Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200700

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
300 AM EDT SAT AUG 20 2016

Warm and humid conditions will continue today as subtropical high
pressure lingers over the area. A cold front will cross the
region from the northwest late Sunday bringing cooler and drier
air for most of next week.


As of 245 AM EDT Saturday: Plenty of mid-level cloudiness across the
area at AFD time, with some low stratus across the mountain valleys
and across the Upstate where widespread heavy rain fell yesterday.
Case in point, KAVL had 1.53" of rain which set a record and brings
that site to within 2" of the yearly normal. We have very little
change to the overall pattern today, with a deep trough pushing
across the Upper Mississippi Valley today and the remnants of an old
front stretched across the Carolinas, providing additional focus for
convective development. A bit of a shortwave impulse will move
across the Mid-South this morning and near-term guidance is wanting
to bring some sort of remnant MCS or MCV across the Tennessee
Valley, which may induce shortwave ridging in the lee of the
Appalachians. This shows up slightly with some midlevel warming
in sounding progs for this afternoon, and would really be the only
inhibiting factor for convection today.

PWs are high, though not anomalously so, ranging between 1.75-1.8"
and thereabouts, with a very tropical-looking profile. As with
yesterday, heavy rain will be the main threat with these garden-
variety thunderstorms. Should be enough movement to the storms (with
30kt westerlies aloft) that stationary storms wouldn`t be an issue,
but rather training as clusters of showers/storms continue to develop
upstream. Expect pretty healthy coverage especially across the
mountains, and with the rain that fell across the Upstate yesterday,
the additional boundary layer moisture may provide some localized
surface moisture this afternoon that will help with convective
initiation. All that said, the new HRRR is backing off somewhat on
coverage of afternoon convection so we will have to monitor this
through the day.

Should see convection begin to wane somewhat late in the evening and
overnight, much like tonight, but with the approach of the longwave
trough late tonight, convection may be able to fill in again fairly
rapidly as we push into the short term.

With widespread cloud cover and expected convection today, should
see a reduced diurnal trend in temperatures with highs a couple of
degrees below seasonal normals.


As of 200 AM EDT Saturday...The Short Term starts at 12z Sunday,
with a cold front just west of the Appalachians. The band of
moisture associated with the front will become narrower, as flow
veers to westerly and cuts off the better moisture transport out of
the Gulf. There will be decent upper support with a fast-moving
upper trough pushing across the Ohio Valley and Great Lakes.
However, the bulk of the deep-layer Q-vector convergence and
upper-level divergence will lift north of the area. So while a band
of showers and tstms is expected to accompany the front, severe
weather looks limited. If the developing downslope flow Sunday
afternoon, mixes out the dewpts, much of the Piedmont may see little
rain. For now, guidance keeps dewpts in the upper 60s to lower 70s,
and results in SBCAPE of around 1500-2000 J/kg. The operational
models vary in the bulk shear, especially east of the mountains,
generally in the 25-40 kt range. So some degree of organization is
possible, but the new Day 2 Convective Outlook still has our area in
a general thunder risk. A gradient of PoPs from categorical along
the TN line to mid-range chance in the southern Upstate will be
featured with this FROPA.

Dry air will filter in quickly behind the front Sunday night, so by
daybreak Monday most of the area should notice slightly cooler
readings and less humidity than we`ve seen in weeks. Lows Monday
morning are expected to dip into the 50s to lower 60s in the
mountains, and mid to upper 60s across the Piedmont. Under mostly
sunny skies and dry conditions, highs Monday may actually be a
category below normal with most of the area in the 80s.


As of 230 AM EDT Saturday...An upper-level ridge will build back
over the eastern CONUS, as a trough digs over the Rockies and the
Northern Plains. Meanwhile, the relatively dry surface high will
gradually drift east from the Appalachians early Tuesday to off the
Eastern Seaboard by Thursday. Low-level flow will struggle to tap
into much deep moisture. However, the medium range guidance does
bring higher dewpts back into the area and a gradual return of
diurnal convection, mainly across the western half of the CWFA
Tuesday thru Thursday. I think the PoPs may be overdone, but
Superblend has chance PoPs for diurnally driven convection across
the west by Wednesday. A cold front will approach from the northwest
Friday, with PoPs back into the high-end chance range in the
mountains. Temps will be seasonably cool for late August...near to
slightly below normal Tuesday and Wednesday, then return to slightly
above normal by Friday.


At KCLT and elsewhere: Convection has dissipated for the evening,
but with plentiful low-level moisture, expect patchy MVFR fog around
the area in the pre-dawn hours. Cannot rule out brief IFR conditions
especially if low stratus across the Upstate manages to work its way
over toward KCLT. Otherwise biggest concern is afternoon convection
but of the entire area, KCLT has the least chances so continued
PROB30 TSRA wording, and will refine as necessary with the 12z TAFs.
Otherwise, generally SW winds through the period around 5kt or less,
though expect some LGT/VRB overnight.

Elsewhere: As with KCLT, convection has dissipated for the evening,
replaced by increasing low stratus across the mountain valleys and
creeping into the Upstate. KAND/KAVL already IFR/LIFR respectively
and expect this will continue through the overnight hours. Included
TEMPO IFR conditions for KGSP/KGMU as well. KHKY the wild card since
they did not see any rain yesterday so kept MVFR vsby and hinted at
cigs with FEW004. Kept previous trend of PROB30 everywhere this
afternoon but went prevailing -SHRA with some VCTS as well. As with
KCLT, generally SW winds expected (LGT/VRB overnight).

Outlook: A cold front will push in from the northwest on Sunday,
enhancing the chance for convection. Much drier air will filter in
behind the front Monday, resulting in fair weather through midweek.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  83%     High  87%     High 100%     High 100%
KGSP       High  90%     High  98%     High 100%     High 100%
KAVL       Med   62%     Med   79%     High  97%     High 100%
KHKY       High  95%     High  99%     High  96%     High 100%
KGMU       High  95%     High 100%     High 100%     High 100%
KAND       Low   56%     High  90%     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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