Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 182229

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
629 PM EDT Tue Jul 18 2017

A stalled frontal boundary will slowly dissipate over the next
few days as a strong upper ridge gradually builds into the region.
This will bring hot temperatures and a reduction in the coverage of
diurnal showers and thunderstorms through the end of the work week.
Temperatures will begin to moderate by the weekend while enhanced
thunderstorm chances return to the forecast early next week.


As of 530 pm EDT: Scattered thunderstorms continue to develop
across the region, with most of the activity over the mtns/foothills
and then over the NC Piedmont, with a large area with very little
activity over the middle of the fcst area. The environment suggests
that convection may continue to develop and move south across the
Piedmont east of I-77 through sunset, and then across the western
Upstate/upper Savannah Basin/srn mtns of NC. Precip prob will be
adjusted accordingly. Cannot rule out a few more severe storms
given all the mid-level dry air seen on water vapor imagery and
dCAPE above 800 J/kg across the entire fcst area.

Otherwise...a closing upper-level low pressure system will migrate
slowly east across the piedmont and coastal plain of the Carolinas
tonight through Wednesday. Meanwhile, heights will steadily recover
from the west. A weak vorticity lobe will cross the mountains on
the back side of the main trough late today and this feature, along
with weak upslope flow, will generate scattered convection into
the evening along and east of the Blue Ridge. After the convection
wanes, PoPs will diminish greatly later this evening and overnight,
with the main overnight issue likely being low clouds and fog
in the mountain river valleys. Low clouds east of the mountains
should be less widespread given the slightly drier boundary layer.

On Wednesday, profiles exhibit less instability, lapse rates are
more modest, and triggering should be weaker. Scattered ridge top
coverage still seems likely as sbCAPE rebounds to 1500+ J/kg in
many areas. 850 mb flow will acquire a light downsloping component
to further limit PoPs east of the high terrain. Anticipate max
temps another 2 degrees above current day maxes, with Heat Index
values likely reaching the upper 90s.


As of 230 PM EDT Tuesday:  The short term fcst period kicks off on
Wednesday evening amidst gradually building heights from the west
as a large upper anticyclone strengthens over the plains, while an
H5 depression/low slowly meanders along the Carolina coastline.
At the surface, high pressure anchored over the western Atlantic
will remain dominant, likely suppressing convection to some degree.
With that, the fcst will kick off to feature low end pops,
mainly along the high terrain where inhibition will be weakest.
Said pops will taper into the overnight leading into a dry fcst for
Thursday morning.  By that time the upper ridge will have shifted
over the MidSouth region with enough warming aloft to suppress
convection across much if not all of the fcst area for Thursday
therefore pops will remain below mentionable levels.

Meanwhile, the aforementioned H5 depression is progged to
retrograde westward along the SC/FL coastline, eventually moving
inland by Friday.  This feature looks to complicate fcst somewhat
as it pertains to excessive heat, especially on Friday when the
upper ridge weakens slightly as these two features interact.
As a result, fcst pops will be slightly higher on Friday as deep
subsidence is weaker across the mtns/fthills, however remaining dry
over the piedmont.  As for temperatures and heat indices, expecting
highs in the lower/mid 90s across the low terrain, with upper 80s in
the mtn valleys on Thursday.  Further warming is fcst on Friday when
maxes push into the mid/upper 90s for the low terrain, while lower
90s are favored in the mtn valleys.  Given very moist dewpoints
and minimal mixing, heat index values for Thursday and Friday will
be critical, with values approaching 100 and 105 respectively,
the highest of those indices to be observed along/south of I85.


As of 230 PM EDT Tuesday: Large shifts in the guidance with this
medium range package. Previous forecasts hinged on the expansion of
a strong upper level ridge into the southeast as an upper level
trough axis shifted eastward into the Atlantic. For the latest
cycles though, all major models are now showing the upper trough
over the east coast pinching off an upper level disturbance that
hangs over the southeast and migrates towards the Gulf coast through
the weekend. This will prevent major height rises over the weekend,
so temperatures were trimmed down a few degrees on Saturday and
Sunday, while pops were raised to come closer to climo. Upper level
troughing will return early next week, and with it more moderate
temperatures and a surface front that will provide a focus for some
enhanced diurnal convection, especially Tuesday afternoon.


At KCLT and elsewhere: Convection has thus far has been confined to
the immediate Blue Ridge area this afternoon, but the convection
allowing models bring some of this activity east over the foothills
and into the piedmont through early evening. TSRA chances will be
best from KAVL to KHKY, but will continue to ride on TEMPO TSRA at
KCLT given the light northwest steering flow. Will maintain VCTS at
the Upstate sites. The mountain valleys will once again see the best
chance of low clouds and fog, but less difficulty is expected east
of the mountains where the boundary layer will be a bit drier. Will
keep the forecast dry through 18Z Wed with any isolated to scattered
mountain storms expected later in the day. Light winds will remain
mainly southerly, turning more westerly on Wed.

Outlook: Isolated to scattered afternoon and evening thunderstorms
are expected on Wednesday, with better coverage over the mountains.
Morning fog development can be expected in the mountain valleys and
in areas that receive rainfall the previous afternoon/evening. Drier
air will likely limit convective coverage for Thursday, with typical
mid-summer conditions returning by Saturday.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  91%     High  91%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  91%     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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