Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
950 am EDT Wednesday Jul 30 2014
Canadian high pressure over the area will lead to well below normal
temperatures more typical of late Summer or early autumn. As the
surface high weakens...moisture will return for the weekend
increasing the chances of widespread showers and thunderstorms.
Near term /through tonight/...
as of 940 am...current grid suite verifying very well this morning. This
update will make only very minor adjustments based on latest
As of 600 am EDT Wednesday...current forecast remains on track.
Although all time record low at kavl has officially been
tied...latest satellite difference product and observation indicate
development of low level stratus in the region. This will likely
inhibit any further cooling thus protecting the record. Tweaked
temperatures and dewpoints in the extreme near term to reflect
latest observational trends.
As of 300 am EDT Wednesday...upper vortex centered over eastern
Ontario continues to rotate while its trough remains draped to the
south across the eastern United States. As the primary trough axis
shifts eastward...expecting a slow rise in upper heights across the
southern/central Apps. At the surface...high pressure building in
from the northwest will slowly slide east through the period. The
combination of departing upper trough...rising heights aloft...and
dominate high pressure at the surface will once again yield a nice
day across the region. Forecast will feature no mentionable probability of precipitation
through the period with the exception of a few hours this afternoon
where weak upslope showers are possible in the climatology favored regions
of the upper French Broad.
Aside from that...expecting partly cloudy skies as moisture in the
low levels condenses leading to scattered fair weather cumulus. Northerly
winds through the period will remain in the 5-8kts range with some
minor gusting possible in/around the French Broad valley due to
channeling. Temperatures through the near term will remain below
normal. That said...there will be some moderation in the mountains
late in the period as overnight lows on Thursday morning will be a
few degrees warmer...yet still below normal. These slightly warmer
temperatures combined with light/calm winds overnight could lead to
reemergence of patchy Mountain Valley fog by middle morning Thursday.
Short term /Thursday through Friday night/...
as of 315 am...the deep broad upper trough over the eastern Continental U.S. Remains
the dominant weather feature through the short range. The axis of
the trough sets up over the MS River Valley and sharpens through the
period. In addition...the ridge over the Atlantic builds west toward
the East Coast. This develops then strengthens the southwesterly flow over
the County warning forecast area Thursday the period. A series of short waves move through the flow
and across the area through this time...while the right entrance region
of the upper jet sets up across the area increasing upper
divergence. Low level S to southeast flow develops across the area in
response to a weak 850 mb low slowly moving into the area. Although the
low level moisture becomes widespread...the flow is not very strong.
At the surface...the pattern takes on a wedge like configuration as high
pressure ridges over the area from the NE while a surface front moves
north to the Atlantic coast. The front becomes stationary there as
weak waves of low pressure move along the front. All this leads to
slowly increasing chance pop across the County warning forecast area. However...there is still
some question on where the best precipitation coverage develops. Better
coverage could develop to our southeast closer to the surface front...it could
be along the Blue Ridge in the better upslope flow...or it could
even be west of the Blue Ridge across the NC mountains where better
forcing from the 850 mb low moves. For now...chance pop mountains and slight
chance elsehwere Thursday as moisture and lift will only be slowly ramping
up. Pop becomes likely across the mountains and northern tier with solid chance
pop elsehwere Thursday night...Friday and Friday night. Instability does slowly
increase through the period as middle level cap erodes leading to an
increase in isolated to scattered thunderstorms and rain. Quantitative precipitation forecast amounts are in question as
well. For now...have the highest quantitative precipitation forecast along and west of the Blue
Ridge...but no heavy rain. Highs around 5 degrees below normal Thursday
fall to around 10 degrees below normal Friday. Lows near normal Thursday
night fall to around 5 degrees below Friday night.
Long term /Saturday through Tuesday/...
as of 330 am...southwesterly flow remains across the area early in the medium
range as the upper trough axis only slowly moves east. This keeps the
moist southerly flow and wedge like ridge-front pattern in place on Sat
then slowly breaks it down on sun. This means solid chance to likely
pop and below normal temperatures continue Thursday Sun night.
The guidance then develops an upper low to our west on Monday. The GFS
slowly moves the low into the area by Tuesday...while the European model (ecmwf) has the
low developing near the Gulf Coast and retrogrades it on Tuesday. Even
though the guidance differs...both models show scattered diurnal convection
Monday and Tuesday. The main difference is the GFS has much more quantitative precipitation forecast. For
now...have diurnal scattered coverage each day. Temperatures show a slow warming
trend both days.
Aviation /14z Wednesday through Sunday/...
at kclt...VFR conditions will prevail through the period. Increasing
lapse rates this afternoon through shallow moist layer will yield
fair weather cumulus coverage. Kept mention of low VFR clouds into the
overnight as latest guidance indicate weak low level moisture
advection late in the period as boundary layer flow begins to veer
more easterly. Otherwise...winds will remain northerly and in the
Elsewhere...much the same as kclt above with VFR conditions
prevailing through the period with the exception of possible MVFR
level fog/stratus at kavl Thursday morning. Scattered/broken fair weather VFR cumulus
will highlight the skies today with some of this sky cover lingering
through the period...especially at the SC sites and khky where
moisture begins to advect in near the end of the period. Winds will
remain generally out of the north with some northeasterly veering
flow at the SC sites this afternoon as high pressure slides east
across the central Apps.
Outlook...moisture will increase through the remainder of the work
week as east southeasterly flow resumes. This will yield unsettled
weather with enhanced precipitation chances along with morning
fog/stratus through the weekend.
13-19z 19-01z 01-07z 07-13z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% 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 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: (must be lower case)
record lows for Wednesday July 30...
Avl...54 in 1897
clt...58 in 1914
gsp...60 in 1914
Record lows for Thursday July 31...
Avl...53 in 1986
clt...56 in 1914
gsp...56 in 1936