Area forecast discussion
National Weather Service Greenville-Spartanburg SC
500 PM EDT Thursday may 23 2013
Synopsis...
an upper level trough will swing through the region tomorrow...
bringing cooler and drier conditions for the weekend. Temperatures
will moderate by the beginning of next week... with a chance of
afternoon showers and thunderstorms each day concentrated over the
mountains.
&&
Near term /through Friday/...
as of 445 PM...scattered convective activity continues across
northern most portions of the upstate...and into the the NC
Piedmont. Visible satellite trends are actually showing a decrease
in the cumulus field across the upstate....therefore adjusted temperature
trends to account for this and any outflow cooling from adjacent
convection. Extended zone of likely probability of precipitation in the near term further
west across the Piedmont as near term guidance keeps current
activity going in this region for the next 3-4 hours. Otherwise...no
other changes were made.
Remainder of previous discussion...
As of middle afternoon...cannot rule out one or two severe cells given
sufficient cape on the latest laps analysis and some dcape as
well...but neither is as good as yesterday. NE Georgia and the
upstate will have the least chance of precipitation because of a lack of
trigger.
The convection will be shunted east by the veering flow ahead of the
main cold front...which is expected to cross the forecast area this
evening as a short wave rounds the bottom of the upper trough and
swings it eastward...carrying most of the clouds and showers with
it. Expect some lingering shower chance on the Tennessee border through
Friday morning with moisture being forced upward by the increasing
northwest flow.
On Friday...it looks like a breezy day with a decent pressure
gradient as high pressure builds in from the northwest. An even more
dynamic short wave will round the base of the trough and carry the
trough axis farther east during the day...but this wave has no
moisture to work with this far south. Mixing will be relatively
shallow because of a strong inversion...which will prevent the
tapping of the much stronger winds around 850 mb. Thus we should
avoid any Wind Advisory issues. High temperatures will be a few categories
cooler than today.
&&
Short term /Friday night through Sunday/...
as of 210 PM Thursday...a strong upper low will move up the East
Coast Friday night as an upper ridge builds over the central part of
the country. This will allow a cool northwest flow to continue across the
southern Appalachians through the weekend. At the surface...a ridge
of high pressure with Canadian origin is forecast to extend from the
Great Lakes southward to Florida Friday night. The high will control
the weather through Saturday producing generally clear and
seasonally cool weather. In fact...some sheltered mountain valleys
could see some some scattered frost Friday night particularly in the
little Tennessee Valley.
On Sunday...an upper impulse in the northwest flow aloft will cross the
area along with a weak backdoor surface front. Low level moisture
will be lacking...but model time heights show middle and upper level
moisture increasing which means an increase in cloud cover for Sat
night into Sunday. Model quantitative precipitation forecast response is quite minimal on Sunday and
anything that does fall would probably be just a few sprinkles over
the mountains.
Used a model blend for temperatures which advertises well below average
temperatures for the entire weekend. Min temperatures Friday night will fall
into the upper 30s over many mountain locations with 40s common
elsewhere. Maximum temperatures will be in the 70s...except middle 60s to around
70 mountain valleys.
&&
Long term /Sunday night through Thursday/...
as of 145 PM Thursday...the 06z/12z GFS and 00z European model (ecmwf) are now somewhat
at odds in regard to the upper level flow pattern. The main
difference is that the GFS is quicker moving the anomolously strong
upper low through New England and out of the picture. This allows
the flow to deamplify over the southeast by Monday with an upper ridge
building over the region by midweek. The ecm on the other hand keeps
the southeast U.S. In a northwest flow regime to the east of an upper high it
holds stationary over the lower Mississippi Valley. This allows weak
impulses to translate across the region in the northwest flow making for a
rather unsettled period until late week when the ecm finally builds
the ridge to the east. The latest GFS ensemble supports the GFS
operational runs.
Wpc has favored the ecm in its medium range. However...I have hedged
the forecast toward a blend of our gsp official/GFS/ecm which would
put a diurnal harmonic to the forecast favoring scattered afternoon
convection mainly over the mountains. The exception is Monday afternoon
when some scattered showers/thunderstorms may also occur over the NC
foothills/Piedmont as the backdoor front that slipped through the
area on sun returns north as a warm front. Temperatures will begin the
period a little below climatology...then warm to about a category above
climatology by the end of the week. If the GFS solution verifies...maximum
temperatures would be about a category higher each day.
&&
Aviation /21z Thursday through Tuesday/...
at kclt for 21z update...have held onto a two hour window of
thunderstorms from earlier taf issuance in the 21z to 23z time range
per the 4km WRF model and recent radar trends. Thus...kept previous
tempo group as is with the only change being to winds with mention
of vrb10 gusting to 20kts adjacent to any thunderstorms. The front
should finally cross the region during the middle part of the
evening which will clear out the low clouds and shift the wind from
SW to northwest...by 04z at the latest. Conditions should be VFR from that
point Onward. On Friday...once the winds come up during middle
morning...expect frequent gusts from the northwest.
Elsewhere...similar chances for MVFR ceiling until 20z or so when
the boundary layer lifts the cloud bases above 030. Otherwise...
less of a chance for convective activity. Only khky warrants a
vicinity thunderstorm for the time being. Expect VFR conditions
after frontal passage...which should be late afternoon to early
evening. Some upslope showers could occur along the Tennessee border...but
this is not expected to affect kavl. Winds should be strong and
gusty from the northwest on Friday.
Outlook...VFR through the period.
Confidence table...
21-03z 03-09z 09-15z 15-21z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 92% medium 76% high 85% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% high 93% 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)
Www.Weather.Gov/gsp/aviation_tables
&&
Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.
&&
$$
Synopsis...level
near term...cdg/PM
short term...lg
long term...lg
aviation...cdg/PM
