Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
959 PM EDT Thursday Sep 18 2014
high pressure will build to the north of the region through Friday
as a weak area of low pressure develops off the southeast coast and
lifts northeastward through the weekend. A cold front will approach
from the west early next week...with more high pressure expected
Near term /until 6 am Friday morning/...
Canadian high pressure will continue to build from the north
overnight...resulting in slightly dryer and cooler conditions for
much of the region. Aloft we find a broad middle level trough of low
pressure that remains over much of the eastern Continental U.S. With its axis
extending southward off the southeast coast.
The bulk of the convection has pushed into the Atlantic late this
evening within an earlier southeast steering flow. However...the flow is
changing and we are already seeing signs that it is starting to
veer around and gradually turning onshore. This will be courtesy
of easterly trajectories within much of the lowest 6-8k feet. Low
level convergence will strengthen...and we anticipate an increase
in the coverage of marine-based showers and T-storms to occur.
Rain chances will be generally in the 20-30 percent range over our
coastal counties....as we figure the convection will diminish as
it moves further inland with slackening convergence. If any
persistent bands of convection develop we might need to raise probability of precipitation
over an isolated section near the coast...and also watch for
heavier rains to occur.
We have maintained patchy fog across most inland sections after 3
or 4 am...in sync with the lowest condensation pressure deficits
and further removed from any mixing with the onshore trajectories.
If skies remain void of too much clouds long enough we might need
to bump coverage into areas of fog later on...with even a risk of
locally dense conditions.
Lows tonight will be similar to last night...mainly 65-70...except
for some lower 70s east of the intra-coastal.
Short term /6 am Friday morning through Sunday/...
Friday and Friday night...aloft the region will be under the
influence of a baggy trough that will slowly attempt to close off
into a broad upper low across the southeastern Continental U.S. Late in the
overnight. At the surface...several features will be in play
including high pressure shifting eastward across New England...and a
subtle surface trough along the southeast coast. The larger scale
steering flow will become uniformly easterly through the period
which will help to increase deeper moisture across the forecast
area. This Atlantic moisture feed will result in precipitable waters increasing to
around 1.75 inches late. Models show the subtle surface trough
pushing back to the west with time and bringing with it enough
convergence to generate good coverage of showers and isolated
thunderstorms. The convergence zone will primarily be focused along
the coast so the highest probability of precipitation are over the waters and along the
immediate coastline...but have generally been held in the 30-40
percent range. Overnight...probability of precipitation increase into the likely range
across the coastal waters where the moisture/convergence align the
best. Temperatures will be tempered by plentiful cloud cover through
the period. Highs will be kept in the low to middle 80s...which is
actually below normal for this time of year. Lows will be in the middle
to upper 60s.
Saturday through Sunday...the broad upper low will begin to open
back up and lift to the northeast as it begins to feel the height
falls to the north from a vigorous trough diving into the Great
Lakes region. The main feature of interest through the weekend will
be a weak area of low pressure prognosticated to develop off the East Coast
of Florida on Saturday. The models disagree a bit on the timing of
the surface low development...but they do agree that it will be well
off the Georgia/South Carolina coast and will steadily lift
northeastward through Sunday. The NHC currently gives this system a
low chance of tropical development. From a precipitation
standpoint...any rainfall associated with this feature should
primarily be over the coastal waters and right along the coast.
There will be a relatively tight moisture gradient on the western
periphery of the low and any low level convergence will favor areas
further east. The other impact will be a tightening pressure
gradient which will drive a freshening northeast flow...especially
on Saturday. Expect breezy conditions at the beaches on Saturday.
Temperatures will continue to be below normal...with highs topping
out in the low 80s and lows falling into the middle/upper 60s.
Long term /Sunday night through Thursday/...
a strong shortwave trough will sweep across the New England and
Middle Atlantic States on Monday...helping to drive a cold front
through the forecast area. Medium range models are in decent
agreement that the front will be associated with little significant
precipitation...and will thus continue to cap rain chances in the
slight chance range during frontal passage. Expansive surface high
pressure following in the wake of the cold front will have origins
from the northern plains states...and will be accompanied by a
more notable change in air mass. Ahead of the front...temperatures
on Monday will actually warm several degrees within frontal
compression and downslope flow. Conditions will then quickly
cool...with low temperatures Monday night falling into the low to
middle 60s. Expect high temperatures Tuesday through middle week to only
peak in the low 80s. Nighttime lows will range from the low to
middle 60s. With dry high pressure possibly lingering through the
extended period...have generally kept rain chances in the slight
chance range...highest along the coast and offshore.
Aviation /02z Friday through Tuesday/...
both kchs and ksav will remain VFR through much of the night. For
late tonight and Friday morning there could be some low stratus
and/or fog...with the potential for flight restrictions to
occur...especially at ksav. Any rain showers moving onshore of Charleston
County will likely fizzle out before reaching kchs late tonight
and early Friday.
A deepening and moderate easterly fetch off the Atlantic will
generate isolated to scattered rain showers and maybe a thunderstorms and rain during Friday
afternoon/evening. Too far out to include anything worse than low-
end VFR ceilings at this time...but sub-VFR weather is possible.
Depending upon how thick the cloud cover gets on Friday we do
expect some east/NE wind gusts into the lower and middle teens.
Extended aviation outlook...low clouds/fog will be possible with
periods of MVFR or lower ceilings each morning through at least
Saturday. Otherwise...mainly VFR outside of any isolated to
scattered showers each day. Improving conditions with mainly VFR
Sunday and Monday.
overnight...a large 1027 mb high dropping southeast through southeast Canada
will extend south to the northern Gulf by late...resulting in
steadily rising pressures and fairly constant NE and east winds at
less than 10 or 12 knots across the marine community. The wave
spectrum will remain with generally long period swell
energy...with significant wave heights holding around 2 or 3 feet.
Showers and a few T-storms will increase in coverage...aided by
the warm and moist flow over the local waters and Gulf Stream.
While the vast majority look to be low-topped convection...a few
T-storms could be strong with gusty winds...heavy rains and
Friday through Tuesday...with high pressure positioned to the north
of the area and a weak area of low pressure developing to the east a
tightening pressure gradient will result in strengthening
east/northeast flow through at least Saturday night. In fact...as
early as Friday night conditions could become supportive of small
craft advisories thanks to the increasing winds and building seas.
The strongest period for winds/seas will likely be Saturday before
subsiding Saturday night through Sunday as the aforementioned area
of low pressure moves northeastward and away from the area.
Thereafter...conditions will become much quieter early next week. A
weak cold front will move through on Monday followed by high
pressure into Tuesday.
Rip currents...lingering back swell from tc Edouard will still
impact the beaches Friday...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
with a tightening pressure gradient expected to support increasing
east/northeast flow Friday and Saturday...increased tidal departures
are anticipated. In fact...departures may be high enough to support
tide levels approaching shallow coastal flooding thresholds with the
evening high tides this weekend. Saturday will likely be closest as
that is when the strongest winds are expected. To reach coastal
Flood Advisory thresholds...departures would have to about 1.2 feet
which is pretty significant. Due to the marginal nature of the
potential...no mention will be added to the severe weather potential statement at this time.