Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
806 am EDT Sat Jul 4 2015
a broad upper level trough will remain over the southeast through
Monday...providing lift and maintaining elevated rain and
thunderstorm chances each day. Slightly drier conditions return
Tuesday and Wednesday...before another front pushes across the
southern Appalachians by the end of the work week.
Near term /through tonight/...
as of 700 am EDT Saturday...tweaked probability of precipitation/sky to account for
current light precipitation coverage. Also adjusted temperatures and
dewpoints to better reflect most recent observation...no other changes
were made as current forecast remains on track.
As of 325 am EDT Saturday...rather robust band of convection
associated with deep layer instability continues to slide along the
i20 corridor from eastcentral Georgia into the SC midlands. Latest Storm Prediction Center
mesoanalysis continues to indicate a modestly unstable environment
over these areas...while showing a rather sharp decreasing gradient
northward into the SC upstate. Further west another band of broken
convection stretched in a north/south orientation from southeast Tennessee
southward to around Columbus Georgia. It is this convective element that
latest cams forecast will survive/advance into northeast Georgia and the
western Carolinas by around daybreak.
For the remainder of the Holiday...the pattern is somewhat similar
to that of yesterday as it is highlighted by a broad positive tilted
upper trough...and a quasi stationary convergence axis across the Tennessee
Valley. This surface feature combined with lobes of vorticity
around the upper trough...all enhanced by rear entrance region of
upper jet...will support scattered/numerous showers and
thunderstorms once again today. Also similar to that of yesterday
will likely be the evolution of the instability parameters across
the region which will be greatly impacted by am
convection...especially over the southern tier of the forecast area.
20-30kts 0-6km bulk shear will once again be present in the
profiles...however its seems the above mentioned faults in
instability will prevail leading to only a few strong storms at best
this afternoon. Precipitable waters remain around 2 inches which will lead to
continued flash flooding threats where any cell training occurs.
Plans are to expand/extend current Flash Flood Watch to account for
most recent precipitation/ffg values in the near term...and beyond into the
short term. Latest quantitative precipitation forecast remains very similar to that of last forecast
cycle which focuses mainly on the SW mountains of western NC...and
northeast Georgia where upwards of 1-2 inches is featured.
However...those numbers could be understated if/when training of
All said...the Independence day forecast features scattered to numerous
probability of precipitation for rain showers/thunderstorms and rain this morning across the upstate and upper Savannah
River valley...before tapering to chance levels this afternoon. At
that time the highest probability of precipitation will reside over the high terrain...and
also along and north of i40 in the western NC Piedmont where likely
levels are mentioned. Skies will be mostly cloudy with below normal
Short term /Sunday through Monday night/...
as of 315 am Independence day...on Sunday the key features will
continue to be a broad upper shortwave centered over the upper tenn
valley...and a weak low level boundary over the Carolinas.
Differences in the pattern are minor compared to Saturday. Deep
moisture will actually be slightly better...characterized by precipitable water
values above the 90th percentile per Storm Prediction Center radiosonde observation climatology. This is in part
due to a better Gulf connection...on account of low level winds
backing to southwest immediately ahead of the upper trough. This will
enhance upslope forcing into the SW portion of the County warning forecast area as well.
Furthermore wind profiles will feature weak and nearly unidirectional
shear. The concern accordingly will continue to be for thunderstorms
producing heavy rain and potentially training. With the ingredients
continuing to be in place for a flood threat...we have extended the
Flash Flood Watch another 24 hours...now ending at 00z Monday. Also
the area has expanded to all of Jackson Colorado...Oconee Colorado...and the
remainder of our NE Georgia counties. These areas have the lowest ffg
values as well as greater potential for upslope enhancement. Maximum
temperatures will continue to be below climatology...similar to Saturday.
On Monday the trough will lift northeastward across the area...and begin to
merge with a larger scale shortwave entering the upper Mississippi
Valley. The influence of the trough on lift will be lesser...and
profiles are less conducive to heavy rain production. However
isolated flooding cannot be ruled out. Probability of precipitation in general will be lower
particularly over the southern half of the County warning forecast area. Despite less cloud cover
and precipitation...temperatures may be slightly cooler due to lower thicknesses
beneath the trough. So values still top out a category or two below
Long term /Tuesday through Friday/...
as of 130 am Independence day...heights start to rise Tuesday as
upper low is absorbed into the northern stream and subtropical ridge
regains strength over the southeast. Midlevel lapse rates accordingly
weaken from day to day...so diurnal convective potential will return
to near climatology...with mainly just showers mentioned. As a shortwave
moves across Ontario/Quebec a cold front will sag into the Ohio Valley
Wednesday...with high pressure building eastward behind it. The presence of this
feature limits our confidence going forward. It is a seasonably weak
boundary...however global models hint that a wave will develop along
it somewhere to our west...though they differ in the details. GFS/Gem
depict the wave tracking along or north of the Ohio River...mainly
keeping the front on our northwestern fringe. The ec on the other hand
develops a stronger wave over the Southern Plains...leading it to delay
its push across the mountains until Friday. However this also implies a more
bullish front. Will bring probability of precipitation back up slightly Thursday and Thursday night.
Temperatures will warm back to near normal by Wednesday.
Aviation /12z Saturday through Wednesday/...
at kclt...VFR with the exception of morning/afternoon convection
leading to restrictions. Stalled boundary in the vicinity
of the Carolinas continues to yield an unsettled pattern with
scattered rain showers/thunderstorms and rain favored. Thus...the taf initializes dry under
broken middle clouds and gusty SW winds with a 4hr tempo for rain showers. Winds
will remain elevated out of the SW through the day with gusts as
high as 20-25kts possible during mixing hours. Prob30 for thunderstorms and rain will
prevail for the afternoon. Conditions will improve into the early
evening where broken middle and overcast high level clouds prevail.
Elsewhere...similar to that of kclt above where multiple rounds of
rain showers/thunderstorms and rain are forecast at all sites on Saturday. All sites feature weather
mention in the form of rain showers/vcsh along with MVFR ceilings this morning
associated with initial round of convection...with further rain showers/thunderstorms in the vicinity
mention this afternoon to account for additional activity.
Increasing SW winds at the SC sites will lead to yet another
windy/gusty day with gusts as high as 25-30kts possible. Expecting
conditions to improve into the early evening as stated above.
Outlook...precipitation coverage will remain high through
Sunday...then the pattern will transition to a more diurnal
pattern...but with continued moisture in the area. Restrictions will
be confined mainly to thunderstorms and then early morning stratus
where heavy rain fell the previous day.
12-18z 18-24z 00-06z 06-12z
kclt high 100% high 100% high 100% high 100%
kgsp high 99% high 100% high 100% high 100%
kavl high 81% high 96% high 100% medium 72%
khky high 97% high 99% high 100% high 100%
kgmu high 99% high 100% high 100% high 100%
kand high 100% high 100% high 100% high 97%
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)
Georgia...Flash Flood Watch through Sunday evening for gaz010-017-018-
NC...Flash Flood Watch through Sunday evening for ncz048-051-052-
SC...Flash Flood Watch through Sunday evening for scz001-004.