Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
443 PM EDT Friday Aug 29 2014
a weak nearly stationary front will linger over North Carolina
tonight and then dissipate by Saturday night. 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 Saturday/...
430 PM EDT update...the forecast is on track and needs only minor
tweaks. Middle level warming is keeping cumulus field in check with some
enhancement seen across the mountains in mech lift. Isolated showers
will likely form across the ridgetops over the next few hours...but
most locales will remain dry through the afternoon.
As of 130 PM EDT...low stratus are finally dissolving over our
portion of the northwest NC Piedmont though overcast skies remain just NE of
Charlotte. These clouds are expected to continue to break up through
middle afternoon. The nearly stationary boundary on the southern fringe of a
wedgelike surface ridge which caused these clouds to form early this
morning remains near the NC/SC border. Dewpoints are currently highest
in an arc from Asheville to Spartanburg to Charlotte...a result of
moisture pooling along the boundary. This has implications for the
forecast tonight. However...though analyzed cape values are notably
higher within the area of higher dewpoints...low to middle level lapse
rates are not good enough east of the mountains to expect deep
convection. Have maintained isolated thunderstorms and rain mention over the mountains and southern
Escarpment areas...but that/S it.
Tonight the upper ridge continues to shift eastward into classic Bermuda
high position as trough moves into the Mississippi Valley. Any
remnant of the stationary boundary appears to wash out...at least
enough so that any upglide will diminish...so low clouds not
expected over the boundary tonight. Very weak southerly low level flow
will prevail again over the area which does prompt thoughts of low
stratus or fog creeping up from the coast...but moisture does not
look especially good this far inland. A better case can be made for
fog to form where surface moisture is pooling along the front this
afternoon...given good guidance support for temperatures cooling below the
expected crossover temperatures as well as with the weak southerly flow.
Will advise evening shift to consider threat of patchy dense fog
That upper trough does bring some weak disturbances northward into the
tenn valley tomorrow afternoon but these will have minimal impact on
forcing over our area. Lapse rates and model cape values are best
over the southern mountains and adjacent Blue Ridge...where the upper heights
are a little lower. Southerly flow into these areas may also enhance
the coverage...so I will maintain the highest probability of precipitation there. Further
east subsidence capping will keep the forecast dry. Mainly diurnal
cumulus will be seen for sky cover with maximum temperatures near normal.
Short term /Saturday night through Monday/...
as of 200 PM Friday...a weakening short wave trough ejecting from
the Great Plains will act to nudge the southeast Continental U.S. Upper high
off the southeast coast early in the short term...while generally
resulting in an overall lower amplitude upper pattern. A Bermuda
high will become established within this regime...which will act to
increase moisture and instability across the western Carolinas and
north Georgia. Additionally...a plume of deep moisture accompanying
the short wave trough/weakening frontal zone will approach the
forecast area by Sunday morning. This should result in increasing
chances for deep convection beginning across the west Sunday
night...with chances increasing and overspreading the remainder of
the area by Sunday afternoon. Terrain effects and Lee trough
interacting with lingering moisture plume...along with respectable
SBCAPE values (generally 1500 to 2500 j/kg per forecast soundings)
should support scattered to perhaps numerous convection...with the
highest probability of precipitation occurring near the NC Blue Ridge. Shear will remain
quite weak throughout the period...so severe storm chances will be
limited to the isolated pulse variety.
With frontal zone expected to weaken across the eastern Continental U.S....
expect a more typical diurnal widely scattered convective trend to
return by the end of the period. The breakdown of the ridge and
increasing cloud cover/convection should spell a period of slightly
below normal maximum temperatures/near normal mins through the period.
Long term /Monday night through Friday/...
as of 150 PM Friday...the medium range period begins 00z Tuesday
with a nearly zonal pattern across the nation as an upper ridge
departs east from the Atlantic coast. A subtropical ridge being
suppressed south over Florida at 00z Tuesday will retrograde over the
deep south being centered over the Gulf states in middle week. This
ridge will expand into late week creating a greater amplitude
pattern Thursday and Friday with the jet stream from California to the
Dakotas to New England. There appears to be a trend in the
models...especially in the GFS...of moving the center of the ridge
even farther west at the end of next week. If the center of the
ridge sets up around Oklahoma next weekend...our pattern may shift
from being in a ridge to being in northwest flow which would bring mesoscale convective system
systems from over the top of the ridge to the Carolinas.
A weak cold front will drop down from the north in middle week but it
is questionable whether the front will reach our area before
dissipating. This boundary will provide a focus for convection in
middle week especially in the PM hours. Low level wind will be light
and vary from day to day depending on the influences of passing
shortwaves to the north...a persistent Lee trough and
whether the front makes it through or not.
Temperatures are forecast to be above normal through the period
under the upper ridge. Maxes above by 5 to 7 degrees in middle week and
3 to 5 above on Friday.
Aviation /21z Friday through Wednesday/...
at kclt/khky/kavl/kgsp...VFR should prevail into this evening...but
a few patches of MVFR level stratus will linger nearby early this
afternoon over the Piedmont. Weak but persistent stationary front near
NC/SC border will pool moisture this afternoon and keep dewpoints from
mixing out appreciably. Given the expected calm and few-scattered sky
cover tonight...radiational cooling should allow temperatures to easily
approach crossover values. Kavl has the best support from guidance
for dense fog so have prevailed LIFR around daybreak there. I think
the other sites have a better shot at mixing out some later
today...plus there are mixed signals from guidance. Thus only IFR tempos
there. Except at kavl which is already southeasterly...the other sites will
flirt with east-southeast winds later this afternoon before going calm
overnight...then come up from the southeast Saturday.
At kgmu/kand...expecting VFR to prevail this period...though kgmu
will be nearer the area of better moisture described above and hence
has a small chance of a restriction around daybreak. Winds will
mainly stay east to southeast this afternoon...going calm by evening...and
prevailing southeasterly again Saturday.
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.
20-02z 02-08z 08-14z 14-20z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 98% high 100%
khky high 85% high 100% high 87% 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)