Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181830

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
230 PM EDT Thu May 18 2017

Moisture will continue to increase atop the region through Saturday
resulting in increasing chances for mainly afternoon and evening
showers and thunderstorms each day. Also on Saturday, a cold front
may drop down from the north into the region before stalling
Saturday night, then wash out Sunday. Another cold front approaches
the region from the west Sunday night, and then should move across
the region on Monday.


Considerable cloud cover has limited instability this afternoon,
with sbCAPE of only 1000-1500 J/kg (much less than early morning
model runs were indicating) depicted across most of the area per the
latest SPC mesoanalysis. A few very isolated showers have popped up
across the southern Appalachians and the upper Savannah River Valley
this afternoon, and activity should gradually expand in these areas
through the afternoon. However, the severe weather potential, such
as it is, appears to be waning, as it is doubtful that instability
will increase appreciably over what is currently being observed. Any
convection should diminish fairly quickly this evening, giving way
to another unseasonably warm night.

Less cloud cover and improving mid-level lapse rates should yield
considerably higher instability Friday afternoon, esp across the
mtns. Meanwhile, some of the lower theta-e air within the ridge axis
associated with Bermuda high is expected to find its way into the
southern part of the forecast area. Therefore, a fairly tight
gradient in pop, ranging from 50-60 across the NC mtns, to less than
slight chance southeast of I-85 appears in order. More robust
buoyancy along with slightly improved shear profiles may also yield
improving severe weather chances, although the threat should tend
toward the pulse end of the spectrum. With less cloud cover
anticipated, all areas should resume the trend of max temps 5-10
degrees above climo.


As of 210 PM Thursday: The southeast ridge will become slightly
resurgent Friday night through Saturday as the deep plains system
lifts northeast toward the western Great Lakes. Near the surface, a
backdoor boundary will sag southward toward or into the western NC
piedmont by late Saturday. Some model differences remain, with the
NAM keeping the boundary stalled near Davie County while the GFS
allows for better progress southwest toward the SC state border. A
blend of model solutions will be featured with a swath of 2000+ J/kg
sbCAPE likely developing somewhere along or south of I-40 just ahead
of the front. In addition, mountain lapse rates will steepen to
nearly 7 deg C/km by late in the day on Saturday. All told, will
advertise solid chance convective PoPs for most of western NC
Saturday afternoon/evening and taper down to slight chance across
the Upstate and GA mountains and dry in the lower piedmont. Max
temps will remain 5 to 10 degrees above climo all but near the I-40
corridor Saturday afternoon.

The low level upslope component will increase into the eastern and
southern slopes of the mountains Saturday night through Sunday. The
increased cloudiness will lead to cooler daytime temperatures and a
steady uptick in PoP into the likely range across the mountains,
with chance east, by late day for showers with embedded
thunderstorms. Deeper forcing ahead of the low lifting into the
western Great lakes will then arrive in the southern Appalachians
late Sunday into Sunday night. The period of heaviest rainfall
Sunday night just ahead of the approaching cold front should be of
short enough duration to preclude any significant hydro issues
around the forecast area. However, a nearly 2-inch QPF maximum will
is forecast in the southerly upslope areas. Shear will also briefly
improve Sunday night, but nothing too impressive is expected as the
better dynamics lifts well to the northwest, and instability will be
limited overnight. The cold front should reach the mountains by
daybreak Monday.


As of 130 PM Thursday...Reasonable confidence exists with respect to
the timing of the eastward translation of moderately forced frontal
zone which is progged to be downstream of the cwfa by Monday night
along with the respective fast decrease in additional rainfall
chances.  For the new Day 5-7 period, the next round of unsettled
weather is shaping up for some period during the middle of next week
as rather deep mid-Conus troughing wobbles it`s way eastward. At
some point, deeper moisture and warm conveyor belt pcpn should
overspread the region with national guidance and blended model
solutions carrying the greatest chance on Wednesday.  Small pops
will be maintained on Tuesday and also introduced for Thursday
although either one or both of these days could easily wind up being
dry. Temperatures are slated to be much closer to climo versus what
we have been experiencing this week.


At KCLT and elsewhere: convection was developing over the high
terrain of western NC early this afternoon, but coverage was spotty
at best owing to considerable cloud cover that has lingered since
this morning. Coverage is expected to gradually expand, mainly
across the mtns and into the Upper Savannah River Valley, and
VCSH/VCTS has been included this afternoon at KAND and KAVL. Cannot
rule out a stray shower elsewhere, but see no evidence that a
mention of convection is warranted at the other terminals.

Otherwise, lingering MVFR cigs should lift to VFR at the Upstate SC
terminals by 20Z or so. Areas of VFR clouds will likely continue
into the overnight. So while moisture levels are expected to be high
enough to support some light fog, have omitted any mention other
than a 6sm here and there. The exception is at KAVL, where MVFR is
advertised, but conditions could be better or worse depending upon
the degree of cloud cover. Otherwise, winds will remain SW through
the period.

Outlook: chances for diurnal convection and restrictions will
increase on Friday and Saturday ahead of a backdoor cold front that
approaches the forecast area from the north over the weekend.
Another front is expected to bring more significant chances of
showers/storms/restrictions early next week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  89%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  91%     High  97%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  90%     High 100%     High 100%     High 100%
KAND       High  90%     High 100%     High  97%     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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