Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222341

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
741 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.


730 PM Update...Cold front is finally beginning to work in from the
west. A few of these storms have decent cores and are likely
producing small hail and gusty conds. With the earlier deep mixing
instability remains quite low across the FA. These storm shud remain
sub-severe thru the next update...but an isol svr cell is possible.
Will begin monitoring for possible hydro threats as antecedent conds
are a little wetter than normal across NC.

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 pessimistic TAF period in store with
evening cold front pushing in from the west and then a wedge front
moving in from the north aft 09z or so. Have MVFR CIGS with initial
-shra/tstms then decreasing to IFR as BL moistens overnight.
Moisture will be locked in most of the day with periods of elevated
tstms possible late in the period. Aft 09z...winds outside the mtn
valleys will align ne/ly and with a strong sfc ridge building
in...low-end gusts are probable thru the remainder of the
period. KAVL will remain generally down valley thru 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 improve by Tuesday with dry air working in behind
the departing low.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     Med   78%     Med   72%     High  81%
KGSP       High 100%     Med   61%     Med   78%     High  87%
KAVL       High 100%     Med   61%     Med   78%     Med   62%
KHKY       High 100%     Med   66%     High  83%     High  87%
KGMU       High 100%     Low   55%     Med   72%     High  87%
KAND       High 100%     High 100%     Med   61%     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:


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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