Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220011

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
811 PM EDT Mon May 21 2018

A moist air mass will remain in control of our weather through at
least Tuesday, bringing a continued high chance of showers and
storms in the afternoon and evening. A weak front sliding down on
Thursday may allow for some brief drying toward the end of the work
week. Expect another round of tropical moisture to lift across the
area from the Gulf for the weekend.


As of 730 PM EDT: Coverage remains fairly minimal around the region
as many foothill and piedmont locations saw dewpoints mix out into
the lower 60s. Coverage will be best through the mid evening across
the NC piedmont where outflows will interact with some lingering
1500+ J/kg sbCAPE values. Slow storm movements and backbuilding
favored profiles could still yield some isolated flooding through
02Z. Expect the convection to diminish quickly from 02Z to 04Z.

We will see a steady S to SW flow continue across the area
overnight, as 500mb heights fall slightly with a shortwave entering
the Ohio Valley. Several guidance sources develop light showers over
the southern part of the area as 850mb moisture advection increases,
though forcing really remains rather weak overall. A slight chance
PoP will be retained for mainly souther mountain sections overnight.
Some patchy fog is possible if the mid/high clouds thin sufficiently
and lowering stratus may return from the southwest toward daybreak.

Given rebounding dewpoints and ever so slightly better lapse rates
aloft thanks to the aforementioned shortwave, precip chances are
much better Tuesday than those today: likely for the western half of
the area and a nearly 50/50 chance over our east. Mean PWAT
anomalies from the SREF rise above 1 SD for the whole area, with
parts of the Piedmont above 2 SD. Given weak storm motion vectors,
weakly organized clusters of cells producing heavy rain will
continue to be our main concern from the developing convection. Max
temps will be near normal in the east and slightly below normal
elsewhere due to cloud cover.


As of 210 pm Monday: Frontal zone/pre-frontal trough will ooze
into/across the forecast area on Wednesday, providing the primary
focus for diurnal convective development, supporting 40-60 pops
across the area during the afternoon and early evening. Moderate
CAPE and weakly sheared environment will support a localized pulse-y
severe threat as well as the potential for additional localized
hydro issues in locations that have poor antecedent conditions.

On Thursday, short term guidance pushes the lower theta-e/post
frontal air mass more solidly into the forecast area, supporting
lower coverage of diurnal convection, especially across northern
areas. Nevertheless, ingredients are sufficient to support 20-40
pops across much of the area during the afternoon/evening. Max temps
will be a couple of degrees above climo and min temps about 5
degrees above climo through the period.


As of 225 pm Monday: Weak boundary will remain over the area early
in the forecast period, possibly providing focus for another round
of diurnal convection on Saturday. Meanwhile, the axis of the upper
ridge is forecast to build over the area during the weekend, while
some guidance builds the low theta-e air mass associated with
Bermuda high-associated surface ridge into the Carolinas. Although
this would argue for less overall coverage of diurnal convection,
there are also indications that a SE upslope flow may develop as
well. Therefore, diurnal convective chances are maintained Sat

For late in the weekend into early next week, much depends upon how
tropical weather will evolve over the next few days, specifically
regarding the area of disturbed weather currently located across the
western Caribbean. There is a decent consensus among the ECMWF and
Canadian global models (as well as the GFS ensemble system) that a
tropical cyclone will be in the vicinity of the lower Miss Valley on
Sunday. These guidance sources also depict a rather large shield of
moisture associated with the cyclone, which would possibly impact
our area for Days 6/7. For the time being, this will be reflected in
the current official forecast as enhanced chances (generally 60-70%)
for mainly diurnal convection Sunday afternoon into Monday evening.
However, forecast trends will need to be monitored closely for a
potentially more organized heavy rainfall threat early next week.
Otherwise, temps are forecast to remain above normal through the


At KCLT and elsewhere: TSRA appears confined mainly to the KCLT
to KHKY area but with activity moving very slowly. Additional TSRA
should miss KHKY, but a short fused TEMPO for TSRA could be needed
soon at KCLT if the storms continue to drift into Mecklenburg County
from the south. Otherwise, TSRA should diminish through late
evening. Profiles exhibit some risk of developing MVFR stratus
overnight in the low level moist southerly flow, but the models and
MOS have been overdone with this and the fog potential recently, so
will just hint that way at all but KAND and KAVL overnight, where
MVFR conditions are more likely in returning moisture. Convective
chances across the area will be notably higher on Tuesday, given the
return of better sfc moisture and slightly better mid-level lapse
rates. This warrants a fairly broad SHRA/TSRA mention for the
afternoon hours. Light S to SW flow will continue.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under periodic showers
and thunderstorms. Morning stratus/fog are possible especially
following heavy rain the previous day.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  98%     High  88%     High 100%
KGSP       High 100%     High  85%     Med   79%     High  86%
KAVL       High 100%     Med   68%     Med   77%     High  98%
KHKY       High 100%     High  93%     High  91%     High  97%
KGMU       High 100%     High  86%     Med   78%     High  86%
KAND       High 100%     High  93%     High  87%     Med   72%

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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