Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
210 PM EDT Wednesday may 6 2015
Atlantic high pressure will prevail today while an area of low pressure
develops north of the Bahamas. Low pressure will slowly lift northward
well off the southeast coast tonight into Friday...and may become
nearly stationary just off the North Carolina and South Carolina
coast on Saturday. The low is then expected to lift north across
the North Carolina Outer Banks into Monday. A weak cold front may
push through the region by the middle of next week.
Near term /through tonight/...
today...previous forecast seemed to be in pretty good shape so no
major changes were made. Surface high pressure to the north and
east will slowly break down as broad low pressure near and just
north of the Bahamas begins to organize and track northward.
Middle level dry air will keep the area rain-free today although
cannot rule out a light shower/sprinkle...mainly along the Georgia
coast where the deeper moisture will be. Should be decent coverage
of cumulus and stratocu as the sea breeze develops. Far inland
areas will generally see the most sunshine and warmest temperatures this
afternoon. Northeast to east winds will be breezy/gusty...mainly
near the coast where gusts will be near 25 miles per hour.
Tonight...models continue to indicate that the surface low
lifting north along the Gulf Stream nearing 30 degrees north by
daybreak may begin to take better tropical characteristics as
upper shear decreases over the developing system. Ahead of the
low...deep tropical moisture will shift north and northwest into
coastal South Carolina late. Probability of precipitation could quickly ramp up along
immediate coastal areas east of line from Beaufort to Summerville
to Jamestown late as a flux of deep Atlantic moisture pivots into the
northern zones sometime nearing daybreak. The pressure gradient will
begin to tighten significantly late tonight producing building
northerly breezes along coastal areas.
Short term /Thursday through Saturday/...
as expected...the details for the short term forecast period
Thursday through Saturday hinge greatly on the area of low pressure
developing off the southeast coast and its eventual
track...intensity...and proximity to the coast. The good news is
that the suite of models appear to be starting to converge on a
similar solution. The general consensus track now takes the low
slowly northward with only a minimal westward slide...and has it
centered somewhere near 33n/78w by late Saturday. Such a position
would put the center about 75 miles east of the far eastern portion
of Charleston County. For the details of the forecast...the 00z/06
European model (ecmwf) was the favored solution as it follows this slow northward
track very closely. The reason the European model (ecmwf) is favored is due to its
minimal westward jog with the system given the very weak steering
environment under the deep ridge aloft. Also...it stands to reason
that having the low retrograde away from the warmer waters of the
Gulf Stream seems unlikely in the absence of a prominent steering
feature to the east.
Now that we have started to solidify the preferred solution for the
general track of the system...the question of the characterization
remains. The environment under which the system will be developing
seems to be pointing towards either some type of subtropical system
or perhaps even purely tropical. As it drifts northward...the deep
layer shear drops off considerably and remains as such through
Saturday. Sea surface temperatures along and east of the Gulf Stream
are in the upper 70s to around 80 as well. Furthermore...phase
diagram analyses continue to favor a warm core system. The National
Hurricane Center has increased the outlook to moderate for
subtropical development in its 5 day outlook...and a reconnaissance
aircraft may explore the storm later today.
However...its important to keep in mind that regardless of whether
the system remains purely extratropical or becomes a classified
subtropical or tropical cyclone...the impacts will be the same given
the currently expected slow northward and well offshore track. Such
a track would solidly put the forecast area on the weaker and drier
side of the storm. In fact...the preferred European model (ecmwf) solution would
limit all rain chances mainly to the tri-County and especially along
the eastern Charleston County coastline. In fact...the low level
flow would increasingly become north and then northwest with time
which would favor a warming trend into the first half of the
weekend. Therefore...probability of precipitation have been adjusted and have essentially
been limited to just the tri-County region through Saturday. Given
how far off the coast the system is expected to be...the strongest
winds remain out over the coastal waters though some periods of
breezy conditions will be possible Friday and Saturday afternoon
with the onset of the best heating. If this preferred solution works
out...it appears the main time period for coastal hazards such as
shallow coastal flooding and rip currents will be Thursday into
Friday before the flow turns more north and then northwest and
offshore. All are urged to keep up to date on the latest forecast
information over the next couple of days as well as any guidance
from the National Hurricane Center.
Long term /Saturday night through Tuesday/...
the offshore low is expected to lift northward toward eastern North
Carolina Sunday and Sunday night with a prevailing southwest flow
setting up for Monday and Tuesday ahead of an approaching weak cold
front. The front may push through the area by Wednesday with high
pressure building in from the north. The overall trend for the long
term period is for warming and well above normal temperatures with
highs in the middle/upper 80s and lows in the upper 60s. We may also
see isolated to scattered diurnal convection each afternoon with the
strong surface heating taking place.
Aviation /18z Wednesday through Monday/...
VFR conditions through tonight. Low clouds may fill in some as low
level relative humidity increases overnight...but expect VFR conditions to continue
as ceilings will remain above 10 kft. The surface inversion will
lead to decreasing winds overnight...but winds should become
gusty shortly after sunrise tomorrow morning and the mixed layer
develops. Showers will become increasingly likely at kchs
beginning around daybreak as deep moisture builds ahead of the
developing low pressure offshore.
Extended aviation outlook...flight restrictions are possible Thursday
night through Saturday...mainly at kchs...based on the eventual
track of the developing low pressure system off the coast. Otherwise
VFR conditions are expected to prevail.
a complex and lower confidence coastal waters package taking shape
later today and tonight. This is due to the developing low to the
south with a potential for the low to become a tropical cyclone
by early Thursday as it moves to the north. We know the gradient
will begin to pinch as the high to the north maintains some weak
inland ridging while pressures fall along and off the coast ahead
of the low. We continued to show a deterioration forecast through
tonight as waves slowly build today and then sharper increases in
winds and seas occur this evening through later tonight. A Small
Craft Advisory is in effect for offshore Georgia waters today and
then advisories will go into effect this evening and overnight for
the rest of the waters. Seas should build to 5-6 feet within 20 nm
while outer Georgia waters closer to the stream perhaps reach or
exceed 10 feet by dawn on Thursday.
Thursday through Sunday...the details of the forecast are highly
dependent on the area of low pressure well off the coast and its
eventual track and intensity. As of now...the preferred track is
for the system to pass well east of the local waters as it slowly
lifts northward into the weekend. This will bring a tightening
pressure gradient into the waters on Thursday and continuing
through at least Thursday. Current model trends do not explicitly
favor the possibility for gales...but confidence is low given the
possibility for this system to become tropical in nature. At the
very least...solid Small Craft Advisory conditions are expected
for all of the waters...and possibly for the Charleston
Harbor...with strengthening winds and increasing seas. Advisories
have been hoisted with ending times beginning late Thursday night.
The worst conditions are expected across the Charleston County
waters which will be closest to the center of the low. Stay tuned
to forecast updates as well as details from the National Hurricane
Center regarding the track and intensity of this low pressure
Rip currents...moderate risk for rip currents today due to
increasing onshore flow and some 2-3 feet swell every 8-9 seconds.
Robust longshore currents should continue today all beaches. An
elevated risk for rip currents will likely continue Thursday and
Friday as an area of low pressure slows off the coast. The risk
may then begin to diminish as the low level flow turns more
offshore and north and northwest with time.
increasing north winds tonight will result in above normal tides
along the coast. Astronomical tides are 5.9 feet MLLW in Charleston
Harbor the next couple evenings. The north winds and swell energy will
push tides above that level...though there will probably not be
enough surge to push that to 7.0 feet at downtown Charleston. Tide
levels will continue to be monitored.
Marine...Small Craft Advisory from 8 PM this evening to midnight EDT
Thursday night for amz352-354.
Small Craft Advisory until noon EDT Friday for amz374.
Small Craft Advisory until 8 PM EDT Friday for amz350.