Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211842

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
242 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 230 PM: Destabilization is underway across the area, though
cu field remains very spotty over the Upstate, where SBCAPE is
smallest on SPC mesoanalysis, and dewpoints remain relatively
low. Over the mountains and I-40 corridor, however, moisture
pooling near a weak sfc boundary has resulted in more instability
and more frequent initiation. Still looks like t-storm chances
are best in these areas; another PoP update has been completed to
highlight this. That said, we can`t rule out an isolated shower
or even t-storm over the southern half of the area. Max temps are
on track to top out a little above normal.

This evening and overnight, we will see a steady S to SW flow
continue across the area, 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 this
evening as 850mb moisture advection increases, though forcing
really remains rather weak overall. Activity should diminish after
midnight, but we`ll retain a slight chance in all zones since we`ve
seen a few nocturnal showers each night in this moist pattern. By
morning models begin to key on the enhanced upslope lift over the
western third of the area, increasing PoPs earlier than the diurnal
curve. Overall, given rebounding dewpoints and ever so slightly
better lapse rates aloft thanks to that shortwave, precip chances
are much better 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 this afternoon are most likely
along and north of Interstate 40. This warrants VCTS for
KAVL/KHKY. Elsewhere a few SHRA could occur though impacts should
be low. Light southerly low-level flow will continue tonight and
could produce evening showers over the SC sites and KCLT. Again,
impacts from these are not expected to be remarkable except
perhaps brief MVFR vsby. While the aforementioned flow will bring
additional moisture overnight, forcing will be very limited, and
guidance expectations for low clouds look overdone. Only the more
southerly sites, KGSP/KGMU/KAND, are expected to see restrictions
from stratus. Following persistence, KAVL looks to experience
fog development again. Convective chances across the area will be
notably higher on Tuesday, given the return of better sfc moisture
and slightly better midlevel lapse rates. This warrants SHRA VCTS
at some sites, and PROB30 TSRA at others.

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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High  97%
KGSP       High 100%     High 100%     High  95%     High  91%
KAVL       High 100%     High  97%     Med   62%     Med   76%
KHKY       High 100%     High 100%     High  96%     High  92%
KGMU       High 100%     High 100%     High  95%     High  89%
KAND       High 100%     High 100%     High  93%     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:




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