Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
748 PM EDT Wednesday Sep 17 2014
a Cool Ridge of high pressure will slowly build southwest across the
area through tonight...then intensify on Friday. This pattern is
expected to persist through the weekend...before a cold front
crosses the area early next week.
Near term /through Thursday/...
as of 730 PM...a few showers have managed to develop over the southern
mountains and upper Savannah basin on account of instability
afforded by sky that cleared early enough in the afternoon and some
weak low level convergence. Will extend and enlarge the precipitation
chance area to include more of NE Georgia and the western upstate. The
showers should diminish by 02z or so. Otherwise...clouds continue to
diminish...but that trend should halt with sunset and then reverse
as a weak upslope flow acts on available low level moisture to bring
the low clouds back.
As of 235 PM EDT...visible satellite imagery this afternoon is
showing steady erosion of the low cloud layer in the weak cad region
north of the recently departed backdoor front. This should continue as
VAD wind profiles indicate little residual upglide over the cad.
However...boundary layer moisture is allowing some cumulus
redevelopment as scattering occurs...and weak passing 500 mb shortwaves
aloft could trigger some isolated shallow convection along the most
unstable southern/eastern mountains and southeast Piedmont through early evening. The
best focus of any upper forcing should be with a 60 to 70 knots upper
jetlet crossing the region in west-northwest flow aloft...placing the best
upper divergence over the midlands and Piedmont east of our forecast
area toward evening. Otherwise...anticipate mostly dry conditions
with low clouds and patchy fog filling back in across the region
overnight as surface winds slacken.
A train of additional weak shortwaves will continue to push into the
eastern trough on Thursday...but again with weak triggering and limited
moisture. Will feature low end isolated probability of precipitation for Thursday afternoon as 850 to 500
mb lapse rates remain reasonably steep at 6.5 degree c/km. Low clouds
should scatter a bit more quickly Thursday...resulting in maximum temperatures a
degree or two warmer in most areas...but still a little below climatology.
Short term /Thursday night through Saturday/...
as of 245 PM EDT Wednesday...the short term forecast period
initializes Thursday evening amidst possible ongoing
shower/thunderstorm activity with probability of precipitation starting out at slight chance
levels along the Blue Ridge with the remainder of the forecast area
being dry. Probability of precipitation will taper overnight...with an increasing pop trend
into the afternoon hours on Friday as high pressure begins to wedge
further south leading to increasing upglide potential.
Thus...forecast will feature regionwide slight chance probability of precipitation for
Friday afternoon before tapering for the overnight as The Wedge
extends further south along with best upglide potential. Forecast
will remain dry through Saturday as high pressure continues to
strengthen with cooler northerly flow prevailing. Meanwhile...the
next upper trough will be digging across the upper Midwest with a cold
front draped southward along the Mississippi River valley.
Temperatures through the short term will remain seasonably cooler
leading to below normal levels each afternoon.
Long term /Saturday night through Wednesday/...
as of 310 PM EDT Wednesday...no significant changes for the extended
range portion of the forecast which initializes Saturday evening
amidst deep surface wedge and prevailing northerly flow. Forecast
will remain dry through the overnight into Sunday afternoon before
gradually ramping up to slight chance levels across the western
zones of the forecast area where prefrontal/upslope shower activity
will remain a possibility. Probability of precipitation will increase on Monday as upper
trough and associated cold front swing through the southern/central
Apps. Forecast will feature chance level probability of precipitation across the mountains
along and just ahead of the frontal intrusion Monday afternoon with
only slight chance probability of precipitation highlighted further east. Considering
returning southerly flow on Sunday as above mentioned wedge
retreats...along with improved shear associated with approaching
upper trough axis...did go ahead an include slight chance thunder
mention on Monday afternoon. Beyond that...a dry forecast will
prevail for the rest of the extended range as another high pressure
ridge slides by to the north and extends southward in yet another
wedge configuration. Seasonable cooler temperatures will prevail
through the period.
Aviation /00z Thursday through Monday/...
at kclt...visible imagery shows only scattered clouds remaining
across the Piedmont...so VFR conditions should prevail through late
evening with a light north wind. The low level flow should come around a
bit more easterly after sunset...so expect the low clouds to return
at some point during the overnight hours given the amount of low
level moisture present. The previous progression in the taf still
looks good...with clouds/visible initially in the MVFR range starting
around 05z...earlier than guidance...and then eventually lowering to
IFR around 08z. Not convinced that we will see low IFR as upslope
does not look all that great...so prefer to trend more favorable and
amend later. The low clouds will be slow to lift and scatter again
on Thursday...but it looks like conditions will improve to VFR much
earlier than today...around 17z...but still with a broken ceiling. Wind
should stay light NE.
Elsewhere...similar to kclt...a VFR break through evening...with low
clouds and fog returning late evening or early Thursday morning. The
mountains...kavl...have the best chance of seeing IFR visible restrictions
and the guidance shows a long period of vlifr. The taf reflects more
optimistic thinking that some high clouds will prevent dense fog.
Elsewhere...expect weak upslope component to bring ceiling/visible down
to MVFR and eventually IFR before daybreak. The clouds will be slow
to lift and scatter as on Wednesday.
Outlook...the potential for late night/morning fog/stratus will
persist into the early part of the weekend...especially in the mountain
valleys. Conditions may finally dry out for the latter part of the
weekend. A cold front may bring increasing chances for showers/thunderstorms
early next week.
23-05z 05-11z 11-17z 17-23z
kclt high 83% medium 69% medium 71% high 90%
kgsp high 100% medium 71% low 57% high 100%
kavl high 97% medium 77% medium 71% high 87%
khky high 100% medium 79% medium 64% high 80%
kgmu high 100% medium 73% low 57% high 100%
kand high 100% medium 67% medium 66% 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)