305 
FXUS62 KCHS 221803
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
203 PM EDT Tue Jun 22 2021

.SYNOPSIS...
A cold front will move through the area tonight, then stall
just off the Georgia coast Wednesday. A coastal trough will
persist through early next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
A cold front that is associated with a broad and deep layered 
trough that covers much of the eastern half of the country is 
currently located over southwest Alabama into central Georgia.
A rich plume of sub-tropical moisture, with PWat of 2 inches or 
more within the warm conveyor belt region will exist across the 
forecast counties throughout the day.

The highest coverage of rainfall has passed as earlier 
convection helped stabilize the atmosphere. Current radar shows 
only stratiform rain remaining over our SC and coastal GA 
counties, while some weak convection widely covers the offshore 
waters. Clearing could allow the atmosphere to destabilize 
somewhat and as the front becomes closer in proximity from the
west this evening, more support from the right entrance region 
of the upper jet over the Southeast, could support another round 
of scattered showers and possibly a few rumbles of thunder.
Coverage should remain inland and diminish as the front 
gradually moves east.

The grounds are saturated, more so now from the rainfall this 
morning. The 0-10 cm Soil Moisture analysis from NASA is as 
great as the 80th or 90th percentile across the local area. So 
it won't take much to generate at least minor flooding problems.
But since the instability is limited, any threat looks to be 
highly localized.

Tonight: The final short wave pulls offshore early on, and the 
cold front at the surface limps through the area almost parallel
to the flow aloft. There is a slight decrease in PWat, and with
further stabilizing influences within the nocturnal environment,
we do anticipate less activity after dark. We'll still go with
scattered coverage in association with the nearby cold front.

The saturated atmosphere and winds diminishing, plus low 
condensation pressure deficits will lead to the formation of 
stratus build down. Latest projections are that it won't be low 
enough to produce any significant reduction in visibilities, 
but we will reevaluate through the day to see if fog should be 
included in the forecast. Lows tonight will get down close to 
actual dew points, which will be in the upper 60s and lower 70s.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
The cold front will stall off the coast Wednesday morning, then
transition into a coastal trough. Moderate moisture will reside
over the area Wednesday into Thursday while the trough maintains
weak low-level convergence. A late afternoon sea breeze is also
expected both days which could trigger additional showers and
tstms.

The trough is expected to move inland on Friday, accompanied by
a weak upper low. We expect to see an increase in moisture along
with the trough, pushing PWATs close to 2". Additionally, an
early afternoon sea breeze could provide an additional trigger
for convection. Conditions favor pretty good coverage of showers
and tstms on Friday.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Atlantic high pressure gradually strengthens over the weekend
into early next week, resulting in a slow warming trend with
convective coverage more scattered in nature.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
A disturbance has moved offshore allowing some clearing to 
occur over the terminals; however, some showers are moving 
northward toward KCHS/KJZI where mention of -RA/SHRA is included 
through 20Z. Shower activity could return by the evening ahead 
of an approaching cold front from the west, hence mention of 
VCSH, yet guidance shows any returning convection to stay west
of the terminals. Since most of the greater rainfall coverage 
has passed, gusty winds have nearly diminished and models 
indicate build down of low stratus/stratocumulus which will 
result in MVFR/IFR ceilings through the night. There could be 
periods of VFR if enough clearing occurs in the late afternoon, 
but chances are that it will be brief. Models show a likely 
chance for flight restrictions to last through the night though 
they seem to struggle with the timing/possibility of returning 
to VFR. The current TAF reflects lifting of stratus around 14Z; 
this will hopefully be refined in the 18Z TAF issuance.

Extended Aviation Outlook: Occasional, brief flight restrictions
are possible in showers and thunderstorms, especially through 
Wednesday.

&&

.MARINE...
Today: The gradient remains rather tight between the sub- 
tropical Atlantic ridge extending across Florida and an upstream
cold front that is situated in the Southeast. There is also a
little mixing of the 30-35 kt geostrophic winds throughout the
day. Together this supports a continuation of the Small Craft
Advisories for all waters outside of the Georgia waters 0-20 nm
offshore. Conditions there will need to be closely watched
through the day, but latest indications are for any 25 kt gusts
to be too infrequent and too limited in areal coverage to raise
the advisory. Also of note, the Charleston Harbor has 
occasionally been struggling to achieve winds of 20 kt sustained
and/or 25 kt gusts. So we do have some reservations about the 
ongoing advisory. We'll leave it in effect and see what happens 
when we get enough daytime warming to mix more winds down.

As an aside, while most of the wave energy today will be short
period and wind driven, there has been an occasional 16-18
second swell at both the Grays Reef Buoy and the St. Marys 
Buoy. Swell periods of that length are rather unusual in this
part of the Atlantic, but appear to be backswell from the former
TC Claudette.

Tonight: The advisories will remain in effect early on, but 
come down during the late evening and overnight as the gradient 
slackens with the approach of the cold front. Due to already 
low cloud decks, the nocturnal environment will result in clouds
lowering further through the night. There is a concern that at 
least a minor reduction in visibilities will transpire.

Wednesday through Sunday: A northeast flow will develop later 
on Wednesday behind a cold front, then persist into Thursday 
before shifting to the southeast. Persistent southeast flow then
expected through early next week. The current forecast keeps 
winds/seas below advisory thresholds.

Rip Currents: At least some swell energy will impact the 
beaches today. That along with moderate winds that are roughly
parallel to the shoreline, astronomical influences and reports
of rip currents of late, supports a "Moderate Risk" of rip
currents at all beaches today. Additionally, a strong south- 
southwest to north-northeast longshore current will also occur.

A 1-2 ft swell combined with onshore winds in the afternoon 
will result in a moderate risk for rip currents along the SC 
coast on Wednesday.

&&

.TIDES/COASTAL FLOODING...
A northeast wind will develop on Wednesday and continue through
Thursday evening before shifting to the southeast. Tidal
anomalies should gradually increase during this period. Given
the approaching full moon and the associated increase in
astronomical tide levels, when adding another 0.5-0.8 ft
departure, we expect to reach minor coastal flooding in
Charleston Harbor with the evening high tides Wednesday through
Friday.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for AMZ330.
     Small Craft Advisory until 10 PM EDT this evening for AMZ352-
     374.
     Small Craft Advisory until 1 AM EDT Wednesday for AMZ350.

&&

$$

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