite mild 10 to 15 degrees above normal. Highs 
Thursday will show the wedge pattern, with near normal readings 
along and south of the I-85 corridor and the TN border counties. 
Highs will be below normal along the Blue Ridge into the NC 
Foothills and NW Piedmont.


As of 2pm EDT Wednesday: First of a series of strong dynamical 
spring systems reaches the area Thursday night/Friday morning with a 
round of showers and thunderstorms.  Current expectations are for 
very marginal instability with CAPE below 500 j/kg and LI of -1, 
combined with deep layer shear to 55 kts.  Showers and some thunder 
are a good bet Thursday night/Friday morning, but severe potential 
will depend on some improvement in parameters over what is now 
forecast.  As the system moves eastward on Friday, diurnal warming 
will increase instability, with improved chances for SVR as the 
system exits out of eastern parts of the CWA.  Region should receive 
useful amounts of rainfall with this first system Thursday night and 
Friday morning with storm total rain amounts of around a half inch 
to an inch, with locally heavier amounts with the deeper convective 

Wedge/CAD regime in place on Thursday brings some low-level moisture 
from the southeast into the area with some upslope showers in the 
higher terrain.  Somewhat cooler air and CAD-related cloudiness will 
limit the development of surface-based CAPE.  CAD will be disrupted 
by wet passage of major frontal system Friday morning, with 
low-level winds veering to westerly and eventually northwesterly 
behind the front.  Frontal passage will be accompanied by gusty 
winds, especially at higher elevations with 850mb winds of 30 to 40 
kts. Passage of front will scour moisture and create drying 
conditions for the balance of Friday and clearing by Saturday 
morning; with fair weather on Saturday.

Temperatures on Friday will warm some over Thursday pre-frontal, 
with post-frontal temperatures recovering rapidly on Saturday in 
clear skies.


As of 215 PM EDT Wednesday: the extended fcst picks up at 00z on 
Sunday with upper ridging building back over the region as a very 
deep upper trof digs down over the SW CONUS. The upper trof will 
approach the fcst area on Monday and move across the CWFA early 
Tuesday. The system is expected to lift NE of the area by early 
Wednesday with flat upper ridging spreading back over the region in 
its wake. At the sfc, Canadian high pressure will be pushing into 
the fcst area from the north early Sunday but is expected to drift 
NE of the region later in the day. At the same time, a vigorous low 
will develop over the ArkLaTex region and lift NE on Monday and move 
a warm front over the Carolinas. The system is expected to lift 
north of the fcst area on Tuesday and push a cold front thru the 
CWFA as it does. Things should dry out late Tuesday/early Wednesday 
as the low moves NE of the region. As for the sensible fcst, Sunday 
should be dry with POPs ramping up quickly from SW to NE Monday 
morning. Chances for widespread showers and thunderstorms, some of 
which could be strong to severe, remain high Monday afternoon thru 
early Tuesday with lingering showers possible for the remainder of 
Tuesday. I kept a slight chance POP over the NW half of the CWFA on 
Wed for any lingering showers. Temps will remain above normal, for 
early April, thru the period.


At KCLT...big changes between flight conditions this evening and 
when operations ramp up again in the morning. VFR through late 
evening...with a light wind favoring a SE direction. Once high 
pressure moves off to the east, low level moisture is expected to 
surge from the southeast in the early morning hours, bringing the 
low clouds in by daybreak. Guidance continues to suggest the 
development of MVFR-level clouds after about 08Z and then IFR around 
10Z-11Z. The new fcst keeps the same scenario as the previous one, 
but the onset of IFR was pushed back a few hours. The upslope flow 
will probably not be quite as productive as far east as KCLT, so the 
ceiling was allowed to rise back up to MVFR during midday. This 
depends greatly on how strong the cold air damming wedge becomes, 
and it is almost as equally likely that the ceiling will remain IFR 
through the end of the period. Wind should come around to ENE when 
that happens. Precip chances go up in the afternoon, and may 
eventually require more than just a PROB30.

Elsewhere...upslope flow and isentropic lift should be more 
productive, so low clouds develop a bit earlier and last longer in 
the IFR or LIFR range into Thursday. Precip chances are greater 
closer to the Blue Ridge as well, so a visibility restriction will 
be included with prevailing light rain.

Outlook: The approach of a strong storm system will bring continued 
chances of restrictions as well as a potential for heavy rain 
showers and thunderstorms Thursday night and Friday. Drier 
conditions will return again for the weekend.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z 
KCLT       High  98%     High  88%     High 100%     High  96%     
KGSP       High 100%     High  88%     High  94%     High  89%     
KAVL       High 100%     High  87%     High  94%     High  98%     
KHKY       High 100%     High  87%     Med   78%     High  86%     
KGMU       High 100%     High  89%     High  93%     High  88%     
KAND       High 100%     High  95%     High  84%     High  87%     

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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