Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1029 PM EDT Monday Jun 1 2015
Bermuda high pressure will persist across the region early this
week. An upper low is expected to form over the Tennessee Valley by
Tuesday...remaining just west of our region for much of the
week...while cool Canadian high pressure will build down from the
north. This pattern will help enhance lift and moisture return into
the area...creating elevated chances for showers and
thunderstorms...and keeping temperatures slightly below normal.
Near term /through Tuesday/...
as of 1020 PM EDT Monday...earlier convection sparked along old
outflow boundary near the i85 corridor has lifted northeast into the
NC Piedmont...and continues to weaken as instability is at a
premium. Latest Storm Prediction Center mesoscale analysis indicate roughly 500j/kg SBCAPE
across this region...with elevated cape levels approaching 1k.
Nevertheless...think the atmosphere has been worked over to the
point that any strong/severe convection threats are diminishing
quickly. That said...as the upper low continues to rotate over the
Tennessee Valley...weak DPVA over the region will warrant at least low end
probability of precipitation overnight despite heating loss. Thus the forecast was amended to
highlight current/remnant convection...along with holding onto
slight chance probability of precipitation by morning. Moist/saturated boundary layer will
likely lead to areas of patchy fog overnight...especially in/around
areas that have received precipitation this afternoon/evening.
Otherwise...partly/mostly cloudy skies amidst calming flow and the
above mentioned isolated rain showers will highlight the forecast overnight.
Lastly...tweaked temperatures/dewpoints to reflect latest observations.
As of 150 PM...area temperatures reaching the upper 70s to middle 80s are
resulting in convective initiation...with radar returns blossoming
across the County warning forecast area. A sharp upper trough will begin to close off into a
compact upper low over the Tennessee Valley...enhancing the lift across the
area. Probability of precipitation ramp up to the high-end chance in the southeast to
categorical along the Tennessee border by late afternoon. The 12z gso/ffc
soundings showed a lot of middle level moisture...with weak shear and
moderate lapse rates. So expecting mainly garden variety
convection...with a couple of marginal pulse severe storms
possible...with wind being the main threat.
Convection should diminish to just isolated by the overnight hours...with
plenty of debris cloudiness persisting. The moist ground from the
expected convection will support patchy fog and low stratus again
Tuesday morning. Min temperatures will be a couple categories above normal.
Tuesday...the upper low will slow down over the Tennessee Valley...keeping
our area under the q-vector convergence side of the circulation.
Meanwhile...a surface front will sag southeast into the area...with axis
of deep moisture still in place. So another round of numerous to
widespread showers and thunderstorms is expected with daytime heating. The
severe threat looks even more marginal than Monday...with low dcape
and weak shear. Temperatures will also be held in check by plenty of cloud
cover and relatively early convective initiation...generally a
category or so below normal.
Short term /Tuesday night through Thursday/...
as of 215 PM Monday...closed upper low remains nearly stationary over
the southern Appalachians Wednesday then weakens on Thursday as it moves slowly east
into the County warning forecast area. At the surface...a weak surface boundary moves east of the
area as high pressure builds in from the NE in a cad like
configuration. This should lead to quite a bit of convective
coverage each day...with a diurnal trend. However...copious low
level moisture and a southeasterly low level flow...along with weak elevated
instability...will help keep convection around during the overnights
and the mornings. There will be a chance of isolated severe storms each
day with damaging winds the main threats. However...the chance of
flooding will be on the increase given the high precipitable water values and
possible training or back building of cells. Highs will be a little
below normal each day while lows will be a little above normal.
Long term /Thursday night through Monday/...
as of 2 PM Monday...small differences in synoptic features of
the GFS and European model (ecmwf) for the medium range at least on Friday and
Saturday. Upper low opens into the general trofiness Thursday night
into Friday with the afternoon warmth helping to fuel the more
convective activity. The forecast through the weekend and into
early next week will depend on the timing of shortwaves and the
brief calm of high pressure ridging down from the north. The GFS
and European model (ecmwf) differ the most in the day 6 and 7 portion of the
forecast where the GFS continues almost nonstop activity and the ec
has Sunday and Monday under high pressure. Best not to make much
change in the prior forecast to see more consistency toward the
later periods. PM instability forecasts have trended lower so
perhaps less thunder around next weekend. Temperatures remain to be
a little below normal.
Aviation /03z Tuesday through Saturday/...
at kclt...VFR through the period with the exception of middle evening
rain showers/thunderstorms and rain that is currently firing along an old outflow boundary
across the i85 corridor in the upstate of SC. This boundary extends
northward along the Interstate into the southern NC Piedmont.
However...confidence that it will advect eastward before depleting
is only marginal at best...thus opted to carry MVFR tempo from
02-04z. Otherwise...guidance varies as to MVFR ceilings overnight
across western NC therefore opted to go with the consensus which
featured low/middle VFR stratus with a mention of scattered MVFR clouds
around daybreak. The possibility of MVFR ceilings at this time
remains. Beyond that...pretty much a Carbon copy of today for
Tuesday as upper low continues to rotate overhead and the boundary
layer destabilizes. Thus...with hefty probability of precipitation in the forecast opted to
prevail thunderstorms and rain at 19z through the period however left restrictions out
do to timing uncertainty. Winds through the taf cycle will remain
generally light/southerly...except adjacent to any strong convection
where g20-g30 gusts are possible.
Elsewhere...ongoing convection across the region continues to
warrant tempos for such at the SC sites where old outflow boundary
is providing focus for thunderstorms and rain. Further north into western
NC...convection is less dense with the most organized storms
knocking on khky doorstep...therefore prevailed thunderstorms and rain for 1hr...with
thunderstorms in the vicinity at kavl. Beyond that the tafs are fairly similar to that of
kclt above with low stratus and or fog tonight with the most likely
restrictions to occur at kavl/kand...and possibly khky where
convection this evening moistens the bl. That said...cant rule such
out at kgsp/kgmu. Therefore the taf features MVFR restrictions at
kavl/kand/khky...with a scattered IFR deck forecast at kavl...which could fill
in leading to IFR ceilings. Finally...southerly flow and increasing cumulus
will highlight the day on Tuesday with early afternoon convection
expected...hence all sites carry thunderstorms in the vicinity for such.
Outlook...a middle level low is forecast to remain across the deep
south through the work week. Deep moisture will linger across
the southeast Continental U.S. Through the week. Showers and thunderstorms may
develop each afternoon and evening. Morning stratus and fog will
remain a possibility...especially over wet ground.
02-08z 08-14z 14-20z 20-00z
kclt high 98% high 98% high 94% high 100%
kgsp high 100% high 95% high 95% high 100%
kavl high 100% medium 77% high 97% high 100%
khky high 100% high 90% high 100% high 100%
kgmu high 100% high 97% high 98% high 100%
kand high 100% high 91% high 91% 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)