Area Forecast Discussion
Issued by NWS Charleston, SC

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959
FXUS62 KCHS 241049
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
649 AM EDT Sat Jun 24 2017

.SYNOPSIS...
A cold front will gradually move through the area this weekend,
moving off the coast Monday. Drier high pressure will build in
from the northwest for much of next week before moving offshore
late week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Incoming high resolution guidance has been trending slower with
the arrival of convection this afternoon and evening. Have
adjusted pops accordingly, but did not slow the progression of
showers/tstms as much as some data would suggest. Further
refinements are likely later today.


Convection will be slow to work its way into Southeast South
Carolina and Southeast Georgia today as subtropical high
pressure breaks down ever so slowly. Weak capping noted on
various forecast soundings should hold through early afternoon
before gradually mixing out as temperatures warm through
convective temperatures in the upper 80s. Several broken bands
of showers/tstms are expected to develop across central Georgia
and the South Carolina Midlands along a pre-frontal trough by
early/mid afternoon as synoptic forcing intensifies ahead of an
approaching 700 hPa shortwave. This activity should approach
the far inland areas, roughly north of a Metter to Saint George
line, by late afternoon while rain-free conditions persist
central and coastal areas. Pops will be heavily gradated
mid/late afternoon, ranging from 40-50% well inland to 0% at the
beaches.

Modified soundings are not overly unstable for late June,
likely due to dewpoints mixing out a bit more compared to the
past several days. SBCAPE 1500-2500 J/kg will still support
moderately strong updrafts, but poor mid-level lapse rates and
the lack of strong kinematics will support a mainly pulse
convective mode with the risk for localized damaging wind gusts
being likely confined where mesoscale boundary interactions
occur. Guidance does show 0-6km bulk shear increasing to about
20-25 kt across southern South Carolina closer to sunset which
could support a slightly better potential for loosely organized
convective clusters and thus a slightly better potential for
damaging winds.

It will be another hot and somewhat humid day for the region
with highs warming into the lower-mid 90s inland with mid-upper
80s at the beaches. Dewpoints will mix out into the lower 70s
for most areas with some upper 60s possible in the Savannah-
Ludowici-Darien corridor. This will limited heat indices to the
99-102 range and below the pre-July 1st Heat Advisory criteria
of 105.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY/...
Tonight: The pre-frontal trough will propagate into the area
this evening as a cold front moves into north Georgia and the
South Carolina Upstate. Scattered to numerous showers/tstms will
work their way from west-east for much of the evening hours as
700 hPa shortwave passes through and eventually offshore after
midnight. Although the greater synoptic forcing will occur prior
to midnight, convection will likely linger for much of the
night as a cold front meanders into the far inland areas prior
to daybreak Sunday. Pops will range from 70% across much of
southern South Carolina to 40% across McIntosh County, GA. Will
have to watch Downtown Charleston for possible flooding as
convective rains could fall during the elevated evening high
tide. Lows will range from the mid 70s inland with upper
70s at the beaches.

Instability will slowly wane with the loss of insolation and the
risk for severe weather should end by 9 pm or so. Again, the
risk will be highest across Southeast South Carolina, especially
north of an Allendale-Folly Beach line, where 0-6km bulk shear
of 20-25 kt could support several loosely organized convective
clusters.

Sunday: A broad upper trough will persist across the eastern half of
the country. A cold front will be moving approaching from the
northwest during the morning, slowly moving towards the coast.
Southerly flow will advect plentiful moisture ahead of the front.
PWATs should reach 2.0-2.25", noticeably high for this time of year.
Decent instability should be in place with BLCAPEs expected to reach
2,500 J/kg far inland during peak heating along with Lifted and
Showalter Indices trending in the right direction for storm
development. However, low and mid level lapse rates are not too
impressive. 0-6 km bulk shear increases to around 35 kt late in the
afternoon farther inland. We`re maintaining showers and
thunderstorms in the forecast with the best opportunity for strong
to maybe severe storms being far inland. The main threat would be
damaging winds, but locally heavy rainfall is also possible. The
storms should be moving fast enough to limit the risk of localized
flooding. Thunderstorms will decrease after sunset. The front is
expected to move through in the evening and overnight hours.
Locations along the coast can expect showers to linger through the
night while locations further inland should see improving conditions.

Monday and Tuesday: The front will linger just offshore Monday
afternoon keeping at least a chance for convection for the coastal
counties. POPs taper as one head inland and it very well could be
completely dry far inland. The front will dissipate and move further
offshore Monday night into Tuesday as continental high pressure
builds into the area. The high will bring dry conditions and
slightly below normal temperatures. Dewpoints in the low to mid 60s
will make it feel very comfortable outside.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
High pressure will pass to our north Tuesday night into Wednesday,
bringing dry conditions and slightly below normal temperatures. The
high will shift offshore Wednesday night, allowing southerly flow to
develop. Temperatures will trend back to normal at this time and
moisture will increase, bringing a return to the typical summertime
shower/thunderstorm pattern.

&&

.AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
VFR through late afternoon with increasing probabilities for
showers/tstms impacting the terminals during the evening and
overnight hours. Guidance has trended slower with the
progression/development of convection this afternoon. As is
typical with summer convection, it will be difficult to time the
arrival and specific impacts with tstms. Best chances for tstms
at KCHS look to be 02-05z at KCHS and 03-06z at KSAV. After
which, steady light rains are expected to settle in through
sunrise Sunday. Limited conditions in the tstms to MVFR for now,
but localized IFR or lower is possible within brief, heavier
convective elements.

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

&&

.MARINE...
Today: A potent nocturnal surge is impacting the waters this
morning with winds ranging from 20-25 kt with gusts to 30 kt
along the Charleston County waters with 20 kt elsewhere. Winds
should subside a bit after sunrise as the low-level jet weakens
slightly, but enough will be present to keep winds 15-20 kt
through the day with frequent gusts to 25 kt possible along the
Charleston County coastal waters. Seas will average 2-4 ft. A
Small Craft Advisory remains in effect for the Charleston County
waters. Winds will once again be close to advisory thresholds
in the Charleston Harbor this afternoon as the sea breeze moves
inland. Forecast soundings and marine guidance at FBIS1 suggest
sustained winds will stay just below 20 kt with frequent gusts
below 25 kt, so will hold of on the issuance of a Small Craft
Advisory for now. However, conditions could be rough at times
for recreational craft from late morning into the mid-afternoon
hours as the tide goes out and counterflow conditions develop.

Tonight: There will be a potential for a few strong tstms moving
across the coastal waters this evening as a pre-frontal trough
settles in ahead of an approaching cold front. A few of these
storms could produce wind gusts in excess of 35 kt and Special
Marine Warnings may be needed at some point. Otherwise, a modest
southwest surge of 15-20 kt will persist across the waters
overnight with seas 3-5 ft. A Small Craft Advisory will remain
in force this evening for the Charleston County waters where
frequent gusts to 25 kt will be possible prior to the arrival of
showers/tstms.

Sunday through Wednesday: Atlantic high pressure will remain
east of the waters Sunday as a cold front approaches the area.
SW winds ahead of the front will veer to the NW as the front
moves through Sunday night. No advisories will be needed with
the frontal passage. The front may meander just to our south and
east Monday before it dissipates and is pushed further offshore
late Monday night. High pressure will build into the area
Monday night and Tuesday, passing to the north on Wednesday.
Light winds and low seas are expected during this time frame.

Rip Currents: Early morning spectral data and buoy observations
show a 2-3 second southeasterly swell is impacting the coastal
waters. NWPS Gerling-Hanson plots for Capers Nearshore (41029)
suggest 2 ft, 9 second swell will impact the Charleston County
beaches later today, which combined with breezy onshore winds
and lingering astronomical influences will support a high risk
for rip currents. The risk will be especially high from late
morning into mid-afternoon with the outgoing tide. For the
Georgia and remaining South Carolina beaches, a moderate risk
for rip currents will be maintained as Gerling-Hanson plots for
Fripp Nearshore (41033) suggest only a 1 ft, 9 second swell
will beach the beaches. A similar enhancement can be expected,
especially at Tybee Island, as low tide approaches mid-
afternoon.

&&

.TIDES/COASTAL FLOODING...
There is a chance tides could reach shallow coastal flooding
thresholds with the evening high tide, mainly along the lower
South Carolina coast. A Coastal Flood Advisory may be needed.

Evening high tides will continue to be elevated through Monday due
to astronomical influences. The wind direction is not very conducive
to pushing much surge towards shore, so it`s hard to say if we will
reach 7 ft MLLW at Charleston Harbor during the evening high
tides.

&&

.CLIMATE...
The low so far at KCHS is 79 degrees. If this holds through 1
am Sunday, it would tie the record high minimum for the date that
was last set in 2016.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...High Rip Current Risk through this evening for SCZ050.
MARINE...Small Craft Advisory until 2 AM EDT Sunday for AMZ350.

&&

$$



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