Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
939 am EDT Thursday Jul 31 2014

a ridge of Canadian high pressure remains over the area into the
weekend as a front moves north to the coast. Moisture
increases between these two systems increasing the chances of
widespread showers and thunderstorms. Conditions return to those
more typical of late Summer by the middle of next week.


Near term /through tonight/...
at 930 am EDT Thursday...a closed upper low was located south of
Hudson Bay...with a trough extending SW to the Gulf Coast.
Channeled upper level energy was rounding the base of this trough
over the Tennessee River valley...with a better defined shortwave over
the lower MS River Valley.

Satellite imagery showed extensive cloud cover across much of the
southeast along the wave train at the base of the trough. Radar
showed a band of nearly stationary precipitation form the upper
Savannah River valley to the southern NC foothills associated with
upslope flow and isentropic upglide over a weak surface
well as a weak Lee trough.

Precipiation coverage and intensity are expected to increase with
time as heating...and lift associated with the approaching
shortwave...increase. The potential for flooding will also increase
if precipiation continues to train across the same areas.

1230 UTC update...probability of precipitation and quantitative precipitation forecast were increased per radar trends in
the SC foothills...where upslope flow and moist isentropic upglide
have resulted in greater precipitation coverage.

As of 630 am EDT Thursday...line of upglide induced showers along
and south of the i85 corridor in the western upstate is expected to
continue through morning therefore made some fine tune adjustments
to probability of precipitation. Otherwise...tweaked temperatures and dewpoints in the
extreme near term and left the remainder of the forecast as is for
this update.

Previous discussion...

As of 300 am EDT Thursday...upper trough located over the eastern
United States will remain in place while a short wave impulse passes
through the mean flow. Currently...said short wave is located
across the Central Plains...pushing east into the midsouth.
Consequent cirrus shield continues to advect downstream across the
southern Apps leading to partly/mostly cloudy skies this morning.

Meanwhile...surface warm front continues to spread northwest across
the midlands of SC...spilling into the forecast area by middle/late
morning leading to increasing low level cloudiness. A convergence
axis along the leading edge of this intruding warm front...and weak
wedge like surface ridge will continue to promote weak shower
activity early in the day. Precipitation chances will increase
through the day however as moderate upglide atop wedge boundary
combines with weak elevated instability to support convection.

Forecast will initiate this morning with slight chance probability of precipitation across
the upstate...northeast Georgia...and the NC foothills/Piedmont...with
probability of precipitation increasing to low end likely levels near the end of the
period. As for the mountains...above mention upglide/instability
will be enhanced as weak easterly upslope flow commences leading to
higher probability of precipitation across the terrain. For the mountains...forecast will
feature chance level probability of precipitation initially with categorical probability of precipitation to round
out the forecast. Due to return flow of moisture there will be some
elevated instability across the region today per forecast
soundings. Therefore cannot rule out an isolated thunderstorm with
a primary threat of cloud to ground lighting especially when above
mentioned short wave energy moves in late. Speaking
of...precipitation rates will also improve late tonight into the
overnight with the highest amounts residing along the Blue Ridge
where upwards of 1 to 1.5 inches is possible. Temperatures through
the period will remain 8-10 degrees below normal as radiational
heating is limited due to abundant sky cover and precipitation.


Short term /Friday through Saturday night/...
as of 330 is looking more and more likely that the first
part of the weekend will be wet. Southwesterly upper flow increases through the
period as a trough axis to the west sharpens and the Atlantic
sub-tropical anti-cyclone moves west toward the coast. Short waves
move through the flow and across the area keeping q-vector convergence
in place. In addition...upper level divergence remains over the area
as the right entrance region of an upper jet becomes parked nearby.
A series of 850 mb lows move over the area bringing periods of southeasterly low
level flow. These keep low level and deep moisture across the area
as well. Best pop will be along the Blue Ridge where upslope flow
maximizes. However...likely pop will be featured all areas given the
moisture and forcing. Scattered thunderstorms and rain should also develop...mainly along and
south of I-85 as elevated instability develops. Forecast hodographs show
quite a bit of speed and directional shear develops. Right now...
instability remains relatively weak and elevated...but will have to
keep an eye on this.

Saturday will be similar to Friday except for the weakening surface
ridge leading to increased instability and better chance of scattered thunderstorms and rain
across the area. Although instability will be greater...shear will
be much less. Still too early to tell if areas of heavy rain will
develop either day...but it cannot be ruled out. Highs Friday will
be 10 to 15 degrees below normal...then rise about 5 degrees on Sat.


Long term /Sunday through Wednesday/...
as of 340 am...the northern part of the upper trough slides east of the
area leaving a weak upper low over the southeastern US. The upper low then
slides across the area leaving zonal flow in place. At the surface... a
weak front or baroclinic boundary remains along the coast with weak
high pressure over the Appalachians. Likely pop on sun drops to
diurnal chance pop for the rest of the period. Highs sun around 10
degrees below normal slowly rise to near normal by the end of the


Aviation /14z Thursday through Monday/...
at kclt...a mixture of VFR/MVFR conditions will prevail through this
taf period. Initialized taf with broken low/middle level ceiling which
carries through 22z. At that time...marginal upglide combined with
weak instability could produce rain showers/thunderstorms and rain at the airfield...therefore
opted for thunderstorms and rain prob30 from 22-02z. Expecting ceilings to recover as
precipitation subsides during a brief lull before upper wave moves in near
the end of the period initiating rain showers and MVFR ceilings. 5-8kt
northeasterly winds will prevail through the period with an
occasional easterly component early on this morning.

Elsewhere...a mixture of VFR/MVFR conditions will prevail at all
sites through the period with kavl actually lowering to IFR/LIFR
levels late in the period. Line of upglide induced showers across
the western upstate will likely continue through much of the morning
before spreading as weak destabilization occurs by late morning into
the early afternoon. Thus...prevailed vcsh mention at all the SC
sites under low VFR ceilings through around noon where vcsh gives way to
thunderstorms in the vicinity as the atmosphere destabilizes somewhat. There will most likely be a
lull by early evening as heating is lost...however weather mention
returns late in the period as upper wave slides east providing weak
upper support to the persistent upglide regime and convergence along
quasi stationary warm frontal boundary. Included IFR/LIFR ceilings at
kavl late in the period as guidance is in agreement regarding low
stratus development along the high terrain overnight.
Lastly...reduced visibility at all sites late in the period associated
with rain showers as the boundary layer moistens and winds subside
overnight. Winds through the period will remain light and southerly
at kavl...and east northeasterly elsewhere.

Outlook...moisture will continue to increase through the
remainder of the work week as a warm front approaches from the east
and southeasterly flow continues. Unsettled weather with enhanced
precipitation chances along with morning fog/stratus are expected
through the weekend.

Confidence table...

13-19z 19-01z 01-07z 07-13z
kclt high 100% high 100% high 96% high 94%
kgsp high 91% high 93% medium 77% high 100%
kavl high 97% high 85% medium 77% high 91%
khky high 100% high 100% high 97% medium 78%
kgmu high 92% high 93% medium 77% high 100%
kand high 91% medium 78% high 80% high 98%

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)



Fire weather...
the planning forecast has been updated for lower mixing heights due
to slightly lower forecast temperatures.


Gsp watches/warnings/advisories...


near term...cdg/jat
short term...rwh
long term...rwh
fire weather...jat

National Weather Service Glossary of Abbreviations