Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210607

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

A wedge of cool high pressure will remain entrenched over the
Carolinas and northeast Georgia through early tomorrow. A cold front
will then sweep across the area later in the day, leading to partial
clearing. Drier high pressure slowly builds into the region early
next week.


As of 145 AM: Base of upper trough is now over the lower Ohio
Valley, with attendant sfc low centered over eastern KY. Some
degree of dynamic lift, combined with a small amount of buoyancy
atop the wedge inversion (per RAP) continues to drive light
showers over parts of the CWFA. Said buoyancy diminishes toward
dawn, so coverage is expected to decrease over the next few hrs.
The low stratus has lifted in some areas, but skies are expected
to remain cloudy until at least a couple hrs after daybreak. Dense
fog has not yet developed on any area ASOS/AWOS readings, and
guidance does not give a strong signal that it will occur even if
wedge boundary does slip northward. Some of the meso models key on
slightly cooler/wetter conditions over the I-77 corridor and
depict an expanding area of dense fog after 09z. That is plausible
and has been reflected in the grids, still using "areas" wording.

Saturday morning, the low pressure and associated cold front will
exit east of the area, switching the winds around out of the NW.
As is typical with CAD erosion, temperatures should be warmer
*behind* the cold front than ahead of it, with actually a fairly
pleasant day in store for Saturday, at least for the most part.

However, lapse rates will steepen through the day as the upper
trough continues to dive south, with intermittent vorticity lobes
rotating around it. With the steepening lapse rates plus breaks
in the clouds leading to increased sunshine and thus increased
instability, we should see a corresponding increase in thunderstorm
chances Saturday afternoon as well. Best chances will be either
over the mountains (closer to the upper trough) or across generally
eastern Upstate SC and the I-77 corridor ahead of the mesolow that
is progged to form in the lee of the Appalachians sometime midday.
Both the GFS and NAM increase sbCAPE across those eastern zones to
up to 1500 J/kg, with corresponding deep-layer shear of 45kt. SPC
Day2 outlook only has us in general thunder but thinking that
chances for strong to even severe thunderstorms are a little higher
than that, though WRF ARW/NMM reflectivity forecasts only develop
isolated convection over our area. The concern here is that the
best chances of thunder will be near the Charlotte metro area where
there is most definitely a public safety concern over the weekend,
so will further refine this forecast with the overnight package.


As of 230 PM looks like the second half of the weekend
and the first part of next week will be dominated by an upper low
that closes off over VA and then rotates down across the Tidewater
region Sunday night and off the Mid-Atlantic coast on Monday. Within
this flow regime, a fairly strong short wave will rotate down across
the mountains late Saturday night, bringing a swath of DPVA
overhead. For that reason, a precip chance will be retained across
the mountains with a larger expanse than usual through early Sunday
morning. Should be breezy over the higher terrain early Sunday with
the passage of the wave, but not to the point where an advisory
would be needed. Once the wave passes, we should see the usual N/NW
flow upslope cloudiness and shower activity on the TN border thru
Sunday. East of the Blue Ridge, strong downslope should put a damper
on most of the potential for convection, but an isolated shower will
not be ruled out over the western Piedmont of NC. The upslope
showers on the TN border should end after sunset, along with any
activity over the Piedmont. The rest of the period...through
Monday...should be dry. The upper low should keep temps below normal
through the period...altho warming up almost back to normal Monday


As of 100 PM Friday, still expecting a return to more summerlike
conditions as next week wears on.  Temperatures should rebound to
above normal levels on Tuesday, but with only nominal return flow
developing and ridging aloft, deep convection should still be
suppressed. A bit warmer and moist on Wednesday with the suppressive
effects of upper ridging beginning to lessen. Small and/or slight
chances for diurnal tstms will remain limited to the mountains.

For the latter half of the week...the pattern atop the SE conus does
not change very much with the return flow around Bermuda high
pressure continuing.  The above climo temperatures should remain for
Thursday and Friday with daily diurnal deep convective chances each
day cwfa wide.


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...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Med   77%     High  84%     High 100%     High 100%
KGSP       High  83%     High  89%     High 100%     High 100%
KAVL       High  89%     High  84%     High 100%     High  96%
KHKY       Med   72%     Med   77%     High 100%     High 100%
KGMU       High  85%     High  84%     High 100%     High 100%
KAND       High  86%     High  87%     High  98%     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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