Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 212340

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
740 PM EDT Sun May 21 2017

An area of low pressure will move away from the area tonight, with
another system approaching the region Monday night. With the
approach of a deep upper trough and attendant cold front Wednesday,
another round of storms will be possible. Expect dry high pressure
and milder temperatures to return on Friday and into next weekend.


Meso-low is currently located between KGSP and KCLT and the swirl
can clearly be seen on radar. Back door cold front has dropped into
NC keeping the TSRA limited to the I-85 corridor and south. Expect
the showers and isolated thunderstorms to move east of the area this
evening. Until then, isolated flooding cannot be ruled out in the
heavier showers. Will keep the watch in place for now, but may be
able to cancel it this evening when the precip moves east.

Scattered showers along the cold front to our west may move into the
mountains overnight, so have kept a small chance there even as
precip chances end outside of the mountains. Do not expect any heavy
rain from these showers.

Ample low level moisture remains in place overnight. With the
mid/upper levels drying out and a wet ground (esp across western
areas) fog and low stratus may become problematic late tonight/early

Surface boundary loiters near the southern half of the forecast area
into Monday afternoon, with short term guidance (especially the GFS)
depicting some rather healthy sbCAPE. Mesoscale/high res and short
term guidance are a bit at odds regarding the potential for deep
convection tomorrow afternoon, but chance pops are nevertheless
advertised. Max temps should return to around normal levels in all
areas tomorrow.


As of 200 PM EDT Sunday: Cycloidal motion will continue among the
closed low centers over the Great Lakes Monday night, with
associated vorticity lobes digging southward across the Midwest and
sharpening up the central CONUS trough through Tuesday. An MCV, or
other small-scale wave, developing in the base of the trough over
the lower Mississippi River Valley Monday night will likely move
quickly into the forecast area from the southwest around daybreak
Tuesday. The next slug of deeper moisture will accompany this
feature across the forecast area through Tuesday. QPF should be
enhanced as a low level circulation associated with this feature
rides northeast along the stalled frontal boundary and generally
moves up the I-85 corridor. Many locations will see another 1 to 2
inches of rain from late Monday night into Tuesday night, and the
hydrology threat will increase once again. Mid-level lapse rates
will be fairly modest on Tuesday and widespread clouds and
precipitation will limit instability, but backing and strengthening
flow will improve shear and the overlap of any instability with the
brief spike of 30 to 40 kt surface to 3 km bulk shear could produce
some rotating updrafts and isolated severe weather.

The upper trough will then sharpen up to the west Tuesday night
through Wednesday and deepen into a closed low over Illinois. Yet
another round of deeper moisture should surge northward across the
forecast area Wednesday to continue the threat of heavy rainfall.
Locations southeast of I-85 could also see sbCAPE recovery to 1000
to 2000 J/kg by late Wednesday afternoon just ahead of the next
passing cold front. Given the ongoing flooding threat in the near
term, no new Flood Watches will be featured yet for Tuesday or
Wednesday, but the HWO mention of heavy rain will continue.


As of 200 PM EDT Sunday: The sharp upper trough will finally migrate
east of the Appalachians on Thursday. Steep lapse rates of 7.0+ deg
C/km will accompany the passing trough axis, but low level flow will
veer more westerly post-fropa and lower levels will start to dry
out. Scattered showers and thunderstorms may continue during the day
Thursday, and northwest upslope flow moisture may keep some western
mountain showers going into Thursday night.

Broad ridging between systems is expected over the southeast Friday
through Saturday. Northwesterly downsloping flow will aid drying and
warming somewhat on Friday before low level flow starts to back
southwest into next weekend. The ridge will then flatten out on
Sunday with a return of embedded shortwaves in deep layer WSW flow
aloft. Instability will also build over the southeast half ahead of
the next cold front approaching from the northwest and moisture
return will be slightly better over the higher terrain to warrant
chance convective PoPs over the mountains and slight chances in the
piedmont. Temperatures will steadily warm through the period.


At KCLT and Elsewhere: Showers and isolated thunderstorms will
slowly move east of the area this evening, and have already moved
east of KAND/KAVL. Expect MVFR to IFR restrictions until the showers
move east, then a brief reprieve may develop before restrictions
return. Expect MVFR vsby and IFR cigs through much of the night as
fog and stratus develop. IFR vsby expected at KAVL/KHKY. Conditions
slowly improve to VFR vsby and MVFR cigs through the morning with
low VFR expected around noon. Scattered convection is expected again
Monday afternoon, so a PROB30 is in place. NE to E winds will slowly
turn to S then SW through the night as a wave of low pressure moves
east. KAVL will see NW wind. SW wind continues through the day,
except NW at KAVL.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through about Wednesday.
Drying is expected to finally occur Thu through the end of the week.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High  94%     Med   78%     Med   70%     High 100%
KGSP       High  94%     High  95%     Low   36%     High 100%
KAVL       Low   58%     Med   61%     Med   70%     High 100%
KHKY       High  94%     Med   63%     Low   48%     High 100%
KGMU       Med   64%     Med   65%     Low   41%     High 100%
KAND       Med   66%     High 100%     Low   38%     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...Flash Flood Watch until 2 AM EDT Monday for NCZ033-049-050-053-
SC...Flash Flood Watch until 2 AM EDT Monday for SCZ003.


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