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000
FXUS62 KCHS 250810
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
410 AM EDT Sun Jul 25 2021

.SYNOPSIS...
High pressure will be far to the northeast while a weak low
moves towards Florida through tonight. Then, a typical summer
pattern will prevail with high pressure in the Atlantic and
weak low pressure inland.

&&

.NEAR TERM /THROUGH TONIGHT/...
Pre-dawn: We`ll begin the day with cooler than normal conditions
for most places. Expect temps that are down to the mid and
upper 60s far inland, 70-76F over much of the coastal counties,
with upper 70s in downtown Charleston and right along the
oceanfront. The coolest places will be in the Francis Marion
National Forest.

Today: Sub-tropical ridging from the Atlantic at the surface and
in the low levels, along with a west to east oriented ridge in
the mid and upper levels will generate another day with large
scale subsidence and rainfree conditions. This is quite unusual
for the "Dog Days" of summer.

Meanwhile, a weak but well defined area of low pressure that is
about 150 nm off Daytona Beach and Cape Canaveral, Florida. This
system is embedded within a deep layered dry air mass, and as a
result, convection is quite limited. Since the surrounding
environment is so dry, a weakening trend by the majority of the
models seems the most likely forecast as that feature drifts
west or west-northwest through the day.

Skies will average out to be mostly sunny today, with no more
than few to scattered stratocumulus and flat cumulus moving in
from the east, and scattered to maybe broken cirrus arriving
from the north and northwest. 850 mb temps rise a bit today,
reaching 18-19C, which supports max temps of 90-92F near and
inland from US-17, with mid and upper 80s closer to the
shoreline. the associated relative humidity levels will be quite
comfortable for this time of year, with dew points down in the
mid and upper 60s for many places away from the ocean.

Tonight: High pressure from the Atlantic gradually relaxes its
hold on the local region, as the weak low off Florida will
likely open into a trough as it approaches the coast near St.
Augustine and Jacksonville by daybreak. As the system gains some
northern latitude, low level convergence and some increase in
moisture could spur a few showers over parts of coastal Georgia
late at night. It won`t be quite as cool as the previous two
nights with continued air mass modification. But still looking
for lows that are near or a bit below climo.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
The mid-levels will consist of a long-wave trough over the
Northeastern U.S. and a strong and broad ridge to our west. Both the
trough and the ridge will strengthen/amplify with time. The surface
pattern will be typical of summer with high pressure in the Atlantic
and troughing inland.

Monday: Moisture is expected to increase with PWATs creeping towards
2" by late in the day. Most of the models show the afternoon sea
breeze initiating convection as it moves inland. The highest
moisture and POPs are over our GA counties. The highest instability
appears to be just south of our area. But if it does creep into our
southernmost counties late in the day, a pulse thunderstorm with
marginally severe wind gusts cannot be ruled out. Further north,
especially across our SC counties, there won`t be much instability,
especially for late July, so the overall risk for severe
thunderstorms is low. Any convection will dissipate in the evening.
However, more convection may form over the coastal waters late at
night and move towards the coastal counties. Temperatures will be
near normal.

Tuesday: Moisture will continue to increase across our area with
PWATs approaching 2.25" by the afternoon. Instability will also
increase quite a bit. Both of these should cause convection to start
a little earlier in the morning, then increase in both coverage and
intensity into the afternoon with the inland moving sea breeze. The
highest POPs are far inland late in the afternoon. A few pulse
severe thunderstorms are possible with damaging winds. Also, these
storms may not move too fast due to weak winds higher up. Given all
of the moisture, locally heavy rainfall in slow-moving and/or
back building storms could lead to flooding in low-lying and
poorly drained areas. The worst convection should dissipate in
the evening. But showers/thunderstorms are expected to persist
overnight. Temperatures will be near normal.

Wednesday will be similar to Tuesday. Only moisture and instability
could be higher, increasing the threat for thunderstorms and
flooding.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The models have a mid-level trough prevailing to our north and a
strong ridge to our west. The surface pattern will be typical of
summer with high pressure in the Atlantic and a trough of low
pressure inland. The NBM forecast reflects this with diurnal
showers/thunderstorms and temperatures within a few degrees of
normal.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
While we can`t rule out very intermittent MVFR ceilings at KJZI
this morning, VFR weather will prevail there, at KCHS and at
KSAV through 06Z Monday.

Extended Aviation Outlook: Flight restrictions are possible in
showers and thunderstorms.

&&

.MARINE...
Today: Sub-tropical ridging is the main feature of note for the
local waters, while a weak low pressure system off the coast of
northeast and east Florida drifts closer to the peninsula. Even
with sea breeze influences, winds won`t be a great as the past
couple of days, but still enough to get NE and E winds as high
as 15 kt. Seas will be a mix of swells and wind driven waves,
averaging 2-3 feet within 20 nm and mainly 4 feet on the outer
Georgia waters.

Tonight: The low pressure system to the south draws closer to
the immediate coast of northeast Florida by late at night, as
the ridge across the local waters starts to pull east. The low
will likely transition into a trough as it shifts northwest and
continues to encounter considerable dry air. Provided this
feature isn`t stronger than anticipated, we look for E and SE
winds mainly 10 or 15 kt, with seas of 2-4 feet.

Extended Marine: Monday, high pressure far to our northeast will
move further offshore, causing winds to clock around to the
southeast, then south. Then, a typical summer pattern will persist
with high pressure in the Atlantic and a trough inland. Winds will
generally be from the south/southwest. Each afternoon, expect slight
backing of these winds and higher gusts associated with the sea
breeze along the land/sea interface and in the Charleston Harbor.
Each night, expect slight veering of these winds. No marine
headlines are anticipated. However, there may be an increased risk
of waterspouts Wednesday morning.

Rip Currents: Persistent onshore flow, a small swell and
lingering astronomical factors will continue to produce a
Moderate Risk for rip currents on all beaches today. The risk
for rip currents should diminish Monday.

&&

.TIDES/COASTAL FLOODING...
Tides will continue to be well above astronomical levels today
into tonight, and will again produce at least minor salt water
flooding in downtown Charleston and some neighboring coastal
sections. This will likely result in the issuance of a Coastal
Flood Advisory for Charleston and coastal Colleton County for
the late evening high tide.

For Monday, astronomical factors will wane, and winds will veer
toward the southeast. But a final episode of minor salt water
flooding will still be possible along the Charleston County
coast with the late evening high tide.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...