Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201139

Area Forecast Discussion
National Weather Service Charleston SC
739 AM EDT Fri Jul 20 2018

A front across the area will gradually lift north through
tonight as low pressure moves up the coast. A trough of low
pressure will persist into the middle of next week before
Atlantic high pressure starts to rebuild late next week.


As of 7 AM: Radar rainfall estimates indicate a large swath of
4-5 inches of rain has fallen across Downtown Charleston to
Daniel Island. Multiple flooded and closed roads have been
reported across the area. One residence located downtown is
surrounded by waist-deep flood waters, with several inches of
water flooding the house. The band associated with the flash
flooding is lifting north of the area, followed by a wide area
of stratiform rain.

As of 420 AM: Latest sfc analysis indicated a inverted trough
or stationary front running across inland GA east near the SC
coast. Recent GOES-East WV images clear show the axis of a mid-
level trough passing over middle GA. As the mid-level
disturbance approaches later this morning, a frontal wave should
consolidate over SE GA. Once the low develops, near term
guidance indicates that it will track NE within the sfc trough.
The environment ahead of the low will feature PW values as high
as 2.4 inches, CAPE around 2000 J/kg, and WBZ around 15 kft.
Near term models, including many CAMs, indicate that solid bands
of thunderstorms will develop ahead of the approaching low. In
fact, as the low deepens later this morning, these bands appear
to pivot or slow across the Charleston Tri-County area. Rainfall
amounts under these bands are forecast to range from 2-3
inches, WPC indicates the potential for 5 inches. We will
highlight the area with a Flash Flood Watch until 9 PM. See
Hydro section for more details. High temperatures should be
limited by the frequent rounds of rainfall and thick cloud
cover, peaking in the mid 80s across SC and mid to upper 80s
across SE GA.

Tonight: The center of low pressure is forecast to track north of
the forecast area. Dryer air should wrap around on the west side of
the low, resulting in shower and storm coverage to shift east during
the late evening and overnight hours. I will forecast rainfall to
end late tonight. Using a blend of MOS, low temperatures are
forecast to range in the low to mid 70s.


NVA behind a departing shortwave will affect the area Saturday
morning into early afternoon, producing mid-level subsidence and
limiting rain chances during this time. However, modest surface
heating and dewpoints hovering in the low 70s will produce
increasing instability. Things potentially get interesting late
Saturday afternoon into the evening. Models show a potent
shortwave dropping southeast from northern GA/SC into the area.
Bulk shear increases to 35-40 kt while model soundings show 1-2k
J/kg CAPE values and DCAPE exceeding 1.2k J/kg. If a cluster of
convection develops upstream earlier in the afternoon, it would
likely continue moving southeast through at least the northern
half of the forecast area late Saturday afternoon into the
evening. Severe weather is possible, mainly from damaging
straight line winds, though it could be on a weakening trend by
the time it reaches our forecast area. We show the greatest PoPs
later Saturday afternoon into the evening, pushing offshore

An elongated upper trough will slowly dig into the Deep South on
Sunday. A lobe of somewhat drier air is forecast to settle over
the forecast area with PWATs dropping below 1.5" at times. Low-
level westerly flow during the morning will keep the sea breeze
pinned at the coast. Later in the afternoon we should see
moderate convergence along the coast as the sea breeze develops
and interacts with moderate instability.

PWATs are expected to rebuild on Monday as the upper trough digs
farther to the south and becomes slightly negatively tilted.
This will allow the mean flow to become more southerly,
advecting moisture off the Atlantic. Although the best
convergence will remain over the coastal waters, there should be
ample forcing for scattered showers and tstms over land areas.


The upper trough across the eastern United States will gradually
weaken through Thursday then a deep layered Atlantic ridge will
build from the east. Deep moisture and synoptic scale forcing
should support numerous showers and tstms through Wednesday
before coverage becomes more scattered late week.


Broad low pressure developing over eastern GA will track NE
across the forecast area through this evening. MVFR ceilings
may persist over KCHS through the rest of the daylight hours,
with passing showers and VCTS. A round of afternoon
thunderstorms is expected to develop over KCHS this afternoon,
highlighted with a TEMPO from 17Z-21Z. Patchy fog is possible
over the wet soil during the pre-dawn hours Saturday.

KSAV will likely see VFR conditions early this morning, with
MVFR ceilings with morning heating. Winds are expected to shift
from the west during the early afternoon. There is some
potential for MVFR ceilings to occur during the afternoon into
this evening. However, I will keep the TAF VFR after 17Z.

Extended Aviation Outlook: VFR conditions will be interrupted
by periods of flight restrictions in showers/thunderstorms.


Widespread showers and thunderstorms are expected across the marine
zones today, especially off the SC coast. Visibilities may be
reduced to less than a mile during downpours. The sfc pattern will
support SW winds between 10-15 kts. Wave heights are forecast to
range between 2-3 ft within 20 NM to 3-4 ft beyond 20 NM. Low
pressure is forecast to track north of the region tonight. In the
wake of the low, winds will veer from the WNW after midnight across
the marine zones. Wave heights beyond 20 NM are forecast to increase
to around 5 ft. Conditions are expected to remain below Small Craft
Advisory levels.

Saturday through Wednesday, the gradient will be enhanced for
much of the time due to surface low pressure over the Southeast
while weak high pressure remains offshore. South to southwest
winds will range from 10-20 kt for much of the period. In
addition, numerous showers and thunderstorms will produce
locally hazardous conditions.


Several SC coastal areas received between 2-3 inches of rainfall on
Thursday. The heavy rainfall resulted in the stream gauge on Church
Creek near West Ashley (BEES1) to rise 2 feet. Early this morning,
the stage of BEES1 was observed around 1 foot below flood stage.
Today, heavy showers and thunderstorms are forecast to produce 2-3
inches of rain across the CHS Tri-County, WPC indicates the
potential for 5 inches. We have issued a Flash Flood Watch until 9
PM for the Tri-County. The arrival of the heavy rain should result
in flash flooding, especially locations along small streams/creeks
and Downtown Charleston. The next high tide along the SC coast will
peak around 3:30 PM.


SC...Flash Flood Watch through this evening for SCZ044-045-050-052.


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