Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220547

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
147 AM EDT Mon May 22 2017

Low pressure will form today along a stalled front over the northern
Gulf coast and track northeast across our region tonight. A deep
upper trough forms over the Mississippi Valley Tuesday night and
Wednesday with the axis crossing the southern Appalachians early
Thursday. This results in cooler than normal temperatures until an
upper ridge builds over our area for the weekend bringing a return
to Spring temperatures.


Meso-low is currently located near KUZA and the swirl can clearly be
seen on radar. Moderate to isolated heavy showers will slowly move
east of the area during the rest of the evening with only isolated
mainly light showers behind for the rest of the night. Cannot rule
out some isolated flooding until the low moves east, but the
heaviest rain will be over the CLT Metro area and east. Do not
expect any additional heavy rainfall over the watch area. Therefore,
will cancel the watch early.

Scattered showers along the cold front to our west may move into the
mountains overnight, so have kept a small chance there. 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 145 AM Monday:  The axis of the deep upper trough will pass
over the western Carolinas either Thursday morning or around mid day
Thursday as it moves northeast. Although the ECMWF very little
instability at 18Z Thursday, the GFS has a core of CAPE over I-77
just north of CLT. The 18Z run of the GFS had around 600 CAPE and
the newest run now has around 1000 CAPE from north of CLT to near
the VA border. With the lower freezing level under the upper trough,
this core of higher CAPE may help generate some convection which
would result in hail producers. This higher CAPE will shift to the
RDU and north by 00Z Friday with heights building in from the west
as the upper trough leaves our region.

Weak high pressure dominates our weather from Thursday night through
Saturday night. The shallow upper ridge that forms in the wake of
the departure of the upper trough deamplifies late Saturday and
flattens out for zonal west to east flow on Sunday. A series of
shortwaves pass through west to east next week with the first
bringing convection to the mountains by late Sunday and over all
areas Sunday night. Instability will be increasing from the west
little by little but will moisture arrive in time to result in
storms late Sunday. At this time we are going with slight to low
chance of that happening.

Temperatures start out almost 10 degrees below normal Thursday
rising to above normal over the weekend.


At KCLT and elsewhere: Still some lingering -SHRA/VCSH around/E of
KCLT, but otherwise precip has moved out for now. Widespread MVFR to
IFR cigs are expected through the morning hours, though Upstate TAFs
are currently VFR (expect them to drop again). Could see some
occasional LIFR as well before improvement after sunrise to VFR.
Convection chances increase this afternoon and evening and have
extended the previous PROB30s later into the period. Cigs should
drop again to at least MVFR again toward the end of the period.
Winds will be VRB today, but favoring S early in the period for all
but KAVL where WNW winds prevail, veering more SW by daybreak, and
then either backing rapidly SE this evening, swinging around toward
NE toward the end of the period.

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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High  90%     High 100%     High 100%
KGSP       Med   62%     Med   75%     High  95%     High 100%
KAVL       High  81%     High  80%     High  85%     High  94%
KHKY       Med   62%     Med   75%     High 100%     High 100%
KGMU       Med   62%     High  80%     High  95%     High 100%
KAND       High 100%     High  85%     High 100%     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:




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