Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
652 am EDT Wednesday Jun 3 2015
upper level low pressure will gradually drift across the
southeastern states through the remainder of the work week.
Meanwhile a wedge of cool air will remain entrenched over most of
the forecast area through Thursday. The interaction of these
features will enhance the likelihood of showers and
thunderstorms...and keep temperatures a few degrees below normal.
The pattern will slowly transition to more seasonable weather over
Near term /through tonight/...
as of 650 am...clearing taking place across the western tier of the
County Warning Area...based on sate. However...clearing should fill in with stratocu
through 13z. I will update the forecast to adjust sky to observation and
As 530 am...a band of light showers was sliding east across the
eastern SC Piedmont. Conditions east of the line generally featured
low clouds and patches of fog. To the west of the line...little to
no fog with clearing sky. It remains possible that low clouds and
fog could redevelop west of the line as sky clears over wet soil. I
will update the forecast to adjust to the latest position of the
showers. I will also populate with the sky and hourly observations.
As of 340 am...latest surface analysis indicated a stationary front east
of I-85 with a frontal wave across the middle Savannah River valley.
Radar images show several large clusters of rain showers/thunderstorms and rain generally
along and east of the surface boundary. West and north of the
front...light winds have veered from the north-northeast. Based on the surface
pattern and recent observations...it appears that weak cad has
settled over portions of the NC foothills and Piedmont. Based on
lingering convective available potential energy of 100-500 j/kg and the approach of a middle level vorticity
maximum...convection may exist through sunrise...slowly tracking NE.
Today...near term models and recent west/v images indicate that the
closed middle level low will continue to drift east through the
daylight hours. The 0z GFS indicates that the majority of the
forecast area will remain under an area of q-vector convergence
through this afternoon. Based on the position of the surface front and
light NE winds across the western Carolinas...it appears that the
greatest moisture pool should occur over northern Georgia. Forecast convective available potential energy
across northern Georgia indicate the potential for values above 2500 j/kg.
Instability will decrease to the north...with the I-40 corridor
likely not exceeding 500 j/kg. The range in cape values will be
limited significantly by stratus...expected to be widespread during
the early morning...remaining across the NC foothills and northern
Piedmont through the day. I will forecast likely to high chance probability of precipitation
across the County Warning Area today. However...the potential for severe thunderstorms and rain should
remain limited to the upper Savannah River valley. We will mention
the potential for large hail and damaging winds in the
severe weather potential statement...highlighting NE Georgia and the western tier of the upstate of SC.
High temperatures are forecast to range in the 70s.
Tonight...the center of the 500 mb low will remain over the forecast
area...but should continue to drift east. At the surface...a ridge of
high pressure is expected to ridge SW across the western Carolinas.
Low level moisture and NE winds will increase across the County Warning Area
overnight. Weak instability may remain over the lakelands...but near
zero to the north. Weak isentropic lift around 300 k should support
a few rain showers or patches of dz tonight...I will highlight with chance
probability of precipitation. Low temperatures may range from the u50s across the mountains to low to
middle 60s east.
Short term /Thursday through Friday night/...
as of 230 am Wednesday...models have trended later with the erosion of The
Wedge...which seems reasonable. They now feature more interaction of
the slow moving upper low and The Wedge boundary. GFS GOES so far as
to close off a surface low off the Outer Banks Friday morning. Consensus
is to maintain easterly flow through Thursday despite the parent high
pressure moving off the northeast coast. Upslope into the Blue Ridge
will continue to produce above climatology probability of precipitation...particularly along the
ridge itself. Vorticity forecasts suggest the low will be a bit too far
east to provide much extra lift on its own...but should continue to
enhance lapse rates around The Wedge boundary. Thunder will be
advertised on its warm side. Maximum temperatures will stay a couple categories
below normal. Mins Friday morning will be near normal.
A deep northern stream trough moving through eastern Canada will bring a cold
front to the Great Lakes and St Lawrence Valley early Saturday. The
trough seems to phase with the upper low over our region and help
sustain it. Meanwhile a shortwave will round the sharp Southern Plains
ridge and into the Ohio Valley. Past runs have suggested this too may
merge with the trough or at least minimize height rises in its wake.
Low level flow continues to reflect weak offshore low
pressure...favoring northerly. Without good downsloping winds to
scour it...The Wedge could hang on through Friday. A small area of high
pressure is analyzed on NAM/GFS forecasts over the middle-Atlantic with a
wedgelike mslp pattern. Profiles are less unstable but sufficient for
slightly elevated probability of precipitation compared to climatology. Temperatures should rebound to a
couple degrees below normal Friday afternoon...with lows a bit above normal
A return of weak shear implies slow moving storms may enhance Hydro
concerns. Moisture is not exceptionally high Friday or Sat but precipitable water
values are higher than they were Tuesday...when isolated flood issues
sprang up. So the situation will likely still require careful
monitoring...particularly if The Wedge boundary is able to focus
convection Thursday afternoon.
Long term /Saturday through Tuesday/...
as of 300 am Wednesday...another dome of Canadian high pressure will push
southeastward into the northestern Continental U.S. Through Sunday. The upper pattern does not
appear favorable for it to lock in as cad...but easterly flow will be
maintained into the County warning forecast area Sat and most of sun. Will maintain above
climatology probability of precipitation near the Blue Ridge. Maximum temperatures will rebound to just below
normal. A low pressure system will follow the high...the surface low
moving over Ontario Sunday night. 03/00z GFS and 02/12z ec differ in
their depictions of this system...the latter being stronger and
further south with it. Both models effect a return of southerly flow
for our area Monday...and above normal maximum temperatures...prior to the
arrival of the weak cold front from the Ontario system. A broad and
somewhat deep trough digs into the eastern states thereafter...while the
cold front stalls just south of the County warning forecast area potentially prolonging
unsettled weather. GFS features a second...more potent cold front as
the axis of this trough reaches the southern Appalachians Tuesday night. The
ec has been drier...and while the new 03/00z run maintains the
overall trends of the previous run...it has now come in with less
precipitation early next week. Nonetheless on account of the pattern chance
probability of precipitation will be maintained Mon-Tue.
Aviation /12z Wednesday through Sunday/...
kclt...latest surface analysis indicated a stationary front east of
I-85 with a frontal wave across central Georgia. Radar images show several
large clusters of rain showers/thunderstorms and rain generally along and east of the surface
boundary. West and north of the front...light winds have veered from
the north-northeast. Based on the surface pattern and recent observations...it
appears that weak cad has settled over portions of the NC foothills
and Piedmont. IFR ceilings will begin the 12z taf...tempo rising to
MVFR through 14z. Based on the latest MOS and NAM forecast
soundings...IFR clouds may linger into the late morning...then
remaining MVFR through the rest of the day. Winds will remain
generally from the NE through the taf period...speeds should peak in
the low teens this afternoon. It appears that the approach of the
closed 500 mb low this afternoon will trigger rain showers and possibly a thunderstorms and rain
through early evening. I will indicate in the taf -shra through this
afternoon...with a prob30 for thunderstorms and rain from 21z to 1z. Based on
mesoscale models...cad should surge southward after sunset...I will
time the return of IFR ceilings at 3z.
Elsewhere...at 12z...conditions across the western Carolinas ranged
from VFR to vlifr. Between 12z-14z...broken stratus should develop
across the terminals...even areas that observe VFR conditions at
10z. The stratus should linger until middle day across the upstate and
mountains...but should remain through the day for khky. I will feature
-shra for most sites during the afternoon hours...all sites a prob30
group for late this afternoon and early evening for -tsra. After
sunset...cad is expected to surge southward and restrictive ceiling
will become widespread.
Outlook...a middle level low will linger over the area through at least
Thursday...with deep moisture. So numerous showers and thunderstorms
are expected each afternoon and evening. Morning stratus and fog
will also remain a possibility...especially over wet ground...through
at least Friday.
11-17z 17-23z 23-05z 05-06z
kclt medium 62% medium 66% medium 78% medium 66%
kgsp medium 75% high 94% high 89% high 83%
kavl medium 75% high 83% high 89% high 91%
khky high 81% medium 78% high 94% low 58%
kgmu medium 75% high 89% high 89% high 83%
kand high 94% high 83% high 100% medium 66%
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)