Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
150 am EDT Wednesday Sep 2 2015
general high pressure will linger across the region the rest of
the week with weak upper level disturbances crossing overhead. A
cool and moist area of high pressure is expected to settle across
the western Carolinas and northeast Georgia this weekend into early
Near term /through today/...
as of 145 am...rather quiet around the County warning forecast area at this time. Still
thinking that the mountain valleys will at least briefly see fog near
daybreak...but low level profiles are a bit too dry to expect
it to lock in for an extended period. Also increasing cirrus may
inhibit the needed cooling.
As of Tuesday afternoon...although the latest visible satellite imagery
indicates the cumulus field continues to appear quite healthy across the
mountains...have yet to see any deep convection initiate this afternoon.
The atmosphere is weakly unstable...and it/S appearing that coverage will
be quite minimal this afternoon. Nevertheless...Haven/T given up on
convection yet...especially in light of the well-developed cumulus field...thus
probability of precipitation will be retained in the 20-30 percent range across most of the
mountains and foothills. Any convection that does develop will dissipate
quickly this evening. Min temperatures should be about a degree warmer than
this morning/S readings...as heights continue to increase aloft.
Continued gradual air mass recovery/increasing dewpoints and even
more warmth...with prognosticated low level thickness values supporting maximum
temperatures about two categories above climatology...will result in increasing
instability for Wednesday afternoon...although SBCAPE should remain rather
modest at less than 2000 j/kg. Meanwhile...short term guidance is
in good agreement in allowing height falls overspreading the north
central Continental U.S. To kick the area of vorticity...currently sitting over
Indiana...to kick toward the central Appalachians and middle-Atlantic
tonight through tomorrow. Chance probability of precipitation will once again be featured
near the Blue Ridge...but also across the NC mountains and fhills...which
will be closer to the area of upper forcing. Slight chance probability of precipitation
will be featured across most of the remainder of the forecast area.
Short term /tonight through Thursday night/...
as of 145 PM Tuesday...the short term will feature overall height
falls at 500 mb along the appalachian chain...as a large upper ridge
builds over the MS valley to the Great Lakes. The middle and upper flow
will remain rather weak. However...the models agree on some q-vector
convergence atop the County warning forecast area...especially on Friday. At the surface...weak high
pressure will remain centered over the area on Thursday...but then give
way to a weak back door cold front that will push into the area on
Friday. So Thursday looks like another very warm day with only
isolated to widely scattered afternoon convection...mainly in the mountains
while Friday looks more active...as guidance in good agreement on
better instability...with plenty of low level moisture pooling along the
boundary. Probability of precipitation ramp up to solid chance to near likely in the mountains and
low-end chance across the Piedmont by Friday afternoon. Temperatures will be in the
low 90s on Thursday across the Piedmont...and a category or two cooler
on Friday. Low temperatures will be above normal with increasing humidity.
Long term /Friday through Tuesday/...
as of 200 PM Tuesday...an ulvl pattern change is in store over the
weekend. There is good general agreement with with the development
of a closed low dropping south across the north atl enabling a na
Omega block and a weakness or broad closed low developing over the
southeast Continental U.S.. the persistent upper ridge will anchor a surface high across
new eng with surface ridging into the southeast region to some degree. There is
some uncertainty as to the strength of the low level ridge and thus the
quantitative precipitation forecast response in isent lifted atl moisture flux.
Sat looks to have the best chance for scattered to perhaps numerous
-shra...but with varying ideas to the pattern and moisture
transport...will side on the lower end and keep probability of precipitation in the low to
middle chance range. The amount of instability will be another factor
that should be low-end per model soundings and weak mlvl
forcing...thus will lower general thunder mention to isolate coverage.
Decent confidence is had in the layered east/ly flow however...and this
should lead to a modest quantitative precipitation forecast response mainly across the Blue Ridge.
The cad like ridge will remain in place and weaken a little sun.
With less isent lift as the surface ridge weakens...will favor the mountains
with chance probability of precipitation and isolate elsewhere. Instability on Sunday will be
less than Sat as warmer air aloft develops under the ridge
axis...thus thunder will not be mentioned. Maximum temperatures will be held a
couple cats below normal both Sat/sun...with mins right around or a
bit above normal in good nocturnal cloud cover.
The upper pattern shifts sigfntly again on Monday as the southeast upper low
becomes dominant and the Northern Ridge axis pushes east. This will
weaken the surface ridge from the south through the period and decrease
the amount of moisture flux as a broad coastal trough forms. There
will still be a enough general lift and increasing insol for afternoon
shra/tstms. Thus...probability of precipitation were maintained in the seasonal range. Maximum
temperatures and dew point/S will remain relatively low and recover slowly as the
surface flow is held in a NE/ly to east/ly config due to the coastal
trough...so any thunder mention will be limited to the southern Blue Ridge
within mountain top converg. Maximum temperatures should rebound to normal levels by
Aviation /06z Wednesday through Sunday/...
at kclt and elsewhere...VFR is likely to prevail through the
period except near daybreak today in the mountain valleys...with fog
supported mainly by persistence under influence of unchanged
pattern...guidance having not verified well recently. Could see
some fog or low stratus roll off the lakes in upstate SC as
well...affecting kand...though confidence too low to mention
restrictions there. IFR visibility mentioned at kavl only. Surface high
pressure will remain in control today...but a weak shortwave will
drift from the southern Appalachians to the middle-Atlantic coast. This
enhances probability of precipitation in the afternoon...but still keeps chances low enough
to omit at the taf sites. A similarly weak boundary will advance
southward across the area tonight behind the trough. Winds will prevail
southwesterly in the Piedmont but northwest at kavl and khky...veering to north if
not going calm tonight.
Outlook...a pattern of diurnal isolated/widely scattered showers
and thunderstorms is expected to continue through the end of the
work week. By the weekend...a moist easterly flow is expected to
develop...bringing better chances for showers and restrictions. Low
clouds and fog will be possible each night in the mountain valleys.
06-12z 12-18z 18-24z 00-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl medium 72% high 80% high 100% high 100%
khky high 91% high 91% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 91% high 94% 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)