Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 151119

Area Forecast Discussion
National Weather Service Charleston SC
619 AM EST Fri Nov 15 2019

Low pressure will slowly pass offshore into Saturday, then move
away on Sunday. High pressure will then prevail for most of
next week.


The forecast is on track. KCLX is showing what appears to be
some weak, small perturbations moving north across the
interior, which is locally enhancing rainfall rates. RAP cross-
sections of equivalent potential vorticity, geostrophic momentum
and equivalent potential temperature suggest the presence of
conditional instability. While this does not impact the forecast
much, it is interesting to see this type of phenomena.

Surface cyclogenesis is ongoing across the eastern Gulf of
Mexico to off the northeast Florida coast early this morning
with the greatest isallobaric falls noted just east of the
Grays Reef buoy (41008). A surface low will steadily consolidate
off the coast of South Carolina and Georgia today as a phasing
of strong shortwave propagating across the Tennessee Valley and
the remnant southern stream upper low passing over Louisiana
occurs. The combination of favorable jet dynamics, strong
coastal convergence, and deep layered omega/DPVA within a plume
of PWATs in excess of 1.5 inches will support a large swath of
mostly light to occasionally moderate rain this morning. The
rain could become locally heavy at times along parts of the
Georgia coastal counties through daybreak as a band of
concentrated H9-H7 frontogenesis moves up the coast. Regional
radars are already detecting some enhanced reflectivities over
parts of northeast Florida, the Suwannee and extending into the
Darien/Sapelo Island which suggests the area of frontogenesis
exists. Well over 1.25" of rain has fallen over parts of
McIntosh County, GA since midnight.

Rain chances will steadily diminish from southwest to northeast
beginning as early as daybreak near the Altamaha River and
shifting into southern South Carolina by early afternoon as the
corridor of strongest forcing shifts northeast into northern
South Carolina into North Carolina. Pops around 100% looks
warranted for all areas early with pops steadily diminishing
into the 20-50% range by late afternoon, highest across the
Charleston Tri-County area. Even as the more concentrated areas
of rain end, there could still be some patches of light drizzle
lurking about. Highs will range from the lower 50s well inland
to the lower 60s at the beaches. Gusty to breezy conditions will
be possible at the beaches, mainly from Tybee Island north to
Cape Romain as the pressure gradient tightens as the surface low
consolidates and passes offshore. Gusts could approach 30 mph
at times, especially in exposed areas.

The phased upper level system to the west will quickly break
from the main belt of the westerlies and begin to develop into a
potent mid/upper-level low by this afternoon. The upper cyclone
will propagate very slowly to the east tonight passing over
Southeast Georgia early Saturday. Forcing for assent associated
with the cyclone itself will support another area of numerous
to widespread rains as the surface low off the South Carolina
coast quickly deepens. Data suggest most of these rains will
fall over Southeast Georgia beginning this evening, spreading
east to the coast through the night. A secondary area of rain
is expected to drop south out of the Pee Dee early Saturday
morning in response to moisture wrapping cyclonically around the
surface low offshore. This will make for a somewhat complicated
rain forecast with the highest pops (70-80%) confined to
Southeast Georgia this evening through night, with the highest
pops shifting back into Southeast South Carolina early Saturday,
again pops capped at 70-80% (highest in the Charleston Tri-
County area). Lows will range from the lower-mid 40s inland to
the upper 40s/lower 50s at the coast.


Saturday: A mid-level disturbance will pass overhead, moving
offshore at night. Meanwhile, low pressure will be passing offshore,
strengthening as it moves away. Models have trended wetter, so we
raised POPs and QPF. This is the most noticeable across the Charleston
Tri-County during the day. Additionally, gusty winds are
possible, especially near the coast. Conditions will gradually
improve overnight as the low moves away and drier air builds in
from the southwest. Though, remnant showers may persist along
the coast. The cold airmass remains in place with high
temperatures running about 20 degrees below normal. Luckily
minimum temperatures will only be a few degrees below normal.

Lake Winds: Conditions are favorable for a Lake Wind Advisory across
Lake Moultrie Saturday.

Sunday and Monday: The mid-level and surface lows will be moving
away from our coast as drier conditions build into the region.
Despite mostly sunny skies, high temperatures will remain well below
normal for this time of year.


Models are in good agreement showing high pressure building across
the Southeast. This will lead to dry conditions with a gradual
warming trend.


Widespread LIFR-IFR conditions will persist across the region
for much of the 12z TAF period. Cigs will generally remain in
the 300-600 ft range this morning with some slight improvement
to the upper end of IFR possible this afternoon. Cigs are
expected to lower back below 600 ft by early evening and linger
through the night. Vsbys of 3-5SM will persist with brief bouts
below 2SM. Vsby should improve to VFR by late morning as the
bulk of the rains exit the area. Will have to watch for possible
lingering drizzle this afternoon. More showers will impact the
terminals overnight, likely KSAV by 07z and KCHS after 09z.

Extended Aviation Outlook: Low pressure passing offshore will
cause flight restrictions to persist into Saturday night.
Additionally, gusty winds are expected to persist into Sunday.
VFR returns Sunday and prevails through Tuesday.


Today: Winds will steady ramp up today as low pressure organizes
off the coast. The tightest pinching of the pressure gradient
will initially start over the Georgia waters, then spread north
into the South Carolina waters over the next few hours as the
low deepens and becomes better organized. North winds will
average 20-25 kt across all legs with 15-20 kt with gusts to 25
kt in the Charleston Harbor. Small Craft Advisories will remain
posted through today. Seas will build 3-6 ft nearshore and 6-8
offshore, with lower waves occurring near the coast due to winds
tipping to the north.

Tonight: Speeds will initially diminish to 15-20 kt this
evening as the pressure gradient relaxes a bit as the surface
low pushes to a position off the northern South Carolina coast,
but are expected to increase again later in the night as the low
begins to more rapidly deepen. Winds will average 20-25 kt after
midnight and could rise to closer to 25-30 kt over the
Charleston County waters by daybreak Saturday. The greater risk
for gales looks to hold off until after daybreak Friday. The
Gale Watch for the Charleston County waters will be pushed back
a few hours as result with Small Craft Advisories remaining for
all other marine legs. Seas will hold 3-6 ft nearshore waters
with 6-8 ft offshore waters, again lower near the coast with the
offshore wind trajectories.

Saturday through Tuesday: Headlines will be up for all of the
marine zones, including Charleston Harbor Saturday morning. Low
pressure is forecasted to pass offshore Saturday, then
strengthen as it moves away Saturday night. A steep pressure
gradient will result in strong winds, with gusts to gale force
likely for portions of the waters. The worst conditions are
expected to be across the Charleston waters and GA waters beyond
20 nm Saturday evening/night. Steep, wind- driven seas are
expected to peak in the 8-10 ft range in these locations.
Conditions will improve on Sunday as the low moves further away,
allowing both winds and seas to trend downwards. Though, seas
may remain elevated (>=6 ft) for the GA waters beyond 20 nm
through Monday. By late Monday, high pressure will build into
the area, persisting into the middle of next week.


Positive tidal departures are running a solid 1.2-1.4 ft early
this morning with north to northeast winds. Latest trends and
guidance suggest total water levels will top out ~7.4 ft MLLW in
Charleston Harbor with ~9.1 ft MLLW at Fort Pulaski, but the
wind direction will be critical. This falls solidly within
shallow coastal flooding thresholds for much of the lower South
Carolina coast and a Coastal Flood Advisory has been posted for
Charleston, Coastal Colleton, and Beaufort Counties 8 AM through
12 PM. An advisory will not be posted at this time for the
Georgia coastal counties and Coastal Jasper County, SC as levels
look to fall just short of advisory thresholds at Fort Pulaski.
Trends will be carefully monitored, however.

Tide levels are expected to remain higher than normal through the
weekend due to strong winds associated with low pressure tracking
near the Southeast coast. Coastal Flood Advisories will likely be
needed during the morning high tides through the weekend for parts
of the South Carolina coast, and perhaps also for the southeast
Georgia coast. The height of the high tides will depend greatly on
the direction and duration of the sustained winds. We anticipate at
least minor salt water flooding, with at least some chance for
moderate salt water flooding along parts of the South Carolina
coast. If moderate rainfall is occurring near or at the times of the
high tides, the flooding situation could be further aggravated.


Rainfall records for 15 November:
KCHS: 0.95/2012
KCXM: 1.21/2018
KSAV: 2.45/1926

Record low maximums for 16 November:
KCHS: 49/1940
KCXM: 45/1916
KSAV: 49/1901


SC...Coastal Flood Advisory until noon EST today for SCZ048>050.
MARINE...Small Craft Advisory until 4 PM EST Sunday for AMZ352-354.
     Gale Watch from Saturday morning through Sunday morning for
     Small Craft Advisory until 6 AM EST Saturday for AMZ350.
     Gale Watch from Saturday morning through late Saturday night
     for AMZ374.
     Small Craft Advisory until 10 AM EST Saturday for AMZ374.
     Small Craft Advisory until 1 PM EST Sunday for AMZ330.


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