Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220011

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
811 PM EDT Sat Apr 21 2018

Dry high pressure over the eastern United States will maintain
control over the region through early Sunday morning.  Precip
chances gradually increase during the day on Sunday ahead of
a strong and slow-moving low pressure system over the lower
Mississippi River Valley.  A cold rainfall will become widespread
Sunday night into Monday and persist through early Tuesday,
resulting in significant accumulations.  The remainter of the work
week may remain unsettled due to a series of cold fronts.


As of 800 PM...No significant changes needed with this update. Temps
falling slightly more slowly than fcst, due to increasing cirrus.
For now, I will leave min temps as is, but if the low clouds seen on
satellite advance in a little faster than expected, min temps may be
need to be bumped up a tad in the western zones.

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 245 PM Saturday: The first part of next week still looks quite
wet across the region, and this still looks like a troublesome
fcst with lots of places that it could go wrong. Fortunately,
the model guidance remains in generally good agreement with the
overall wet pattern, that being a large upper low moving slowly
east from the MS River valley Sunday night and then lingering over
the TN valley Monday and Monday night. We should be well on our
way to establishing a cold air damming regime at sunset on Sunday
with light precip breaking out from SW to NE to seal the deal in
the evening, given a complex parent sfc high stretching from ern
Ontario to the Mid-Atlantic coast supplying the dry NE boundary
layer flow and strengthening SE moist upslope/isentropic lift over
top of it. Confidence is high that the entire fcst area will have
rain Monday and Monday night given the overwhelming support of
very strong low level isentropic upglide, deep moisture, and deep
forcing as the upper low spins over the TN valley. Rainfall amounts
are the important part of the fcst. Expect the heaviest precip to
develop across mainly the Savannah R basin, Upstate SC, and the
SE upslope areas into the upper French Broad R basin by Monday
daybreak, and then persist in that region as we have something of
a conveyor belt of Atlantic moisture focused on that region into
Monday night. A few inches of rain is likely, at least 2-3 inches,
with higher amts in the usual upslope region. Whether or not this
will be sufficient to result in flooding is questionable as we
may lack the sfc-based convection to really jack up the precip
rates that would cause problems on the small streams. Instead,
it looks more like a duration-type issue, in which case the
runoff will probably not pile up enough to cause problems until
late in the day on Monday. Confidence is not quite high enough
to entertain a Watch for an event that would mainly be in the
4th fcst period, so we will hold off for the time being. Wind
will also be strong, but mainly a problem above about 5k feet,
so a High Wind Watch will also be tabled. Temps will most likely
not make it out of the 50s east of the Blue Ridge with the cold
air damming in place and extensive precip. Expect a low diurnal
range. Tuesday and Tuesday night should see the upper low start
to open up and then get kicked up the OH River valley, so the
demise of the CAD regime becomes problematic. Guidance suggests
that isentropic lift and warm advection will taper off on Tuesday
and deeper moisture will get stripped away as a dry slot attempts
to wrap in from the SW. However, it is one thing to remove the
reinforcement for the wedge and another thing to bring in some
destructive agent. The NAM kills off the CAD during the day as a
triple-point low moves through, but really doesn`t show much of
any other destructive signal. The NAM would be a problem because
it would allow a plume of sfc-based CAPE greater than 1000 J/kg to
advect into the wrn Piedmont in the afternoon with enough residual
shear to organize severe thunderstorms. The GFS, meanwhile, keeps
a low level easterly flow longer into the day which would support
holding onto some remnant cool pool in the usual damming region
into the afternoon. The GFS seems the more reasonable solution
and agrees with the SREF mean muCAPE. That being said, even the
GFS suggests caution in that enough destabilization could occur up
into, say, the CLT metro area in the afternoon with sbCAPE upwards
of 500 J/kg, with a remnant wedge boundary hanging out in the I-85
corridor. At this point, we will keep this idea on the back burner,
as one should at this time of the year. The forcing and precip
should lift out to the NE in the evening ending any threat for
heavy precip/severe storms. Think we will develop something of a
lull early Wednesday before the next system approaches, save some
remaining chance of light precip on the TN border.


As of 230 PM Saturday: A series of short waves will move through the
long wave upper trough over the eastern CONUS through the period.
However, there is considerable disagreement on timing and location
of these systems from model to model and from run to run. This makes
for a low confidence forecast each day. Therefore, have gone with a
model blend due to these discrepancies and the low confidence. Looks
like generally diurnal PoP trends Wednesday and Thursday, with best
chances across the mountains, as moisture lingers and a series of
weak waves and a cold front cross the area. The GFS shows another
weak wave and cold front moving through on Friday with mainly low
end diurnal PoP while the ECMWF shows it on Saturday. Have gone with
the GFS and Canadian for Friday and Saturday given their agreement.
Highs start out near normal, drop a few degrees for Thursday, then
bounce back to just below normal for Friday and Saturday. Lows start
out a few degrees above normal then remain steady near normal
through the end of the period.


At KCLT and elsewhere: Southeasterly flow atop the region will bring
in moisture and a gradual increase in lower clouds starting during
the pre-dawn hours in the west and spread north and east into
western NC during the day on Sunday. The TAFs start out with just
some thin cirrus and a light SE wind. Low VFR cigs should spread in
across the Upstate sites by daybreak Sunday, with MVFR cigs at KAND
by midday with -shra possible. The -shra should spread across the
rest of the area, except perhaps not quite to KHKY before the end of
the 00z TAF period. Lower cigs and vsby won`t really start becoming
widespread across the region until Sunday evening. Winds will start
to back out of the NE at the Upstate sites, due to developing cold
air damming.

Outlook: Conditions will deteriorate Sunday night with widespread
rain overspreading the area. IFR/MVFR conditions will be likely thru
the day on Monday, with some improvement on Tuesday, as precip
largely shifts NE of the area, but with some moisture lingering.
Unsettled weather will continue on Wednesday and Thursday, as a
trough of low pressure persists across the region.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  92%
KAVL       High 100%     High 100%     High  99%     High  92%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High  82%
KAND       High 100%     High 100%     Med   77%     High  83%

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:


Due to the predominant environment, rain rates associated with the
upcoming event are generally NOT expected to be supportive of flash
flooding, where flooding begins within 6 hours of rainfall onset
and/or initial stream response.  Model guidance has consistently
kept 6-hourly rain accumulations generally under 1.5", which lessens
the concern for significant (i.e., flooding of primary roadways
and structures) flooding.  Instead, a more gradual accumulation
of significant rainfall means that any flood threat will gradually
increase Monday afternoon into Tuesday as widespread soil saturation
is reached and persistent runoff over large areas causes significant
rises on area streams.  Low-lying areas that commonly flood due
to poor drainage and overwhelmed drainage ditches will be most
susceptible and such flooding may persist for many hours due to
the steady and persistent nature of the rainfall.

As a result, minor (i.e., parks, farmland, boat access areas,
and other common flood-prone low lying areas) flooding of the
Upper French Broad River and associated tributaries is possible,
but minor flooding of mainstems in the Upstate is also possible,
especially along the Broad and Saluda rivers, with crests not
occurring until Tuesday and perhaps early Wednesday.  Streamlevel
forecasts for these rivers will incorporate the total forecasted
rainfall beginning Sunday morning and will be accessible at




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