Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Charleston SC
730 PM EDT Sat Apr 19 2014
low pressure will shift off the Georgia coast tonight...then
gradually move away from coast Sunday. High pressure will briefly
build into the region Monday. A weakening cold front will push
through the area by Wednesday... followed by high pressure and
warmer temperatures through late week.
Near term /until 6 am Sunday morning/...
a powerful closed upper low over eastern Georgia coupled with a surface
low off the Florida/Georgia coast will maintain unsettled conditions
tonight. Middle-levels are quite unstable given the -17c 500 mb temperatures
and a pocket of warmer air below it...though the boundary layer is
very stable due to extensive stratus and lack of insolation. Weak
high pressure persists across inland areas while extensive low-
level moisture wraps around the northwest periphery of the offshore
surface low. This will maintain moderate to strong isentropic
ascent through the night...shifting from southeast Georgia into
southern SC late. The resulting cinh will keep surface based
convection in check...but given elevated instability with good middle
level lapse rates and showalter indices below zero we have
expanded on the coverage of scattered T-storms south of I-16 this
evening. We have also adjusted pop coverage based on recent
trends...equating to chance probability of precipitation everywhere...with the exception
of likely probability of precipitation over the southeast corner of the County warning forecast area in closer proximity
to the surface low.
Otherwise we expect lows in the 50s with increasing winds after
midnight as the inland high strengthens while the surface low
Short term /6 am Sunday morning through Tuesday/...
Sunday...the surface area of low pressure will slowly move further
out into the Atlantic on Sunday...but deep cyclonic flow and
plentiful low level moisture will keep clouds in the area pretty
much all day. There may be a few breaks in the western part of the
area by late afternoon...but clouds will predominate the sky.
Winds will also be on the increase...making for breezy conditions
out of the north on the back side of the low. With the clouds and
the cool north winds...temperatures will not make it above 70 in
the South Carolina counties...with some low 70s possible in
interior Georgia where they may get an afternoon peak of filtered
The best chance of showers will be east of Interstate 95 where the
moisture will Delaware the deepest...but it certainly will not be as wet
as the recent couple of days.
Sunday night...the low will move further away...but still
slowly...allowing for clearing to move in from the west overnight.
There still could be a lingering shower or Tow along the coast in
the evening...but the rain should move offshore overnight. Breezy
conditions will persist along the coast...but winds will gradually
be dying down inland as the pressure gradient relaxes. Temperatures will
range from the middle 40s inland to middle 50s at the coast...which is a
bit cooler than normal for late April.
Monday...a weak ridge of high pressure should build in for
Monday...providing much more sunshine than recent days. With heights
building aloft as well...a strong subsidence inversion should set up
in the 5 to 10k feet range...with temperatures getting back into the middle
70s away from the coast.
Monday night...heights already start to come back down aloft ahead
of the next shortwave. Partly cloudy skies should develop...mainly
from some higher level clouds moving back in ahead of the said
shortwave...with the left exit region of a modest 90 knot jet at
upper levels supporting some divergence and weak upward vertical
motion. Temperatures will be a bit milder than the night before as
well...ranging from the upper 40s near the lakes along the northern
tier to the middle 50s near the coast.
Tuesday...the modest shortwave approaches from the west during
Tuesday. The weak surface cold front accompanying the disturbance
will make it to the vicinity of the midlands and central Georgia by
sunset. Southwesterly winds ahead of the front will allow for some
low level warm advection...with temperatures getting to 80 or a little
above away from the cooling sea breeze. I do continue with a slight
chance of thunderstorms during the afternoon ahead of the front as
some modest instability develops...but overall forcing appears to be
weak to moderate...so it does not look like a significant convective
event at this time. Precipitable waters will be less than 1.5 inches...so it will
definitely not be a repeat of the heavy rain from the last event.
Long term /Tuesday night through Saturday/...
the upper trough and associated surface cold front will push
through the region Tuesday night. Surface high pressure will then
build over the region from the north...supporting rain-free
conditions and temperatures ranging from the the middle 70s lower 80s
Wednesday and upper 70s to middle 80s Thursday. As the high shifts
offshore Friday...temperatures in the lower to middle 80s will be
common away from the immediate coast.
Aviation /00z Sunday through Thursday/...
IFR conditions will prevail at both terminals tonight into Easter
morning...as a strong inversion remains in place and traps
considerable moisture underneath. Due to the proximity of a
surface low off the southeast coast there could be some rain showers from time to
time...but the risk for any thunderstorms and rain is far too remote to include with
the latest taf package. Winds will be another issue as the
gradient tightens between the Atlantic low and far inland high
pressure. Winds will start to become gusty at both kchs and ksav
by 03-05z and persisting through sunset Sunday evening...peaking
as high as 15-20 knots sustained and 25-30 knots in gusts.
Extended aviation outlook...mostly VFR conditions are expected to
continue into Tuesday. There is a small chance of some IFR/MVFR
conditions in convection late Tuesday or Tuesday night...with
conditions improving back to VFR by Wednesday.
tonight...another surface high begins to wedge down the eastern
Seaboard while the surface low drifts off our coast. We expect
winds to ramp up after midnight out of the north-northeast with solid small
craft conditions in all waters. Seas will be limited some by the
offshore fetch...but still as high as 4-7 feet within 20 nm of the
coast and a foot or 2 higher from 20-60 nm off. Mariners need to
be alert of scattered T-storms through the night...some of which
will produce wind gusts in excess of 35 or 40 knots.
Gales look pretty sure in the offshore Georgia waters on
Sunday...and may well be needed closer to the coast on later
shifts. However...I still have some question about how strong the
winds will actually get in the near shore waters as the usually
more aggressive NAM is this time quite a bit lower than the
GFS...and considering the near shore waters still are not
particularly warm...I do not know if we will really get a clean
mix down...so I will continue with a gale watch for those
locations. Whether reach those levels or not should be immaterial
to recreational boaters as winds near gale force along with high
seas will definitely make boating hazardous for small craft.
Winds will gradually come down later Sunday night and Monday to
levels below 20 knots by Monday afternoon...but seas will remain up
through the day...especially offshore.
After a relatively quiet Monday night...winds will turn to the
southwest and increase again on Tuesday...although they will still
be below any headline criteria.
Rip currents...high risk for Sunday due to increasing swell and
strong winds. Elevated risk likely to persist into early next
north-northeast winds will increase this evening which will maintain or
elevate the already considerable positive tide anomaly. Based on
trends over the last few high tides...we feel confident the
Charleston tide gage will hit 7.0 feet with the midnight high tide
so we issued a coastal Flood Advisory for much of the SC coast
tonight. Chances for enough anomaly to get fort Pulaski Georgia above
its 9.2 feet MLLW criteria are much lower so we held off on a larger
Tides will continue to run high on Sunday...perhaps into Monday
with more advisories likely.
Georgia...high risk for rip currents from Sunday morning through Sunday
evening for gaz117-119-139-141.
SC...high risk for rip currents from Sunday morning through Sunday
evening for scz048>051.
Lake Wind Advisory from 11 PM this evening to midnight EDT
Sunday night for scz045.
Coastal Flood Advisory until 2 am EDT Sunday for scz048>050.
Marine...gale watch from 6 am EDT Sunday through Sunday evening for
Small Craft Advisory until 6 am EDT Sunday for amz350-352-354-
Gale Warning from 6 am Sunday to 6 am EDT Monday for amz374.
Small Craft Advisory until 6 am EDT Monday for amz330.