Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230804
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
404 AM EDT Fri Jun 23 2017

.SYNOPSIS...
Atlantic high pressure will concede to a cold front, which will move
through the area late Sunday. As the front slowly creeps offshore,
building high pressure is expected to bring drier conditions for mid
to late next week.

&&

.NEAR TERM /THROUGH TODAY/...
The region will remain along the western periphery of subtropical
high pressure today. Modified forecast soundings yield moderate
instability for late June featuring SBCAPE 2500-3000 J/kg, LI`s
-5 to -7C and SI`s 0 to -1C, but warm mid-level lapse rates
owing to subtropical ridging and an extensive low/mid-level
dry pocket will limit convection to the isolated to perhaps
low-end scattered categories this afternoon along/ahead of the
sea breeze. Pops will be limited to 20-30% inland with dry
conditions at the beaches. There will be a risk for localized
damaging winds with the stronger storms as DCAPE is forecast to
rise to near 1000 J/kg, but the lack of any meaningful 0-6km
bulk shear will support a mainly pulse severe convective mode.

Temperatures are poised to warm quickly after daybreak with
highs expected to reach the lower 90s inland while a persistent
onshore flow will limit highs to the mid-upper 80s at the
coast. Dewpoints in the mid 70s will yield heat indices of
100-103 for several hours away from the beaches which is just
sky of the pre-July 1st Heat Advisory criteria of 105.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
Tonight: Diurnal convection should quickly dissipate after
sunset with dry conditions prevailing overnight. It will be a
warm and humid night as nocturnal jetting will keep the
boundary layer fairly well mixed, especially at the coast where
breezy conditions will be found at times. Lows will only drop
into the mid 70s well inland while coastal areas should only
drop into the lower 80s. Would not be at all surprised if a few
coastal locations such as Tybee Island, Folly Beach and Downtown
Charleston only cool off into the 82-84 degree range. The
record high minimums for 24 June could be challenged at all
three climate sites. Mainly clear skies are expected prevail
once high-level debris cloudiness gradually dissipates, but some
marine-based stratocumulus could move inland from off the
Atlantic as the low-level flow strengthens, especially along the
lower South Carolina coast where the orientation of the coast
is the most favorable for inland penetration.

Saturday through Monday: The short term forecast starts out
with Atlantic high pressure to our east and a cold front slowly
approaching from the west. SE flow around these features will
pump plenty of moisture into our area ahead of the front. POPS
ramp up Saturday into Saturday night from inland locations to
the coast as the front gets closer to our area. By Sunday deep
moisture will be in place. PWATs should exceed 2.25", which very
high for this time of year. Additionally, there will be
increasing instability and shear. This will support showers and
thunderstorms across our area. Some of the storms could be
severe, with damaging winds being the main threat. The front is
forecasted to move through Sunday night or Monday morning, then
linger just offshore the rest of Monday afternoon. The risk of
showers/storms will be lower far inland, gradually increasing as
one gets to the coast, closer to the front.


&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
Monday night the cold front will be offshore of our area, dissipating
as it moves further away. Meanwhile, high pressure from the
central U.S. will gradually be building towards our area. The
high is expected to pass north of our area Wednesday, then move
offshore Thursday. Dry conditions are expected for most of the
land area. The only shower threat appears to be over the coastal
waters each night and maybe along portions of the immediate
coast. Temperatures will be slightly below normal through
Wednesday, then creeping up on Thursday.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
VFR. Could see a brief stint of MVFR cigs just before sunrise,
but no major impacts are expected. Incoming high resolution
guidance suggests isolated/scattered showers/tstms will likely
cluster west of the KCHS-KSAV corridor this afternoon. It could
be close at KSAV, but with probabilities generally limited to
20-30%, no mention of VCTS or TSRA will be included at either
terminal for the 06z TAF cycle.

Extended Aviation Outlook: Periods of flight restrictions are
expected late Saturday into Monday due to a cold front slowly moving
through the area.

&&

.MARINE...
Today: Southerly winds will persist today as Atlantic high
pressure holds firm well offshore. South winds around 15 kt will
prevail for much of the day, but will begin to increase later
this afternoon as the low-level jetting begins to take hold.
Could see gusts approaching 20 kt in the Charleston Harbor for a
few hours this afternoon as the sea breeze moves inland, but
think conditions there will hold just below Small Craft Advisory
thresholds. This will have to be watched carefully. Thankfully,
the tide will be coming in when winds will be at their highest
so counterflow conditions are not expected. Seas will average
3-4 ft.

Tonight: Nocturnal jetting is forecast strengthen overnight as
Atlantic high pressure buckles a bit after midnight. Nocturnal
jetting looks strongest over the Charleston County waters were
speeds will reach 20 kt at times with gusts 25-30 kt. Elsewhere
Winds look to average 15-20 kt. Seas will also build in the
southerly flow, reaching 4-6 ft over the Charleston County
waters with 4-5 ft elsewhere. Will hoist a Small Craft Advisory
for the Charleston County waters beginning late this evening.
Conditions will be close to advisory thresholds elsewhere,
especially the Charleston Harbor and Georgia offshore legs, but
will let the day crew decide if a Small Craft Advisory is needed
after the 12z guidance is received.

Saturday through Tuesday: Atlantic high pressure will remain east of
the waters this weekend as a cold front approaches the area. The
front is expected to move through Sunday night, then slowly move
offshore on Monday. Winds will be elevated into Sunday, ahead of the
front. It`s borderline whether gusts will reach advisory criteria,
but the elevated winds should produce 6 foot seas beyond 10 nm for
the Charleston waters. As a result, a Small Craft Advisory is in
effect through Saturday night. This is highly dependent on the wind
speeds and the end time may need to be adjusted. Furthermore, if
winds are stronger than expected, advisories may need to be expanded
to the remaining zones due to gusts and seas. Following the frontal
passage early next week, conditions will improve as high pressure
builds into the area.

Rip Currents: The combination of breezy conditions in the surf
zone and influences from the new moon, lunar perigee and ongoing
elevated tides will support a moderate risk for rip currents at
all beaches today. Thankfully, there does not appear to be much
of a swell component which would likely bump the risk into the
high category. An elevated risk for rip currents will continue
through the weekend.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Advisory may be needed for portions of the lower
South Carolina coast during the evening high tide cycle.

Tides will be running high given the increased astronomical
influences and onshore winds could help push levels high enough
to produce shallow coastal flooding around the times of the
evening high tide through the weekend, especially along the
South Carolina coast.

&&

.CLIMATE...
Record high minimums for 24 June:
KCHS 79/2016.
KCXM 85/1998.
KSAV 79/1932.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 11 PM this evening to 2 AM EDT
     Sunday for AMZ350.

&&

$$


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