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000
FXUS62 KCHS 251956
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
356 PM EDT Sun Jul 25 2021

.SYNOPSIS...
High pressure will retreat northeast and weak low pressure will
approach the southeast coast tonight. The low pressure should
push onshore across southeast Georgia Monday. A persistent
trough of low pressure will then support unsettled conditions
into midweek. The trough should then weaken in favor of
building high pressure late week into next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Tonight: Upper-level winds will shift to a more zonal
orientation associated with high pressure. At the surface, sub-
tropical high pressure over the Atlantic will persist, however
ridging over the forecast area will recede as weak low pressure
off the east coast of Florida shifts northwest. Models show the
low pressure opening into a trough as it nears the extreme
southeast GA coast by morning. East to southeasterly flow will
gradually pull in moisture from the system overnight, increasing
dew points to the low to mid 70s and PWATs to near 2".
Lingering subsidence will continue through late evening keeping
the forecast rain-free over the land areas overnight.

Areas of low stratus will develop late tonight with increased
moisture advection. In addition, winds will calm inland as the
boundary layer decouples. Dewpoint depressions will near 0-1 degrees
in the aforementioned area where some light patchy fog could develop
prior to sunrise and linger until daybreak.

By early morning, low-level convergence within an area of
enhanced moisture over the western Atlantic will cause showers
to develop offshore. PoPs increase to slight chance around
sunrise along the coast as scattered showers begin to approach
the coast.

Overnight temps will be a bit warmer than the previous nights,
dropping into the low 70s inland and upper 70s at the beaches.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: Low pressure located off the southeast coast Sunday
afternoon is expected to push onshore, most likely into southeast
Georgia, Monday. This feature should inject greater moisture with
PWATs increasing to greater than 2 inches across the region, and the
chance for showers/thunderstorms should increase. Because
showers/thunderstorms associated with the offshore low remain
disorganized, maximum PoPs during maximum heating Monday afternoon
are capped at 30-40 percent south to 15-20 percent elsewhere.
However, these PoPs could prove to be conservative, especially
across southern counties. Otherwise, outside thunderstorms, expect
greater coverage of dewpoints in the 70s as compared with past
couple of days, accompanied by high temperatures in the middle/upper
80s near the coast and lower 90s inland.

Monday night: Expect a diurnal decrease in convection coverage after
dark. However, the combination of moisture and a surface trough
could continue to support isolated showers/thunderstorms south the
night, especially across southern and coastal counties.

Tuesday through Wednesday: Expect an active period with above normal
PoPs as a surface trough persists and a mid/upper upper level trough
settles into the region. Within this regime, a pool of high PWAT
values exceeding 2.25 inches, deep cyclonic flow and a series of
elongated, progressive shortwave troughs aloft should readily
support at least numerous showers/thunderstorms, with greatest
coverage during the afternoon/evening hours but also some degree of
convection persisting through Tuesday night.

This scenario could support locally excessive rainfall, and
brief/isolated episodes of damaging wind gusts cannot be ruled out
if/where mesoscale boundary/convective element interactions occur.
The latest forecasts remain quite general, and subsequent forecasts
will graduate toward greater detail regarding timing/impacts.

Outside thunderstorms, high temperatures should range from the
middle/upper 80s near the coast to the lower 90s inland. However, as
usual thunderstorms will greatly alter hourly temperatures at any
given location.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Thursday and Friday: While a surface trough will persist across the
region, medium range models continue to depict deeper moisture
shifting south of the region as the upper trough relaxes and an
inland upper ridge expands toward the east CONUS. This scenario
should support above normal temperatures and below-average PoPs
through late week. Then, the upper ridge is progged to retreat
toward the south/west, allowing the persistent NE CONUS trough to
amplify toward our region. As these subtle changes occur, a return
of moisture more typical of this time of year should translate to
normal temperatures and normal to above normal PoPs. In fact,
conditions could eventually become favorable for inland MCSs to
migrate into our region within the NW flow aloft next weekend or
beyond.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Mainly VFR through tonight. Areas of stratus will form inland
late tonight within a moistening onshore flow and the formation
of a nocturnal inversion. Models are hinting at low stratus or
even fog forming inland, which may expand eastward toward the
terminals by early morning, especially at KSAV where the greater
moisture is located. Introduced BR at KSAV just before sunrise,
along with lower (VFR) ceilings. MVFR cigs are possible and at
the worst: brief IFR; however those details will come with later
updates when confidence increases. Late morning Monday there is
a slight chance of scattered showers that could begin to move
onshore which could keep stratus from lifting, hence the reason
the morning prevailing group remains unchanged through 18Z
Monday at all terminals.

Extended Aviation Outlook: Low probability for brief flight
restrictions in showers/thunderstorms Monday afternoon/night. Longer
periods of flight restrictions are possible within/near more
numerous thunderstorms Tuesday through Wednesday. Then, mainly VFR
with a lowering probability for thunderstorms late week.

&&

.MARINE...
Tonight: The low pressure system to the south will draw closer
to the southeast Georgia coast late night, as the ridge across
the local waters recedes 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, expect easterly winds to shift out of the southeast
overnight with speeds 10 to 15 kt. Seas will average 2 to 4 ft.

Monday, the latest marine forecast assumes that low pressure pushing
through GA waters and onshore along the SE GA coast will remain weak
as expected. E/NE winds should veer toward the SE, mainly averaging
10-15 knots with a few gusts 15-20 knots possible. Seas will average
2-4 feet most areas except perhaps briefly up to 5 ft beyond 20
nm.

Monday night through Wednesday, outside thunderstorms S/SW winds
should average 10-15 knots and seas will average 2-4 feet, highest
beyond 20 nm. The potential for thunderstorms with locally hazardous
conditions will increase, especially Tuesday through Wednesday, and
isolated waterspouts could occur if/where mesoscale boundaries exist
of coastal waters early Wednesday.

The chance for thunderstorms should decrease after Wednesday, but SW
winds could increase and seas could build late week, especially
starting Friday. We will continue to assess the potential for SCA
conditions.

&&

.TIDES/COASTAL FLOODING...
Tides will continue to be above astronomical levels tonight,
and could again produce minor salt water flooding in downtown
Charleston and some neighboring coastal sections. A Coastal
Flood Advisory for Charleston and coastal Colleton counties
could be needed for tonight`s high tide, which will occur
around 1015 PM.

For Monday, astronomical influences will diminish, and winds will
veer toward the southeast. Assuming low pressure pushing onshore in
SE GA remains weak as expected, there exists only a low probability
for minor salt water flooding along the Charleston County coast with
the late evening high tide.

&&

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

&&

$$

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