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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KCHS 201723
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
123 PM EDT Sat Apr 20 2024

.SYNOPSIS...
A slow moving front will bring unsettled weather to the region
this weekend. High pressure will then return and prevail into
the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: Zonal flow aloft continues across the southeastern United
States this morning as the mid-level ridge has been suppressed
south towards southern Mexico with a potent long wave trough
across eastern Canada. In between the two features, a cold front
remains stalled with several rounds of convection forecast to
move east along the boundary. Current surface analysis has a
cold front stalled across the Upstate of SC arching back east
across southern NC. Latest observed sounding from KCHS shows a
thin PBL with an impressive 850 MB temperature of 16 degrees C
(or at the 75th percentile for this time of year). This will
easily support high temperatures around 90 degrees for most
locations (except right at the beaches were highs in the mid 80s
are forecast). Through the morning into early afternoon hours,
no precipitation is expected. The deep westerly flow will keep
the sea breeze pinned pretty close to the coast this afternoon.
Forecast soundings suggest dewpoints will mix out into the
upper 50s/lower 60s, but dewpoints should hold/pool into the
upper 60s in the vicinity of the sea breeze during peak heating.
Modified soundings support modest mixed-layered instability
(MLCAPE 1000-1500 J/kg) in the vicinity of the sea breeze, so a
few showers/tstms could pop along or even just behind the
circulation in the absence of any large scale forcing mechanisms
aloft. Pops near 20% were continued across the coastal counties
this afternoon to account for this possibility. However, dry air
in the mid-levels remain, so some modification will need to
occur before multiple deep moist updrafts can sustain
themselves.

Tonight: Southern stream shortwave energy moving along the
north-central Gulf Coast this morning will traverse the area
this evening and overnight as the surface front meanders south
into Southeast South Carolina and Southeast Georgia. Models are
inconsistent with both the depth and intensify of the forcing
with this shortwave. However, with some degree of residual
instability and a surface boundary lurking about, expect a
healthy coverage of showers/tstms to work from southwest to
northeast across the area into early morning hours Sunday. Pops
50-70% were utilized to account for tonight`s convection with
convection first increasing over interior Southeast Georgia
early this evening and exiting off the middle South Carolina
coast early Sunday morning as the aforementioned shortwave
pushes through and eventually offshore.

0-6km bulk shear looks to only average 20-25 kt and with both
the depth and intensity of forcing for ascent ahead of the
shortwave still unclear, expect any convective organization to
be largely driven by mesoscale processes, such as possible
outflow collisions and boundary interactions with the slow
moving surface front and the remnants of the pinned sea breeze
circulation near the coast. Despite the loss of insolation after
sunset, elevated DCAPE values 800-1000 J/kg will support a risk
for damaging winds with a few of the stronger tstms this
evening with the possibility of brief convective
organization/enhancement near boundary interactions. Lows will
range from the upper 50s/lower 60s near the CSRA and Southern
Midlands (north of the front) to the upper 60s/near 70 south of
I-16 into far southern South Carolina (south of the front).

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
A cold front will likely be positioned off the South Carolina
coast and extending back into southeast Georgia Sunday morning.
The front will shift away from the area later in the day and
overnight. Better forcing for ascent with shortwave energy
passing across the region will lead to increasing coverage of
showers through the day. Focus looks to be oriented over inland
areas initially with activity progressing more towards the coast
in the afternoon. Given positioning of the front, instability
and thunder chances should primarily be over the eastern half of
the forecast area. Temperatures will be tricky as there will be
a notable gradient with highs ranging from the mid 60s across
inland and northern zones to upper 70s/near 80 near the far
southeast Georgia coast. Rain chances will decrease Sunday night
as precip activity moves offshore. Lows will mainly be in the
upper 40s to lower/mid 50s.

High pressure builds inland Monday with front well to the east.
A few showers could develop as main upper wave passes through,
though guidance varies on coverage. Kept PoPs limited to 20%
over land. Otherwise it will be unseasonably cool with highs
topping out in the mid 60s. Lows Monday night are forecast to be
in the mid to upper 40s, except lower 50s closer to the coast.

Ridge of high pressure extends into the area on Tuesday. No
weather concerns with temperatures moderating back to the mid
70s.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
High pressure continues into Wednesday. A cold front could move
into the region Wednesday night or Thursday but most guidance
isn`t showing as much of a clear cut frontal passage.
Regardless, little if any impact is expected with a dry forecast
persisting. Temperatures will within a few degrees of normal.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
-- Changed Discussion --
20/18z TAF Discussion:

This afternoon: All terminals are VFR this afternoon with only
some CU in place. A surface cold front is slowly oozing south
across the NC/ SC border this afternoon, but will likely start
to pickup speed as showers and thunderstorms form upstream.
Later today, a sea breeze will start to move inland with only an
isolated storm possible along the coast.

Tonight: Showers and thunderstorms currently ongoing across TX/
LA will progress east and arrive at the terminals between 8 and
10 PM. As this occurs, the surface cold front will be located
just north of KCHS and KJZI. Generally, showers and
thunderstorms are expected to form and traverse east along the
frontal zone, which means KCHS and KJZI have the highest chance
of observing thunder overnight, while KSAV has a lower chance
(compared to the Charleston Terminals). Showers and
thunderstorms will then come to an end between 3 and 5 AM with
the surface cold front nudging south past KCHS and JZI, but
likely stalling right near or just north of KSAV. Behind the
cold front, cigs will rapidly fall into IFR conditions with
winds backing around from the north.

Sunday morning: A cornucopia of cigs is likely Sunday morning,
with IFR conditions likely at KCHS and KJZI, while conditions
could be bouncing around between IFR/ MVFR/ VFR at KSAV
(depending on the surface cold front position). Showers will
then return to the Midlands first Sunday morning only to pivot
and advect southeast towards the coast by late Sunday morning.

Extended Aviation Outlook: Flight restrictions in low clouds
and showers/thunderstorms are possible Saturday night through
Sunday with a slow moving front. Low clouds could linger into
Monday. Predominant VFR thereafter.
-- End Changed Discussion --

&&

.MARINE...
Today: Broad south to southwest winds trajectories will dominate
the waters today. Some slight enhancement near the land/sea
interface could occur later this afternoon as a pinned sea
breeze circulation develops, but a significant enhancement to
the surface wind field is not expected. Winds will average 10-15
kt with gusts near 20 kt possible at times near the coast and
in the Charleston Harbor this afternoon as the sea breeze forms.
Seas will average 2-3 ft. An isolated shower or tstm could pop
near the sea breeze late in the afternoon.

Tonight: A cold front will sink south into the South Carolina
coastal waters tonight as scattered to numerous showers/tstms
push offshore. A few of these tstms could be strong producing
wind gusts in excess of 35 kt and cloud-to-water lightning.
Southwest winds will veer to the west this evening all waters,
then shift northwest to north behind the front as it drops south
to near the Savannah River Entrance by daybreak Sunday. Speeds
look to average 5-10 kt, but will surge to 10-15 kt in the South
Santee-Edisto Beach nearshore leg a few hours before daybreak
Sunday. Waves will average 2-3 ft.

Sunday through Thursday: A cold front draped across the area
Sunday morning will push east with time, turning winds around
to the north/northeast and increasing in speeds. Seas also build
and Small Craft Advisories will be possible for portions of the
waters Sunday night into Monday, especially the Charleston
county and outer Georgia waters. The other nearshore zones are
not out of the question but looks more marginal at this time.
Winds ease on Tuesday with no additional concerns through
Thursday.

&&

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

&&

$$

NEAR TERM...Haines
SHORT TERM...
LONG TERM...
AVIATION...Haines
MARINE...