551
FXUS62 KCHS 132027
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
427 PM EDT Tue Aug 13 2024

.SYNOPSIS...
A cold front will move slowly south into the area today, then
pass through on Wednesday. High pressure will build from the
north into late week. Another cold front may approach the area
later this weekend or early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Rest of the afternoon and into the evening: Ongoing convection
features a mixture of showers and thunderstorms that developed
along the sea breeze in Charleston County, with additional
thunderstorms moving in from upstream associated with Midlands
convection. The coastal convection has exclusively been a heavy
rainfall threat while the upstream convection organizing along
an outflow boundary has the best severe potential. Instability
across southeast South Carolina did feature MLCAPE on the order
of 1,500-2,500 J/kg, but that has been significantly disrupted
thanks to ongoing convection and resulting debris clouds. DCAPE
values aren`t particularly impressive and the main severe threat
will exist along outflow from the upstream convection. We might
need another Severe Thunderstorm Warning or two this evening,
but the overall threat is low. Locally heavy rainfall will
continue to be an issue, and we have already seen a few
automated gauges report 1.00-1.50 inches around the Charleston
area.

Ongoing convection should dissipate through the evening as it
moves toward the coast. Overnight, there are some indications in
model data that the subtle boundary will continue to linger
along the coast and could provide enough low-level convergence
for showers and storms over the coastal waters and perhaps along
the Charleston County coast. We have kept a 20-30 percent
chance along the coast as a result. Also, there could be some
shallow ground fog around, especially in areas that receive rain
in the afternoon and evening. But coverage isn`t expected to be
enough to warrant carrying fog in the forecast. Lows are
expected to be a bit lower than previous nights, with low to mid
70s more common away from the coast.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Wednesday: This will be the wettest day to an otherwise rather
unusually dry period. A weak backdoor surface front will be
pushing just south of the area by later in the morning, with
surface ridging building over the mid Atlantic and northeast
states, resulting in northeast low level northeast winds. The
area will still be under the backside of an upper level trough,
departing off of the mid Atlantic Coast. This feature, combined
with modest deep layer moisture of PWs of 2+ inches, will result
in scattered showers/thunderstorms, especially south of the
Charleston Tri-County area. Highs noticeably cooler over the SC
zones, in the mid to upper 80s, and around 90 to the lower 90s
southern GA zones and near the Altamaha. Expect any isolated to
scattered convection to diminish by the evening. Lows cooler
than they have been in quite some time, in the upper 60s to
around 70 inland, and lower to mid 70s near the coast.

Thursday and Friday: Deep layer ridging shown by all models to
build from the west and north, helping to drive drier air
southward through the area. PWs falling below 1.5 inches and
mean RH values generally below 50%, which is below normal for
this time of year. Have no mentionable precipitation chances for
both of these days, which will allow for some needed drying out
of the many flooded/saturated regions across our area. Highs a
tad below normal, in the mid 80s to near 90. Lows in the upper
60s inland/west of I-95 and lower 70s closer to the coast.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The deep layer ridging over the southeast U.S. is shown by
global models to weaken and push southward as a large/broad
upper trough forms over the Great Lakes Saturday, pushing into
southeast Canada and northern New England Sunday and Monday.
This will translate to a gradual increase in precip./convection
chances through the period. Have slight chance PoPs Saturday and
into Sunday, then chance by Monday, which is around normal for
this time of the year. Given the presence of a relatively strong
upper level trough axis for mid August, will continue to
monitor any potential threat for strong to severe storms,
especially by later in the weekend. Temperatures warming
slightly through the period, but guidance keeps highs around
normal in the lower 90s and lows in the lower to mid 70s, except
mid to upper 70s near the coast.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
At 18z, showers continue to develop along the Charleston County
coast and could at times move close to KCHS and KJZI. The main
forecast challenge for the TAF`s concerns a cluster of
thunderstorms moving to the southeast out of the Midlands. This
cluster of storms could reach KCHS and KJZI around 20-21z and
bring a brief shot of IFR visibility and gusty winds. The chance
of storms is much lower at KSAV. Thunderstorm activity should
shift offshore during the evening hours. Then overnight, we
could see additional showers around KCHS and KJZI as a weak
boundary lingers in the area. We could also again see an area of
stratus late tonight, but confidence in any ceilings developing
at the TAF sites is low.

Extended Aviation Outlook:

Wednesday: Brief flight restrictions possible with scattered
mainly afternoon and early evening showers and thunderstorms,
especially KSAV.

Thursday and Friday: VFR to prevail with much drier conditions
expected, with less than 15% chance for afternoon
showers/thunderstorms.

Saturday and Sunday: Some moisture starts to return with
isolated afternoon showers/thunderstorms Saturday, and scattered
by Sunday with a higher chance for brief flight restrictions.

&&

.MARINE...
This evening and tonight: Thunderstorms will continue moving
out into portions of the South Carolina coastal waters this
evening. These storms could produce strong wind gusts, and
possibly even a waterspout. We could need a few more Special
Marine Warnings. Otherwise, a weak boundary will linger near the
local waters through the night. Winds north of the boundary
will turn northeasterly while winds to the south will continue
to be southerly. Speeds should top out in the 10-15 knot range,
mainly within the northeast flow in the Charleston County
waters. Seas are expected to average 2-3 feet.

Wednesday through Thursday: No highlights expected as a
backdoor cold front pushes south of the waters through this
period. Expect east-northeast winds of 10-15 knots with some
gusts to near 20 knots possible. Seas generally 3 to 4 feet.

Thursday night through Saturday: Seas expected to increase,
especially beyond 20nm offshore as swells from Ernesto, which
will remain well east of the area, arrive into the marine area.
Guidance indicating seas building to 4 to 6 feet, possibly up to
7 feet by later Friday and into Saturday. Decreasing northeast
winds through Friday, becoming south and southwest Saturday and
Sunday. Winds during this period generally 10 to 15 knots or
less and seas 3 to 5 feet.

&&

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

&&

$$

NEAR TERM...BSH
SHORT TERM...RFM
LONG TERM...RFM
AVIATION...BSH/RFM
MARINE...BSH/RFM