Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
109 am EDT Wednesday Aug 20 2014
a broad upper level ridge of high pressure will build across the
Mississippi River valley over the next few days and linger for the
rest of the work week...resulting in above normal temperatures. A
weak cold front will then push into the area from northeast late
Friday into Saturday and bring some minor relief from the heat.
Near term /through today/...
0515 UTC update...winds were updated from the latest NAM and adjmav.
Sky cover was updated from infrared satellite imagery. More cloud
cover was added to the northwest NC mountains around daybreak ... and
advancing mesoscale convective system will make a close pass...if not hit directly. Probability of precipitation
were initially lowered per radar trends...then raised on the Tennessee
border to account for the mesoscale convective system.
As of 1030 PM...one final update this evening based on radar trends.
Area of thunderstorms that moved down from SW Virginia into the northern
foothills and northwest Piedmont should rain itself out near khky through
04z. A few other isolated showers noted over upstate SC and the
upper Fr broad valley should also dissipate...although one or two more
strays cannot be ruled out over the Savannah River basin owing to
residual cape around 1000 j/kg. Backed off on pop along Tennessee border
based on lack of upstream convection. Temperatures look OK.
Remainder of previous discussion...
Deep layer drying associated with eastward shift of a middle-level
short wave trough should result in diminishing of convection fairly
quickly this evening. The only exception may be along the tenn
border. Late tonight...attention turns to short wave trough and/or
associated (possibly organized) convection approaching the area in
veering upper flow. The short term models...despite picking up on
this short wave...do not develop much in the way of convection
across the area early Wednesday.
By Wednesday afternoon...drier air aloft/steeper lapse rates is resulting
in appreciably stronger buoyancy in forecast soundings across the
area. Nevertheless...the short term models are rather muted in their
quantitative precipitation forecast response Wednesday afternoon. Nevertheless...based upon the degree of
instability...and the potential for weak waves rippling through the
northwest flow...will feature 30-50 probability of precipitation across much of the area...with the
highest favored across the western mountains...in light of persistent low
level west/northwest flow acting on a potentially very unstable air mass
across the tenn valley. The potential for severe convection should
be a bit higher Wednesday owing to the drier air aloft/more unstable
Short term /tonight through Friday/...
as of 215 PM Tuesday...a strong upper ridge axis will Blossom over the
middle and lower MS River Valley Wednesday night through Thursday...and then
expand eastward toward the Appalachians through Friday. A 90 knots 250 mb
jet streak riding over the building ridge axis on Thursday will produce
increased upper divergence southward along the appalachian chain...but
there is considerable uncertainty whether or not the best forcing
with this will reach far enough S to affect our area. What is more
certain is that instability in the deep layer northwest flow should be
sufficient for upstream mesoscale convective system generation Thursday...but with the bulk of
the activity likely passsing north and east of our area. A general higher
chance north to lower chance S pop pattern will be needed for our forecast
area Thursday afternoon and evening. Temperatures should warm to a couple of
categories above normal...but will keep maximum values below MOS and
lean toward the bias corrected MOS maxes which have been
consistently cooler this Summer.
The deep layer flow component may become more nearly around the
strengthening ridge on Friday...which could drive any upstream
convection farther to the south from it/S genesis region in the Ohio
Valley and southern Great Lakes. In addition...the steeper lapse rates
that developed on the eastern periphery of the ridge Thursday will persist
through Friday. A similar pattern of diurnal...higher north/lower S probability of precipitation
will be featured for Friday along with continued above climatology/below MOS
maximum temperatures in the 90s.
Long term /Friday night through Tuesday/...
as of 140 PM EDT Tuesday...the medium range picks up at 00z on
Saturday with the upper ridge axis nearly stationary and just to our
west and broad upper troffing just off the Atlantic Seaboard. For
the remainder of the period...the ridge axis remains largely in
place while the upper trough becomes nearly stationary and morphs into
a closed 500 mb low over the weekend. The latest model guidance
continues to suggest that some flattening of the upper ridge is
likely towards the end of the period...yet this is expected to be
At the surface...a strong high centered over far eastern Quebec and a
deepening low moving into the North Atlantic will establish a cooler
air wedge pattern just to our north on Sat. This provides for more
persistent northeasterly low level flow over the forecast region as the high is
slow to move eastward and the low remains off the northeast coast
through early next week. Previous model runs had The Wedge pattern
breaking down by early next week...however the latest runs suggest
that the Canadian surface high will actually move southward early next
week and maintain The Wedge through the end of the medium range. This
solution is better supported by the 00z European model (ecmwf)...with the latest 12z
GFS actually spinning up a tropical cyclone and moving it into the
Gulf of mex by new day 7. This causes the flow to become more southeasterly
over the Carolinas by that time. As for the sensible weather...I kept a
solid chance pop for most of the zones on Sat and sun with the
deeper moisture expected to remain over the area and some weak
shortwave activity providing some degree of upper support each
afternoon/evening. The deeper moisture is expected to shift eastward
on Monday and Tuesday and The Wedge weakens and thus widespread shower and ts
activity is not as likely over the western Carolinas. Temperatures start
out above normal on Sat and cool through the weekend...with values
expected to rebound a few degrees by early next week.
Aviation /05z Wednesday through Sunday/...
at kclt...rain earlier today will favor fog formation by dawn. MVFR
ceiling and visibility will be introduced before daybreak. Model time heights
Show Low VFR clouds lingering through the forecast. Although a cold
front will approach form the NE overnight...it is unclear if the
front will reach kclt or not...and serve as a focusing mechanism for
both clouds and convection. Currently...very little cloud cover is
associated with this boundary. At this time guidance veers winds
from SW to NE overnight...then backs them to SW today...and only
veers them northwest tonight. Convective chances are too low to warrant a
mention in the taf this afternoon...and low VFR ceilings will not be
Elsewhere...guidance favors fog at daybreak...but may be too
aggressive. For now daybreak MVFR visibility and ceilings will be carried at
all sites but kgmu. Winds veer from SW to NE overnight as a cold
front approaches form the NE...but never veer NE at any taf sites.
Winds back to SW today...then veer northwest tonight. The proximity of this
front could serve as a focusing mechanism for clouds and convection
during the day...but at this time little clouds cover was noted with
this feature. Afternoon convection will be featured at kavl only...
with low VFR ceilings and visibility carried with convection.
Outlook...scattered afternoon convection and patchy early morning fog will
remain in the forecast through the week. There will also be
increasing chances for remnant organized convection approaching the
area from the northwest during the late night/early morning hours.
05-11z 11-17z 17-23z 23-05z
kclt high 85% high 85% high 100% high 100%
kgsp high 87% high 100% high 100% high 100%
kavl high 98% high 96% high 100% high 100%
khky high 91% high 100% high 100% high 100%
kgmu high 100% high 91% 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)