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000
FXUS62 KCHS 260256
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1056 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/...
As of 1055 PM: An update to remove headlines for the Coastal
Flood Advisory. Forecast for inland patchy fog and marine
showers remains on track.

As of 8 PM: Issued a quick update to add headlines for the
Coastal Flood Advisory for Charleston and coastal Colleton
Counties. High tide will occur around 1015 PM.

As of 7 PM: KCLX detected a few weak return beyond the outer GA
and SC waters, no activity detected over the forecast area. The
forecast area should remain under a mostly clear sky through the
rest of the night. Temperatures should cool to the lower to mid
70s by daybreak Monday.

Previous Discussion:
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 /03Z MONDAY THROUGH FRIDAY/...
VFR conditions expected across the terminals through the 0Z TAF
period. A weak surface ridge will remain tonight, then will
give way to an inverted trough as an area of low pressure
reaches the Southeast U.S. coast Monday morning. Moist southeast
llvl flow may yield a period of restrictive ceilings or fog
inland of the terminals during the pre-dawn hours. Weak showers
may develop near the terminals late Monday afternoon, coverage
appears too limited to highlight in the TAFs at this time.

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...
A Coastal Flood Advisory has been issued for Charleston and
coastal Colleton counties 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...NED
SHORT TERM...SPR
LONG TERM...SPR
AVIATION...NED/SPR
MARINE...BRM/SPR
TIDES/COASTAL FLOODING...