Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230603

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
203 AM EDT Sun Apr 23 2017

A backdoor cold front slides into the area this evening with
numerous showers and thunderstorms and locally heavy rainfall.
Meanwhile, a deep and vigorous closed low moves across the TN Valley
Sunday inducing a lengthy period of moderate rainfall that persists
through Monday morning.  Widespread rainfall accumulations of 2-4
inches are likely.  Following this event, next week will be dry
and much warmer.


As of 200 AM EDT: A band of showers continues to weaken as it drifts
east off the Blue Ridge into the Piedmont. Another round of
categorical PoP coverage rain is expected to develop during the
pre-dawn hours per the CAM consensus. So I will leave the PoP trends
as is with this update. Otherwise, only minor tweaks to the winds
and sky grids for the 06z TAFs.

By daybreak Sunday categorical/likely pops are featured north/south
of I85 respectively, where upglide atop the intruding wedge front,
upsloping along the escarpment, frontal convergence induced
convection, and increasing synoptic lift all work to produce
widespread shra and perhaps tsra southward in the warm sector.  Said
pops will remain elevated, all the while expanding to categorical
levels across the entire CWFA through the day on Sunday.  Total QPF
through the period will range from nearly a half to 1 inch south of
I85, increasing to the north/west where the high terrain and
northern NC Piedmont could see rainfall amounts upwards of 2-2.5
inches, perhaps with some locally higher amounts.  Given these
totals, as well as further rainfall beyond the scope of this fcst
period, a Flood Watch will be issued.  Said watch will initialize at
midnight tonight, running through the near term for all counties in
western NC with the exception of Union.  Temperatures on Sunday will
be quite a bit cooler than today as the wedge settles into the
region with highs topping out nearly 8-12 degrees below normal
amongst abundant sky cover and widespread rain.


As of 155 PM Saturday: Precip event will be ongoing at the start of
the period, with widespread convection expected within strongly
forced regime associated with upper low/surface cyclone passing west
through south of the forecast area through Monday. Forcing will reach
a nadir Sunday evening, when there will be a moderately strong E/SE
flow into the Blue Ridge along with strong isentropic lift atop
shallow/in-situ cold air damming air mass. Although PWATs will not
be extremely high (generally just 1-2 standard deviations above
climo), ample MUCAPE  within the strongly forced regime will support
elevated thunderstorms, with an organized heavy rainfall threat
possible along and near the Blue Ridge, especially the northern mtns
and foothills of NC, which are more susceptible to heavy rainfall in
easterly flow. Additional mesoscale areas of heavy rainfall will
also be possible with convective bands developing east through north
of the upper low. Expect the heaviest storm total rainfall from I-85
north, where low level forcing will be maximized at the height of
the event, and 2-4 inches are forecast in this area (again, the
highest amounts occurring along the I-40 corridor). Locally higher
short term rainfall rates could result in localized flash flooding,
although the larger hydro threat should exist within slower response

While the forcing will diminish considerably (especially in the
lower levels) by early Monday, mainly light showers will remain
possible through Monday, especially across eastern areas, as
deformation zone lingers across the region. Abundant cloud cover and
NE flow will result in very cool temps on Monday, which are expected
to average a good 10 degrees below climo.

Pops gradually taper off through early Tuesday, when a consensus of
deterministic guidance depicts the upper low moving off the Carolina
coast. This will result in decreasing cloud cover and a developing
northwest flow Tuesday which should result in a considerable warmup,
with max temps expected to average 10-15 degrees above Monday`s
readings, or around 5 degrees above climo.


Thankfully, the weather will quiet down significantly following
our heavy rainfall event.  No appreciable rainfall is expected
and temperatures will rebound to 5-10 degrees above normal through
the period.

Following the upper-level low, heights will build across the
Southeast as a subtropical ridge develops and strengthens later
next week.  With sprawling high pressure centered between Bermuda
and the NC coastline, deep-layer southerly flow will dominate
the period.  The resultant moisture advection will gradually
increase dewpoints and our low temperatures through the week, with
lows 10-15 degrees above normal by Friday night.  The increase
in atmospheric moisture and favorable upslope flow will gradually
bring back a diurnal chance of isolated showers and thunderstorms,
especially across the mountains, later next week as a weak front
approaches the area overnight Thursday into early Friday.


At KCLT and Elsewhere: Periods of widespread showers will continue
across the area over the next 24 hours, with lowering CIGS and VSBY
expected thru daybreak. Cold air damming should result in NE winds,
occasionally gusty, at the piedmont sites, and E to SE winds at
KAVL. Moisture will be locked in most of the day with periods of
elevated tstms possible late in the period.

Outlook: An area of low pressure will cross the southern CWFA thru
Monday while strong sfc ridging builds in from the north. Widespread
low clouds and precipitation are expected to persist thru this time.
Conditions should gradually improve Tuesday thru Wednesday morning
as dry air works in behind the departing low.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  90%     High  83%     High  84%     Med   79%
KGSP       High  87%     High  85%     High  84%     High  91%
KAVL       Med   78%     High  81%     Med   78%     High  87%
KHKY       Low   50%     Med   79%     High  84%     Med   78%
KGMU       High  87%     Med   75%     High  84%     High  84%
KAND       High  94%     High  87%     High  97%     High  95%

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:


NC...Flood Watch through Monday evening for NCZ033-035>037-048>053-


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