Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
752 PM EDT Thursday Aug 21 2014
a broad upper level ridge of high pressure will continue to provide
very warm temperatures across the region into Saturday. A back door
cold front will then push into the area from northeast ushering in
cooler temperatures starting Sunday.
Near term /through Friday/...
as of 750 PM...Blue Ridge convection has dissipated as expected.
However...convection to the north continues to move southeast toward
the area with isolated convection developing ahead of the line. That
said...the convection is diminishing as it moves southeast. Latest cam and
synoptic guidance also show these trends. Therefore...have removed
pop south of the Tennessee border and I-40 corridor...but left pop in for
those areas this evening. Taper pop off to non-mentionable chance after
midnight as the convection should dissipate. Cannot rule out and
additional strong or severe storm before midnight...but chance will
rapidly diminish after sunset. Had to make some significant
adjustments to dew points as readings were not following the usual
diurnal pattern. Otherwise...updates mainly for current conditions.
As of 430 PM...latest cam and synoptic guidance not very bullish on
convective chances this afternoon. All the guidance shows the
current convection over the Blue Ridge dissipating in the more
stable air over the foothills and Piedmont as the convection moves
southeast. In fact...they are even suggesting the convection over the Kentucky
and WV will dissipate as it moves southeast toward the area. Will not go
that far...but will keep the current pop trend of diminishing the
Blue Ridge pop and increasing pop across the northern tier for the
convection to our northwest. That said...would not be surprised if pop is
trimmed even further at the 730 PM update if these trends continue.
Otherwise...updates were for current conditions.
As of 215 PM EDT...a prominent 500 mb ridge axis will persist over
the length of the MS River Valley tonight through Friday. Convection
arising from impulses riding over the upper ridge has already filled
in on regional radars from southeast Kentucky to WV this afternoon. However...the
convection allowing models still show the best afternoon thunderstorm triggering
over the southern mountains where laps surface based cape values are running 3000
to 4000 j/kg. Then...the mesoscale models depict the weak convective
system to the north passing mainly north of the area this evening. Some
trailing convection from this northern system may brush the northern tier from
21z to 03z. Mainly middle and high debris clouds should linger through
the nighttime hours...with any fog and low cloud issues confined to
the SW mountain valleys. Expect another night of mild mins...with mainly
low to middle 60s in the mountains and around 70 elsewhere.
The flow pattern between the ridge and a New England low pressure system
will amplify on Friday...with more north-northwest flow developing along the
Appalachians between these features. This should bring the northern tier
of the forecast area a bit more into the path of any upstream mesoscale convective system
activity from the Ohio Valley late Friday. In addition...surface based
instability and lapse rates look just a touch more robust for Friday
afternoon...but still relatively unimpressive for severe weather compared to
upstream areas. Otherwise...westerly downsloping 850 mb flow through
the period will make it a struggle to get much convection going from
other sources...and maximum temperatures will likely gain a degree or so over
Thursday values. Southern Piedmont heat index values could surpass 100 Friday
afternoon if we see slightly less dewpoint mixing.
Short term /Friday night through Sunday/...
as of 210 PM Thursday...strong upper ridge will persist over the
miss valley through the period...placing much of the southeast and
middle-Atlantic within moderate north/northwest upper flow. Our area will
therefore remain susceptible to /ring of fire/ convection developing
near quasi-stationary frontal zone across the Ohio Valley/ctrl
Appalachians region...which could drop into the Carolinas in
somewhat organized fashion during the afternoon and evening.
Thermodynamic profiles will remain sufficient to support isolated
severe weather Saturday afternoon/evening.
By the end of the period...the frontal zone may begin to back door
its way into the area by early Sunday...as the still-building upper
ridge begins to squeeze the downstream upper low off the East
Coast...allowing weak surface ridging to build down the eastern
Seaboard. Therefore...in addition to the continued threat of
migration of upstream convection into the area...the backdoor front
may provide an additional focus for convection on Sunday. However...
exactly where the best chances will occur is still subject of
debate...as the air north of the boundary may be too stable to
support much in the way of precipitation chances. The highest probability of precipitation will be
favored across the southwest zones...where confidence in adequate
instability is highest...and a weak easterly flow may provide an
additional source of lift near the Blue Ridge. Temperatures will remain
above normal until Sunday...when maxes will likely be below normal
under more extensive cloud cover and east/NE flow.
Long term /Sunday night through Thursday/...
as of 1230 PM Thursday...the medium range model consensus provides
good continuity from inherited forecast to start off the workweek. Still
expecting surface ridging to be nosed sswd into the County warning forecast area while upper
ridge axis builds into the eastern Continental U.S.. sensible weather will feature
maximum temperatures about 5 degree f below climatology with possibly a few
showers forced by the easterly low level flow. Suppressed orientation to
amopshere looks to continue into Tuesday with effectively minimal
shower chances and temperatures perhaps warming a few degree f.
Deeper layered ridging is prognosticated to break down during the latter
half of the period...allowing temperatures to climb back to normal
and perhaps thunderstorm chances returning by Thursday.
Aviation /00z Friday through Tuesday/...
at kclt...expect convection to remain well north of the airfield then
dissipate this evening...in line with cam and synoptic guidance.
That said...a brief turn to northerly wind is possible if any outflow
boundary can make it that far south. Otherwise...expect VFR
conditions through the period as guidance has been over-forecasting any
fog formation. Should see light northerly wind with mixing Friday morning then
a turn southwesterly for the afternoon. Main area of convection should remain
north and west of the airfield once again.
Elsewhere...khky has the best chance of seeing any thunderstorms and rain this evening
with a small chance at kavl. Otherwise...expect convection to dissipate
this evening...in line with cam and synoptic guidance. Should see VFR
clouds through the period. Guidance has been over-forecasting fog and
expect they are again...but did keep kavl with a period of MVFR visibility
as they are the only taf site that has a decent chance of fog. That
said...the normally more foggy non-taf sites could see restrictions.
Expect light northerly wind with mixing Friday morning then a turn to west or SW
for the afternoon...northwest at kavl. Once again...kavl/khky have the best
chance of seeing any convection...so prob30 included there.
Outlook...a similar pattern is expected Friday night...with mainly far
northern tier scattered convection possible. A backdoor cold front will sag
into the region through the weekend...with chances of restrictions
increasing at khky and kavl. A more diurnal pattern of early morning
low clouds/fog and scattered afternoon thunderstorms may return early next week.
00-06z 06-12z 12-18z 18-24z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 91% high 91% high 100%
khky high 100% high 95% high 91% high 100%
kgmu high 100% high 100% 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)