Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222101

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
501 PM EDT Sat Apr 22 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.


500 pm Update...Continued mixing thru a deep layer has prevented
much of a precip threat this afternoon across the majority of the
CWFA. PoPs were lowered a couple cats with mid-chc left across
the west and north closer to frontal triggering. The mixing has also
lowered temps and td/s from the fcst curve so made some localized
adjs to these grids. Expect an uptick in conv activity across the
wrn NC mtns thru the next update.

As of 250 PM EDT Saturday:  A sharp positively tilted upper trof
continues to slide east across the Plains this afternoon, with deep
swly flow prevailing out ahead across the southeast states.  At the
surface, a quasistationary frontal axis draped across the TN valley
is slowly starting to migrate eastward, likely moving into the
southern Appalachians over the next 4-6 hours.  Meanwhile, the same
front laid almost parallel to the H5 flow across VA will backdoor
into western NC this evening/overnight.  Ample instability out ahead
this afternoon will provide enough buoyancy for iso/sct convection
across the CWFA, however with the greatest chances along the frontal
axis where local shear is maximized.  Therefore the fcst will
feature likely/categorical pops north of I40 as well as adjacent to
the TN line this afternoon/evening, wedging southward as the
backdoor front intrudes.

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:  A rather restrictive period ahead for all
sites as an approaching upper trof drives a series of fronts
into/through the region.  All sites initialize at VFR levels as warm
sector convection continues across the region prompting VCTS/VCSH
through the afternoon accordingly.  Swly winds will remain elevated,
with moderate gusting possible at all sites, especially at the SC
sites and KCLT.  Otherwise, low/mid cigs will dominate as a
quasistationary frontal axis gets on the move across east TN,
eventually moving through the NC high terrain this
afternoon/evening, before backdooring in from the north across the
Piedmont tonight.  Thus, precip chances will ramp up with taf timing
reflective of latest cam guidance, with cigs falling into the MVFR
and eventually IFR range as the backdoor front moves in leading to
wedge development.  With that, winds will back sharply to the ne
with low end gusting possible by around daybreak Sunday, with shra
prevailing at all sites.

Outlook: A strong area of low pressure will cross the region Sunday
afternoon into Monday, with widespread low clouds and precipitation
expected to persist. Conditions should improve by Tuesday with dry
air working in behind the departing low.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     Med   79%     Med   70%     High  84%
KGSP       High 100%     Med   72%     High  81%     High  89%
KAVL       High 100%     Med   68%     High  98%     High  82%
KHKY       High 100%     High  81%     Med   74%     High  89%
KGMU       High 100%     Med   70%     Med   70%     High  89%
KAND       High 100%     High 100%     Med   74%     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:


NC...Flood Watch from midnight EDT tonight through Monday evening
     for NCZ033-035>037-048>053-056>059-062>065-068>072-


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