Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
638 am EDT Friday Aug 29 2014

Synopsis...
a cold front will drop down into North Carolina from the north this
morning...lingering over the region briefly before dissipating
tonight. A Bermuda high will set up for the weekend and early next
week...bringing typical summertime heat and humidity...with
scattered afternoon showers and thunderstorms each day.

&&

Near term /through tonight/...
as of 610 am EDT Friday...low stratus continues to expand across the
NC Piedmont region along stationary boundary as previously forecast
per latest observation and Sat imagery. Further west into the
mountains...fog and low stratus has occurred and continues in and
around the little Tennessee River valley...however has been resistant to
develop in the French Broad valley. Dewpoint depressions at kavl
are currently at 2 degrees and its looking less likely that any fog
will occur. Tweaked temperatures/dewpoints/sky to reflect latest
observational trends and left the remainder of the forecast as is.

Previous discussion...

As of 330 am EDT Friday...upper level ridge centered over the
southeast will continue to dominate the pattern through the near
term forecast period. Meanwhile...deepening short wave trough across
the central/Southern Plains continues to promote pressure falls at
the surface leading to strong moisture advection across the lower
Mississippi River valley. Also at the surface...quasi stationary
back door front remains draped in an east/west orientation across
North Carolina...west into Tennessee. A 1020mb surface high
across the northeast continues slide east and wedge down the Lee
side of the central and southern Apps.

Latest satellite difference product imagery and surface observation analysis
indicate area of stratus development tied to a zone of surface
convergence along the above mentioned back door front and intruding
surface ridge. Guidance has handled this situation fairly well
and also indicates further expansion of low stratus into the
central/southern Piedmont region through late morning.
Meanwhile...calming winds and very moist decoupling planetary boundary layer across the
high terrain will lead to development of patchy fog this morning
with visibilities dropping to less than 2 miles...mainly in and
around the mountain valleys. Expecting above mentioned high
pressure boundary and old back door frontal boundary to be the
primary focus of concern through the near term period. Locations
south of this axis such as the upstate of South Carolina and
northeast Georgia will experience a fairly nice Friday with mostly
sunny skies and dry conditions prevailing amongst weak/increasing
southerly flow and upper level subsidence. As for locations to the
north...surface convergence...better moisture profiles...and weaker
capping will warrant mention of isolated/scattered showers and
thunderstorms. Forecast soundings do indicate modest instability
across the southern tier of western North Carolina were better
differential heating is expected. However this threat seems to be
tied to a rather small area were enough instability will exist
adjacent to the surface convergence axis providing needed forcing.

All said...the severe weather potential statement will mention low end threat for diurnal heating
aided isolated thunderstorms along the i40 corridor with damaging
winds and frequent lightning being the primary threats. Convection
will taper into the early evening hours as heating subsides with all
probability of precipitation being removed by midnight. Temperatures on Friday will reach
normal levels across much of the upstate and northeast Georgia with
highs being slightly cooler across western North Carolina due to sky
cover and possible convection.

&&

Short term /Saturday through Sunday night/...
as of 340 am Friday...upper anticyclone over the southeastern Continental U.S. Slides
to coast Sat...then remains there through the period. The associated
ridging remains in place...but slowly weakens or is suppressed as
northern and southern stream short waves move east over and into the ridge. At
the surface...southerly flow develops as a weak cold front moves toward the
area. The front washes out sun...but southerly flow remains as a Lee trough
develops. With the moisture increase...instability develops across
the mountains and upper sav River Valley...leading to scattered diurnal
convection. Could see isolated convection across the foothills and
western upstate in the transition zone between an unstable atmos and a
capped atmos over the NC Piedmont and eastern upstate. Temperatures will be
near to a little above normal. Moderate instability develops across
the area sun as low level moisture continues to increase. The
weakening boundary and Lee trough should provide low level convergence
while the short waves and upper divergence from the right entrance
region of a jet streak provide synoptic scale forcing. The result
will be numerous coverage of diurnal convection across the mountains and
solid scattered coverage along and north of the I-85 corridor...tapering
off slightly to the south. Precipitable water values increase to around 2 inches
Sunday and could lead to storms with heavy rainfall...and possibly
isolated excessive rainfall. Highs sun will be near normal with lows
Sun night a little above normal.

&&

Long term /Monday through Thursday/...
as of 355 am Friday...the upper anticyclone remains suppressed over
the southeastern Continental U.S. At the start of the period...then slowly retrogrades
across the Gulf Coast states into the middle south with ridging
building over the MS River Valley. Nearly zonal flow on top of the
anticyclone will bring weak short waves near or across the area.

Weak southerly low level flow and a surface Lee trough remain across the area
through the period. A cold front does drop south toward the area Wednesday...
but dissipates before reaching the area. This will lead to generally
scattered diurnal convection each day...with a relatively lull on Tuesday of
low end scattered mountains and foothills and isolated elsewhere. Highs will be
near to a little above normal each day...with lows a little above
normal through the period.

&&

Aviation /12z Friday through Tuesday/...
at kclt...VFR conditions will prevail through the taf period.
Stationary frontal boundary will remain in the area through the day
before lifting north of the area Friday night into Saturday morning.
Still expecting sky cover to increase across the NC Piedmont later
this morning as high pressure wedges southward and interacts with
above mentioned boundary. Thus...low stratus currently to the north
is expected to expand southward and lower over the airfield around
the 14z timeframe with no ceiling restricions forecast at this time as
taf only highlights sct025. This low scattered stratus will linger
through much of the morning...eventually lifting by approx 18z.
Northeast winds will increase into the 5-7kts range through morning
before veering southerly as the boundary pushes north late in the
day or early evening. Carried light southeasterly winds under low
VFR cumulus from 00z through the remainder of the period.

Elsewhere...expecting VFR conditions to prevail through the period
at all sites with the possible exception of fog restricitons at
kavl Saturday morning. Initialized all tafs VFR with an
introduction of MVFR level stratus around 14z at khky as moisture
condenses along convergence axis between intruding high pressure and
old stationary front. VFR will return to all sites by midday with
skies scattered out and winds shifting southerly at the SC sites and kavl
as the above mentioned front migrates north while khky remains
northerly through the end of the period.

Outlook...moisture return through the weekend will lead to
increasing chances for isolated/scattered diurnal rain showers and thunderstorms and rain with
the greatest probabilities residing along the high terrain. In
addition...morning fog/stratus will be possible with the mountains
once again having the greatest chances at restrictions.

Confidence table...

10-16z 16-22z 22-04z 04-10z
kclt medium 78% high 85% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl medium 67% high 100% high 100% medium 62%
khky high 80% medium 66% high 100% medium 65%
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)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...rwh
near term...cdg
short term...rwh
long term...rwh
aviation...cdg

National Weather Service Glossary of Abbreviations