Area forecast discussion
National Weather Service Greenville-Spartanburg SC
351 am EDT Thursday may 23 2013
Synopsis...
an approaching upper level trough will continue to influence the
region today. A cold front moves through the area tonight...before a
cooler and drier airmass settles in for the weekend. The airmass
modifies through the early part of next week.
&&
Near term /through tonight/...
at 300 am EST Thursday...an upper trough axis will cross the Ohio and
Tennessee valleys today...cross the southern Appalachians this evening...
then moves slowly over the eastern Seaboard tonight. The remains of
a cold front will continue to cross our area early today..exiting
the area to the east this afternoon. A strong cold front will
approach from the northwest today...reaching the mountains by this
evening...and crossing the entire area shortly after midnight.
The atmosphere will destabilize again today...with modest wind
shear...and sufficient dry air aloft for downdraft production. Model
time heights show the best lift between 18 and 21z. A veering low
level flow will dry out the lower levels before lift is maximized in
the foothills...but deeper moisture will remain in the Piedmont...
where heavier rainfall can be expected. Maximum temperatures will be
slightly above normal today.
As the cold front crosses the are this evening...winds will veer northwest.
Model time heights show some low level moisture persisting east of
the Blue Ridge...despite a downslope flow...and light rainfall may
persist into the evening. Post frontal precipitation will be best
and most prolonged along the Tennessee border with NC...where a pronounced
moist northwest upslope flow will set up and linger overnight. Minimum
temperatures will be near normal.
&&
Short term /Friday through Saturday night/...
as of 210 am Thursday...axis of an upper trough swings across the area
Friday. At the surface...pressure gradient tightens across the area as a
cold front slowly moves east and high pressure slowly moves in from
the west. Low level moisture lingers over the mountains during the
morning...keeping some scattered rain showers there in the northwesterly upslope
flow. The rain showers dissipate by afternoon as the moisture diminishes.
The wind and cooler air mass will be the bigger stories. Moderately
strong low level winds move in and mixing should be able to tap those
winds during the day. Although a windy day is expected...gusts should
remain below wind advection levels. With the cooler air mass moving in...
highs will be 5 to 10 degrees below normal.
High pressure will continue to build into the are Friday night as northwesterly
flow develops aloft. The gusty winds should come to an end during the
evening. However...the cold air mass will drop lows to around 15
degrees below normal. In fact...temperatures in the NC mountain valleys will be
cold enough for frost. If the wind can diminish enough...which looks
quite possible for the southwestern valleys...then a frost advection would be
needed.
Northwesterly flow aloft remains over the area Sat and Sat night as a few weak
short waves move through. At the surface...a weak back door cold front
drops into the area late Sat or Sat night. Although moisture
increases significantly and there is some forcing...the forecast remains
dry. Highs remain nearly steady outside of the mountains...but rebound a
few degrees over the mountains Sat. Lows Sat night also increase...but
remain about 10 degrees below normal.
&&
Long term /Sunday through Wednesday/...
as of 345 am Thursday...the GFS and European model (ecmwf) are in general agreement
on the overall pattern during the medium range. However...there are
some significant detail differences...especially at the surface. Northwesterly
flow aloft starts out over the area between an upper low over New
England and a ridge over the central Continental U.S.. this flow continues through
Monday on the GFS until the ridge moves east over the area on Tuesday with
the ridge remaining in place through Wednesday. On the European model (ecmwf)...the ridge does
not move in until Wednesday.
At the surface...a back door cold front starts out over the area. The
European model (ecmwf) takes the front south of the area sun...keeps it there Monday
then moves it back north into the area Tuesday and keeps it stationary
Wednesday. The GFS keeps the front over or just north of the area sun and
Monday...then slides it northward Tuesday and eastward Wednesday. The GFS would
indicate more of a diurnal chance of convection each day...while the
European model (ecmwf) suggests more of a steady chance of convection. The GFS has been
pretty consistent with its forecast from run to run...while the
European model (ecmwf) has not. Therefore...since National guidance is closer to the
GFS...have followed those trends as well. Highs a little below normal
sun rise to around 5 degrees above normal by Wednesday. Lows a few degrees
below normal at the beginning of the period rise to a few degrees
above normal by the end.
&&
Aviation /08z Thursday through Monday/...
at kclt...another line of thunderstorms will cross the field
shortly...then activity will diminish. A damp air mass and prior
rainfall amounts support visibility and ceiling restrictions toward dawn...and
MVFR will be carried. SW winds will mix out restrictions after in
the morning. Another round of convection with possible restrictions
is expected on Thursday as a cold front moves through from the
west...and another approaches from the northwest. Convection will move off to
the east by Thursday evening....while SW winds veer to northwest and become
gusty.
Elsewhere...convection will move east our of the foothills shortly.
Earlier rainfall has moistened the atmosphere and ground to the
point that at least MVFR restrictions are expected by daybreak...and
temporary IFR at kavl. By middle morning restrictions will end as the
winds comes up from the SW in the foothills ND northwest in the mountains.
Additional convection and restrictions are expected by midday
between a departing cold front...and another stronger front arriving
from the northwest. Winds will veer to the west during the day in the
foothills and become gusty...veering further to become northwest in the
evening. Precipitation will be mainly confined to the mountains
Thursday evening in northwest upslope flow.
Outlook...restrictions possible with lingering moisture overnight
Thursday night and Friday morning. Generally VFR weather Friday and Saturday.
Confidence table...
08-14z 14-20z 20-02z 02-08z
kclt high 86% high 94% high 100% high 100%
kgsp medium 78% high 98% high 100% high 100%
kavl medium 79% high 94% high 100% high 92%
khky medium 71% high 92% high 100% high 100%
kgmu medium 67% high 97% high 100% high 100%
kand medium 76% high 97% 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)
Www.Weather.Gov/gsp/aviation_tables
&&
Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.
&&
$$
Synopsis...rwh
near term...jat
short term...rwh
long term...rwh
aviation...jat
