Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210737

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
337 AM EDT SAT MAY 21 2016

A cold front will sweep across the area today. Drier high pressure
slowly builds into the region through early next week. A moist
southerly flow develops by mid week and continues into the weekend.


As of 300 AM: Cold air damming is in its waning stage at this point;
the parent high sits offshore from NY/NJ. A deep upper trough will
swing from the Ohio Valley toward the East Coast today, leading
to veering low level winds and a loss of isentropic lift over the
wedge. This process is already underway, as much of the area has
seen cloud bases lift since sunset. As it continues, the low clouds
should thin and begin to break up (by late morning). A few showers
are possible until then, particularly over the I-77 corridor where
moisture remains deepest. Some patchy dense fog may also develop
near daybreak in this area, but confidence on this is still too
low for any kind of headline. I will keep monitoring obs trends.

The best upper Q-vector convergence over the area will be early
this morning, but the presence of the trough overhead later in the
day will bring good mid to upper level lapse rates. Cyclonic flow
aloft could also promote lift to some degree. Model consensus is
that as much as 1500 J/kg of SBCAPE will develop by mid-afternoon
over the Piedmont. Several WRF runs show convection firing along
the Ridge and moving/propagating southeast into this area as
a multicell cluster. Indeed consensus is for 40-50 kt of 0-6km
bulk shear which would support long-lived updrafts and upscale
growth. Freezing levels are fairly low for the season, and the
mid-upper levels become very dry as the trough axis passes. It
appears a few storms may be capable of strong wind gusts and large
hail. Coverage however will be limited by downsloping flow. SPC
has expanded the marginal risk area into much of our Piedmont with
the new Day 1 outlook issued at 06z. Max temps will warm back up
to 3-5 degrees below normal.

Low level winds veer to northwest as the trough moves east of
the area this evening, and this ushers in cold advection and
strengthens downsloping over most of the area. Drying/clearing
conditions are expected, though profiles will still support clouds
and a few showers along the spine of the Appalachians, near the
TN/NC border. Temperatures will be near normal, with downsloping
offsetting the effects of the incoming cooler air.


As of 230 AM Saturday, an upper low drops into Eastern VA Sunday and
to Eastern NC Sunday night. Short wave rotate around the upper low
and across the area. Cold mid-level temps will rotate into the area
as well. Despite the colder temps...surface dew points will be
relatively dry keeping instability weak. Best chance of convection
will be across the NC Mountains where NW upslope flow will help
produce lift. Second area for showers will be the I-40 and I-77
corridors closer to the center of the upper low. Expect windy
conditions across the NC Mountains and breezy conditions elsewhere
as deep mixing taps the relatively strong low level winds. Gusts
should remain below advisory level. The upper low pulls away from
the area Monday and Monday night bringing warmer mid level temps and
rising heights. This should bring and end to any convection chances.
Winds will diminish as well. Below normal temps on Sunday will rise
back to near normal for Monday.


As of 305 AM Saturday, weak upper ridging builds over the area
Tuesday and remains through the end of the period. There will be
some weak short waves moving through the ridge and across the area
from Wednesday on. At the surface, dry high pressure builds in on
Tuesday then slides off shore on Wednesday remaining there through
the end of the period. Weak but increasingly moist southerly flow
develops through the period as a result. This will lead to
increasing chances of diurnal convection with the best chances
across the mountains and foothills where moisture will be greatest.
Temps rise a little above normal on Tuesday then rise a few degrees
through the end of the period.


At KCLT and elsewhere: CAD wedge will diminish in strength this
morning. Cigs are already lifting over the terminals as the damming
high moves further off the East Coast and as low level winds veer
over the wedge. Vsbys acrs the area range from VFR to IFR. There is
a signal from some of the hi-res guidance that dense fog may develop
over the NC Piedmont close to daybreak, where precip continued
well into the evening. This possibility is reflected in KCLT and
KHKY TAFs, with lower-impact fog mentioned elsewhere. Clouds will
lift and thin appreciably after daybreak, but widespread cumulus
are expected to develop with diurnal heating. SCT SHRA/TSRA will
subsequently develop, with convection allowing models in agreement
that initiation will occur near the Blue Ridge in the early aftn
and propagate southeastward into the NC/SC Piedmont. Timing of
PROB30s has been aligned accordingly. Strong wind gusts and a
couple of hail producers are possible.

Outlook: Conditions should gradually dry out Sunday and Monday. AFTN
showers and TSTM chances may return on Wednesday.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       Med   76%     High  86%     High 100%     High 100%
KGSP       High  80%     High  87%     High 100%     High 100%
KAVL       High  86%     High  87%     High 100%     High  97%
KHKY       Low   58%     High  84%     High 100%     High 100%
KGMU       High  84%     High  82%     High 100%     High 100%
KAND       High  85%     High  85%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the schedule 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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