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Area forecast discussion 
National Weather Service Charleston SC
415 PM EST Sat Jan 31 2015

Synopsis...
high pressure will shift into the Atlantic tonight and Sunday...while a
weak trough of low pressure develops offshore and lifts north ahead of a
strong cold front passing over the area Monday. High pressure will then
return to the region by Tuesday...before shifting offshore ahead of
another storm system approaching from the Gulf of Mexico by late in the
week.

&&

Near term /until 6 am Sunday morning/...
high pressure will slide into the Atlantic tonight with a weak
trough of low pressure developing just offshore. The middle and upper
level ridge will also shift eastward ending up over the area by
daybreak. Decent radiational cooling conditions expected at least
the first part of the night with just some high clouds streaming
in from the west and light winds. This will allow temperatures to
drop quickly after sunset...especially away from the immediate
coast. Late tonight clouds will likely be on the increase...mainly
in the middle and upper levels...along with winds above the surface.
This will likely lead to temperatures evening out or even rising a bit
toward daybreak...especially near the coast. Lows should reach
to near freezing inland...especially across South Carolina...with
middle to upper 30s elsewhere except lower to middle 40s closer to the
immediate coast.

&&

Short term /6 am Sunday morning through Tuesday/...
Sunday...surface high
pressure and its associated short wave ridging aloft will pull into
the Atlantic in advance of an gradually amplifying short wave and
it/S Colorado-located surface low moving east through the middle
Mississippi Valley and Midwest. As this transition occurs a very
subtle coastal trough will form out near the western wall of the
Gulf Stream on the west side of the Atlantic progressing
anticyclone. While a few showers will form in proximity to the
coastal trough...they look to remain in the ocean. Deeper moisture
and the best forcing with the upstream cyclone and short wave
aloft will remain west of the region through the day. Thus we are
showing a rainfree forecast. Isentropic lift will be sufficient
enough to generate an increase in marine- induced stratocumulus
clouds within the low level southerly flow...while jet stream
cirriform clouds will persist from the west. Despite this there is
considerable warm advection which will send temperatures far above normal
into the middle and upper 60s inland...maybe even hitting 70 near the
Altamaha river. Sea breeze circulations will limit coastal
communities to the upper 50s and lower 60s. While these temperatures are
not records...they do show a tremendous diurnal range in temperatures of
some 30-35 degrees in most sections.

Sunday night and Monday...this looks to be the most active part of
the short term period...with the region to be impacted by a strong
cold front from the west/NW. A positive tilted short wave aloft will
amplify further as it sweeps into the Ohio/Tennessee valleys by
daybreak Monday...then aligns itself more neutrally tilted Monday as
it swings by to the north Monday afternoon. This pronounced feature
aloft will send a strong and fats moving cold front over the
Appalachians by 12z Monday...into our northwest tier by 15z Monday...to the
coast by 18z and further offshore thereafter. We look for a lowering
and thickening of cloud cover Sunday evening...with showers to
arrive from the west during the late evening west and overnight
further east. This is in response to the approaching cold front...a
150 knots or greater upper jet...a 50 knots or stronger low level jet and
precipitable waters that rise to 1.25 inches or more. While there is still a
little uncertainty in regards to timing...just about everywhere
by sunrise Monday looks to be wet. Rain chances will lingering
into Monday morning...then quickly drop off from west to east for
the late morning and afternoon in wake of the cold front. We have
likely to categorical probability of precipitation going for later Sunday night...with
categorical all zones Monday morning. By 18z the vast majority of
the showers will be offshore and the bulk of afternoon will be
rainfree. Since there are still some timing uncertainties we have
not shown anything higher than 80 or 90 percent...but 100 probability of precipitation
will be necessary at some point once the event gets closer. Quantitative precipitation forecast
looks to about 1/4 to 1/2 inch on average given the progressive
nature to the showers.

While we cannot entirely rule out some elevated convection...poor
thermodynamic profiles suggest that the risk for thunder is low
enough to omit from the latest forecast.

There is also a small window for possible sea fog early Sunday night
over the coastal counties...but latest trajectory forecasts suggests
that if it is able to form it would remain mostly in the nearby
Atlantic as winds increase.

Winds will rise considerably after midnight Sunday night in response
to a potent low level jet passing through...with SW winds to climb
to 15-25 and gusty...greatest over coastal SC. Then a pronounced
rise/fall couplet will develop Monday with the passage of the cold
front and this allows for winds to veer to west/northwest and rise to 20-30
miles per hour with stronger gusts areawide. We/ll need to carefully monitor for
a possible Wind Advisory over coastal SC late Sunday night and
maybe over a larger area Monday. For now we look to remain just
below the Wind Advisory criteria.

Temperatures will fall about 5-10 degrees Sunday evening...then hold steady
or even rise with increased warm advection and insulating effects
of the overcast skies. By Monday morning eastern sections will be
back into the lower 60s. Temperatures won't budge much the rest of
Monday...maybe even falling a few degrees as cold advection Cranks
up. Despite increased sunshine...much of this energy will be
utilized evaporating the standing rain waters.

Monday night and Tuesday...cold advection and clear skies will lead
to a cold night Monday night...with another large diurnal swing in
temperatures to occur. An advective freeze will make it to near the US-17
intra-coastal...with only the barrier islands to hold above
freezing. Although winds will diminish when compared to during the
day Monday...when combined with these temperatures will equate to wind
chills in the lower and middle 20s by Tuesday morning. Canadian high
pressure will settle atop the local area Tuesday with zonal flow
aloft...and despite plentiful insolation the low level thickness
forecast supports highs only in the upper 40s and lower 50s as the
synoptic flow veers around to the NE from off the Atlantic.

Lake winds...gusty winds will develop over Lake Moultrie late Sunday
night with a robust low level jet in advance of a strong cold front
that shifts through Monday morning. Cold advection and large
isallobaric pressure rises will then transpire in wake of the front
before fading Monday night into Tuesday. As a result we/ll likely
require a lake Wind Advisory at a later time for Lake Moultrie.

&&

Long term /Tuesday night through Saturday/...
transient surface high pressure will move into the Atlantic Tuesday
night. A coastal trough will once again develop along the periphery
of the Atlantic high on Wednesday...while a southern stream
shortwave develops far to the west across Texas and supports the
development of a surface low in the Gulf of Mexico. Although the
initial shortwave will dampen late Wednesday...the surface low will
continue to take shape and move east across the Gulf. Numerical
model solutions diverge quite notably regarding the strength of the
low pressure wave and the presence of additional shortwave energy
diving into the Gulf Coast states or southeast late Thursday into
Friday. As a result...confidence in forecast details becomes rather
low for the late week period.

After a period of dry weather early in the week...have reintroduced
rain chances by Wednesday to account for marine showers progressing
onshore from the coastal trough. Have then increased rain coverage
Wednesday night into Thursday...as medium range models are in
general agreement that some degree of rain will be affecting the
forecast area during this time frame. Model solutions that indicate
the second wave of upper level energy late Thursday into Friday
maintain rain coverage across the region accordingly...yet will cap
rain potential no higher than slight chance through Friday until
models come into better agreement. Have indicated rain chances
falling below 10 percent by Saturday...with any coastal low system
likely lifting away by the weekend.

High temperatures will be in the upper 50s to lower 60s Wednesday
and Thursday afternoons...before a notable cooling trend occurs on
the back side of the low pressure system Friday and Saturday.

&&

Aviation /20z Saturday through Thursday/...
VFR with light winds through the period...veering from
north/northeast to start to southerly toward the end of the
period ahead of an approaching cold front near the Mississippi
Valley. Could see some low clouds develop after 12z/01 but any
ceilings should remain VFR.

Extended aviation outlook...VFR conditions should will prevail until
Sunday night into Monday morning...when MVFR or possibly lower
conditions will occur in rain showers and lower cloud decks due to a passing
cold front. Breezy conditions and/or wind shear is also likely
Sunday night/Monday. VFR conditions will then return for the balance
of Monday into Thursday.

&&

Marine...
tonight...generally east winds 10 knots or less as high pressure
slides eastward over the area. Winds could back to more northeast
over the SC waters late as a weak coastal trough of low pressure
develops. Seas will average 1-2 feet near shore and 2-3 feet offshore.

Sunday...the local waters will lie within the western
side of surface high pressure moving further east into the
Atlantic. A relaxed gradient initially will begin to tighten in
advance of a slowly strengthening low and trailing cold front
moving through the middle/lower Mississippi Valley and Midwest. When
combined with sea breeze circulations will notice an increase in
winds and seas during the afternoon...but still below any advisory
conditions.

Sunday night and Monday...hazardous marine conditions will
eventually develop as the aforementioned low and associated cold
front moves steadily east and cross the Appalachians late Sunday
night...moving into and through the local marine district by late
Monday morning and the early afternoon. Mixing is somewhat curtailed
by the warm advection regime in advance of the cold front Sunday
night...but solid small craft advisories seem likely over most if
not all waters. We did consider hoisting a gale watch for late
Sunday night...but after coordination with neighboring weather forecast office/S jax and
ilm we have held off since it is the third period of the forecast.
But initially given a 50 knots or greater low level jet Sunday
night/early Monday followed by a notable rise/fall couplet in
pressures with the passage of the cold front...gales seem likely
across at least amz350-374.

Mariners are also alerted to the possibility of several hours of sea
fog...especially on the southern and central shelf waters Sunday
evening as a warmer and more humid air mass over-rides the cooler
nearby waters. But as winds increase and trajectories veer to the SW
through the night the risk for fog looks to end by 1-2 am Monday.
Also...some stronger T-storms could form out near the Gulf Stream
late Sunday night into Monday morning prior to the frontal passage.

Tuesday through Thursday...much better marine conditions will occur
for middle week under the influence of a transient surface
high...before winds and seas build again Thursday in advance of
system in the Gulf of Mexico. We/ll be well beneath any advisory
conditions Tuesday and Wednesday...but headlines might be required
again Thursday as a strong high pressure wedge begins to form
inland and a tighter gradient develops between this system and the
Gulf low.

&&

Tides/coastal flooding...
strong offshore winds will develop in wake of the cold front
Monday and will likely lead to the development of blow-out tides in
the afternoon with the low tide cycle. Tides might be as much as 1
feet MLLW below predicted levels during that time. Navigate with
extreme care as currents will be stronger than normal and underwater
structures and surfaces will become exposed that otherwise might not
be a problem during higher tide levels.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...Small Craft Advisory from 1 am Monday to 1 am EST Tuesday for
amz330-352.
Small Craft Advisory from 1 am Monday to 5 am EST Tuesday for
amz350.
Small Craft Advisory from 7 PM Sunday to 5 am EST Tuesday for
amz374.
Small Craft Advisory from 4 am Monday to 1 am EST Tuesday for
amz354.

&&

$$
Synopsis...
near term...
short term...
long term...
aviation...
marine...
tides/coastal flooding...

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