Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
1049 am EDT Friday Sep 19 2014

Synopsis...
high pressure will build to the north of the region today as a weak
area of low pressure develops off the southeast coast and lifts
northeastward through the weekend. A cold front will stall just
south of the area Monday through Wednesday with high pressure
building from the north. A trough of low pressure may develop off
the southeast coast middle to late next week.

&&

Near term /through tonight/...
late this morning...the overall setup is quite complex and
confidence in the forecast details is lower than desired.
Currently...shower/thunderstorm activity is focused across
portions of the South Carolina coastal waters and eastern
Charleston County. The land based activity is dissipating as the
morning GOES along and other activity is now slowly sinking in
from the midlands closer to the Savannah River. The synoptic setup
features high pressure over New England ridging down the east side
of the Appalachians with a coastal trough taking shape and
extending northward out of a surface wave well to the south.
Aloft...a baggy middle/upper level trough is in place across the
southeast Continental U.S. With numerous subtle embedded shortwaves and
vorticity impulses moving through the area. For probability of precipitation this
afternoon...much will be determined by the areas that see the best
insolation and surface destabilization as well as boundary
interactions. Right now...this appears to be southeast Georgia
where some decent clearing has taken place this morning and where
outflow boundaries from the convection across the midlands will
likely end up by peak heating. The sea breeze will likely be weak
and muddled thanks to a reduced land/sea temperature differential.
The east to northeast flow will provide a good Atlantic moisture
feed into this area as well...and I have increased probability of precipitation into the
likely range. Thanks to limited surface destabilization the
thinking is that thunder will be isolated at best. However...there
is the possibility for locally heavy rain across the southeast
Georgia area later this afternoon. No significant changes were
made to the high temperature forecast for today...still featuring low 80s
to the east and low to middle 80s to the west where the better
clearing is occurring.

Tonight...low level moisture convergence should shift back to the
immediate coast and beaches and there could be some sustained
downpours along the beaches and barrier islands if any convection
anchors to any one spot for long. Plenty of clouds with lows
in the upper 60s many areas.

&&

Short term /Saturday through Monday/...
the upper low will shift off the southeast coast on Saturday...
pulling the surface low away from the area. Meanwhile cool high
pressure will briefly build down the eastern Seaboard. Extensive
precipitation associated with the coastal low will be ongoing during
the morning...shifting slowly east-northeast as the day wears on with drier air
filtering in from the northwest. Wrap around moisture will maintain
extensive stratus through early to middle afternoon. This combined with
weak cold advection will yield high temperatures in the lower 80s.

On Sunday a zonal flow will develop with weak southerly boundary
layer winds. Downslope flow will allow drier air to mix down with
dewpoints dropping into the lower 60s by afternoon. Mostly sunny
skies expected with any residual precipitation confined to the
coastal waters and mainly during the morning. Adiabatic warming
along with strong insolation will produce high temperatures in the middle
80s.

A shortwave dropping into the area from the Ohio Valley will push a
weak cold front into the southeast United States Monday afternoon.
The upper level energy will become orphaned from the mean layer flow
and linger over the southeast coast for a few days. The cold front
will initially lack much moisture convergence but this will increase
as it sinks slowly south. We are only carrying a slight chance pop
for Monday afternoon due to the front. Pre-frontal compression will
yield high temperatures in the middle to upper 80s.

&&

Long term /Monday night through Thursday/...
cool and dry high pressure will build down the eastern Seaboard at
the surface while a coastal trough lingers just offshore. There will
probably be a sharp precipitation gradient for much of the period with
inland areas dry and considerable shower and thunderstorm coverage over the
waters and immediate coastal areas. There are indications the
coastal trough will strengthen Wednesday or Thursday and shift
inland which would increase sky cover and rain potential farther to
the west. Temperatures will generally be at or slightly below normal during
the period.

&&

Aviation /15z Friday through Tuesday/...
deepening and moderate easterly fetch off the Atlantic will generate
scattered rain showers and maybe a few thunderstorms today. Timing the convection
looks to remain a challenge through tonight as low level moisture
convergence waxes and wanes with the diurnal oscillations along with
complex boundary interactions. We have indicated lengthy periods
of vcsh and will allow radar trends dictate the Ebb and flow of
convective potential this taf cycle.

MVFR ceilings possible through middle/late morning...otherwise VFR
is expected outside of rain areas through tonight. Surface winds
could be a bit gusty at times this afternoon and in shower
activity.

Extended aviation outlook...brief MVFR ceilings possible Saturday
morning at either site. Otherwise VFR expected.

&&

Marine...
high pressure positioning to the north of the marine area and lower
pressure off the Florida coast will result in strengthening
east/northeast flow through tonight. Models in good agreement on
some solid pinching of the gradient along the South Carolina
coast tonight and we have issued a Small Craft Advisory for the
Charleston coastal waters tonight and persisting into Saturday
morning. We will need to watch trends in the Charleston Harbor as
winds peak closer to 20 knots later today and this evening. The
combination of wind and swell wave will still make for a rough
passage through the jetties. The NE surge will be weaker further
south tonight but enough to peak into the 15-20 knots range at times
with seas building to 3 to 4 feet.

A northeast gradient will continue Saturday into Sunday with
marginal Small Craft Advisory conditions possible over the
Charleston nearshore and Georgia offshore waters. A brief southerly flow
expected Sunday night into Monday before a cold front drops through
the waters and high pressure builds from the north. A prolonged
northeast surge expected Tuesday through Friday with small craft
advisories increasingly likely Wednesday and thereafter.

Rip currents...lingering back swell from tc Edouard will still
impact the beaches today...and that along with stronger NE and
east winds will support a moderate risk of rip currents at area
beaches. An elevated risk may persist into the weekend as the
gradient remains rather pinched with enhanced NE and east winds
to continue.

&&

Tides/coastal flooding...
with a prolonged north-northeast flow expected...and astronomical tide
predictions at Charleston of 5.8 to 5.9 feet MLLW...it is looking like
we will be dealing with shallow coastal flooding for a number of
high tide cycles. The late afternoon into early evening high tide is
when the flooding would occur. Moderate chances of seeing enough of
a wind-induced anomaly to reach 7.0 feet MLLW today through Sunday
with moderate-high chances Tuesday through Friday as winds increase.
Will include mention in the hazardous weather outlook.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...Small Craft Advisory from 8 PM this evening to 11 am EDT
Saturday for amz350.

&&

$$

Near term...bsh
short term...jrl
long term...jrl
aviation...
marine...jrl
tides/coastal flooding...

National Weather Service Glossary of Abbreviations