Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 290606
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
206 AM EDT Sat Apr 29 2017

.SYNOPSIS...
A Bermuda high will bring almost summer like weather to the western
Carolinas and northeast Georgia for most of the weekend, until a
strong cold front approaches from the west Sunday night. The cold
front will move across our region on Monday. After the front passes,
weak high pressure will move across the southeast for Tuesday and
Wednesday. However, the atmosphere will reload with another slow
moving system that may bring wet weather for late next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 145 AM EDT: Bermuda high pressure will remain dominant over
the southeastern U.S. today through tonight. Some cirrus slipping
over the area under the ridge early this morning will help to keep
morning fog and low clouds less prevalent than yesterday, but patchy
fog is still likely around daybreak. Otherwise, expect continued S
to SW low level flow to keep dewpoints up in the 60s and boundary
layer moisture relatively high. Temperatures will likely max out in
the upper 80s in many locations, with mid 80s in the lower mountain
valleys. Little to no upper forcing is apparent today, but weak
upslope triggering will be possible near the southern mountains,
with some low-level convergence expected as well in the upper
Savannah River valley. Isolated to scattered coverage is showing up
in the numerical models/CAMs this afternoon and that could prove
troublesome given the model indicated instability. Although there
will be abundant CINH to be overcome at low levels this morning,
model profiles suggest that this is indeed overcome after 18Z and
then tremendous CAPE values of 4000+ J/kg could be realized with any
deep convection that manages to form. All told, this means limited
thunderstorm coverage (best in southwest sections) but with a
reasonably good chance of severe weather with any convection that
does manage to form. Large hail will be quite possible and delta
theta e values of 30 to 35 K suggest a microburst wind threat as
well. Will include an HWO mention for this.

Isolated shower triggering could continue in upslope areas tonight,
but with profiles quickly stabilizing with the loss of diurnal
heating. Another mild night is expected, with fog and low stratus
possible depending on the thickness of any invading cirrus.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
As of 2pm Friday:  Main short-term event will be passage of major
frontal system with precipitation/thunder on Monday.  With 500mb
ridge off the southeast US Saturday evening, little more than a few
upslope showers in the Southern Appalachians are expected with the
southerly low-level anticyclonic flow Sunday through Monday
morning.  SSW flow will maintain low to mid-60s dewpoints and give
warm lows 10 to 15 degrees above normal Sunday and Monday with highs
Sunday and Monday being held-back by extensive cloud cover.  Precip.
chances advance with approach of frontal system.  Current timing
places surface front over the mountains Monday morning, exiting the
area to the southeast by Monday evening. Dewpoints drop 10 degrees
behind the front, and the threat of any precip./thunder will end
with its passage, with Tuesday morning dry and clear and returning to
seasonally normal low temperatures.

Continuous SSW surface flow ahead of front will maintain CAPE
through FROPA on Monday.  CAPE on Sunday is forecast to 2000 j/kg
with weak capping.  However, forcing is nonexistent and deep layer
shear is weak.  While any terrain-related showers on Sunday would be
thunderstorms, extent and severity is expected to be limited.  On
Monday afternoon, significant dynamical factors with surface front
and upper vorticity advection will lead to wide-spread thunder.
However, extensive cloud-cover will hold back temperatures, which
will reduce the afternoon CAPE to 500 j/kg.  However much better
effective shear and more general coverage would make severe storms
more likely on Sunday.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 200 PM EDT Friday: The medium range fcst period kicks off
Tuesday morning amidst longwave H5 troffing atop the east/central
CONUS, while at the surface a cold front moves off the east coast as
weak high pressure slides in from the west across the deep south.
A brief period of cooler/drier air advection is plausible behind
the front on Tuesday before the aforementioned surface ridge
shifts eastward over the western Atlantic, and another upper wave
deepens over the southern Plains leading to surface cyclogenesis
atop the lower/mid MS River Valley for Wednesday.  From that point
forward the fcst will get increasingly wet across the the southeast
states, to include northeast Georgia and the western Carolinas
as upglide/upslope showers will move in Wednesday night into
Thursday morning, while a surface wave and associated warm front
move through the region on Thursday leading to increased chances
for deeper convection and thus tsra.  Meanwhile, the upper pattern
will become increasingly amplified leading to development of a
closed and slow moving upper low atop the Arklatex region on Friday.
With that, llv GOM/ATL moisture fluxes will increase into Saturday
across the southeast leading to higher chances for shra/tsra,
possibly heavy rainfall and flooding as well given the pattern.
Temperatures through the medium range will be a bit variable,
however should hover in the vicinity of normal, leaning mostly
above normal.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Cirrus slipping in under the ridge axis is
keeping the low stratus and fog potential somewhat reserved, but it
is still likely that we will see a narrow window of IFR cigs and or
vsby before daybreak at all TAF sites given the near-ground
moisture. Any restrictions will be quick to burn off this morning,
however, with VFR likely throughout after about 13Z or 14Z. S to SW
surface winds will continue throughout at 10 kt or less, with a few
low end gusts possible with mixing this afternoon. The main concern
will be the potential for isolated, but strong to severe,
thunderstorm development this afternoon and early evening. Chances
appear too low to mention anywhere but KAND, where a PROB30 will be
featured after 19Z.

Outlook: Patchy fog and low clouds will be possible again Sunday
morning around daybreak. Diurnally-based showers and thunderstorms
will be possible on Sunday afternoon. More organized thunderstorms
are likely on Monday with a passing cold front. Drier weather is
expected Tuesday through Wednesday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  86%     High  95%     High 100%     High 100%
KGSP       High  94%     High  95%     High 100%     High 100%
KAVL       High  97%     High  94%     High 100%     High  81%
KHKY       High  95%     High  87%     High 100%     High 100%
KGMU       High  93%     High  92%     High 100%     High 100%
KAND       High 100%     High  93%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...PM
NEAR TERM...HG
SHORT TERM...WJM
LONG TERM...CDG
AVIATION...HG


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