-- Discussion containing changed information from previous version are highlighted. --
962
FXUS62 KCHS 220555
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
155 AM EDT Sat Jun 22 2024

.SYNOPSIS...
Tropical low pressure will move onshore along the Northeast
Florida and Southeast Georgia coast tonight. High pressure will
then become the primary feature this weekend and into next
week.

&&

.NEAR TERM /THROUGH TODAY/...
-- Changed Discussion --
Early overnight update: Satellite imagery and radar data reveals
tropical low center along the Glynn/Camden county coasts showing
minimal movement and a few pronounced shower bands spinning up
into southeast Georgia. No lightning, but given MLCAPE values
running 1-2K J/Kg, a small core of >40 knots effective bulk
shear near the eastern flank of the low, and a small core of
high values of non-supercell tornado parameter...we will be
keeping an eye out for any signs of waterspout development
through the overnight hours into Saturday morning.

Previous discussion...
The observed 0Z KCHS sounding observed CAPE between 2000 to 2500
J/kg and PWAT values around 2.2 inches. Recent radar trends
indicate greater coverage over portions of the Charleston Tri-
County area late this evening. The forecast update will indicate
higher PoPs along the I-26 corridor through midnight.

As of 930 PM, regional radar indicated that the center of a
well defined tropical low was 10 to 15 miles off the coast, near
the FL/GA line. The center of the low is tracking towards the
FL/GA coast; NHC indicates that probs of a tropical cyclone
formation have decreased to 40 percent.

KCLX detected scattered to numerous showers across the forecast
area, greatest near the coast. Recent runs of the HRRR indicate
that the coverage of the inland showers will decrease through
the rest of this evening. Recent SPC Mesoscale Analysis
indicates CAPE values will remain above 1500 J/kg through
midnight. This lingering instability should support scattered
shower coverage this evening. After midnight, PW values will
surge up to 2.2 to 2.4 inches, especially near the center of
the tropical low. HREF indicates that a band of numerous to
widespread showers with embedded thunderstorms will develop
near and just north of the low center late tonight into early
Saturday morning. The combination of lingering instability and
high PWAT should produce a corridor of moderate to even heavy
rainfall south of the Savannah River. In fact, HREF indicates
the probabilities of the 3 hour QPF exceeding 1 inch may peak
at 70% with 30% chance of 3 inches.

QPF was increased to over an inch in spots, primarily between
McIntosh and Chatham Counties. Given the thick cloud cover and
rounds of rain expected overnight temperatures were kept
generally in the mid 70s. However the heavy rainfall over
southeast GA could result in wet bulb cooling, resulting in some
locations getting into the low 70s.
-- End Changed Discussion --

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
Saturday and Saturday night: Aloft, the forecast area will sit
on the eastern extent of the upper ridge. At the surface, the
main feature of interest will be the remnant circulation of the
tropical disturbance that pushed inland along the northeast
Florida and southeast Georgia coast. This circulation and the
envelope of deep moisture associated with it will linger near
the Georgia coast and will be the focus for thunderstorm
activity likely ongoing at the start of the day. The highest
rain chances and greatest coverage of thunderstorms is expected
to be across the Georgia coastal waters, southeast Georgia, and
up along the Savannah River into southeast South Carolina. Rain
chances peak in the 60 percent range, but could certainly need
to be higher depending on how things evolve overnight. This will
yield the possibility of some locally heavy rainfall, given the
presence of the circulation, deep moisture, and potential for
training storms. For this highlighted area, rainfall amounts of
0.50-1.00" will be common, with potential for locally higher
amounts. Overnight, while the focus for nocturnal convection
should shift offshore, we could still see some showers and
storms periodically attempt to push onshore. Though the highest
chances are positioned over the waters, we do maintain 30-40
percent chances along the entire coast overnight. The presence
of clouds and the expected convection should keep temperatures
down a bit along the coast, with upper 80s common there.
However, further inland we could see some low 90s. Overnight,
lows will be typical of summertime with low to mid 70s.

Sunday through Monday: On Sunday, the remnant circulation should
completely dissipate or at least shift off to the northeast.
This should begin the transition to a more typical summertime
pattern as the subtropical high takes hold with a lee trough
inland across central Georgia and the Carolinas. Convection
should become more diurnal each day, favoring the afternoon and
evening time period. Temperatures will begin to increase as
well, with Sunday highs in the mid to upper 90s inland and low
90s elsewhere. Then by Monday, look for mid to upper 90s across
the entire forecast area. The result for Monday will be heat
index values rising to around 105 for much of the forecast area.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Aloft, the pattern will transition to weak and broad troughing
for much of the upcoming week. At the surface, the pattern
should be typical of summertime with subtropical high pressure
in control. Rain chances will be diurnal, with the highest
chances in the afternoon and evening and then transitioning
offshore to the coastal waters during the overnight periods. The
most notable part of the forecast will be highs in the mid to
upper 90s Tuesday and Wednesday, with a few inland locations
possibly hitting triple digits. For Thursday, mid 90s should be
more common then coming down to the low to mid 90s by Friday.
Depending on how dew point values work out, there could be some
heat index values into the 105-110 degree range Tuesday-
Thursday, primarily along the coastal corridor. If this comes to
fruition, Heat Advisories could be needed.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
Tropical low pressure near the southeast Georgia/northern
Florida border will drift along the southeast Georgia coast
through today. This will bring rather unsettled conditions to
the region, particularly to the KSAV terminal overnight and
through most of Saturday...with periodic MVFR conditions
anticipated. We cannot rule out the possibility of some
thunderstorms impacting the KSAV terminal, particularly during
the day Saturday into Saturday night although overall
probabilities are not great. We have elected to not include
TSTMs with the 06Z forecast and will amend as needed.

KCHS/KJZI will be a bit less impacted. But shower activity
should ramp up in and around the terminals during the afternoon
timeframe and could bring brief MVFR restrictions.

Extended Aviation Outlook: Mostly VFR. However, convection will
bring brief flight restrictions, mainly each afternoon and
evening.
-- End Changed Discussion --

&&

.MARINE...
Tonight: Winds are certainly over performing a bit across the
coastal waters this afternoon with tropical low pressure
located just to the southeast of the Georgia offshore waters.
Enhanced winds will linger into this evening then diminish a bit
overnight as the low moves inland along the far northeast
Florida or southern Georgia coast. Winds/seas will diminish
first over the South Carolina waters, including Charleston
Harbor, and the Georgia waters late. However, there is uncertainty
on how quickly winds/seas will diminish, especially over the
South Carolina waters. The Small Craft Advisory has been
extended until 6 PM for the Charleston Harbor, 8 PM for the
South Santee- Edisto Beach nearshore waters and into the
overnight for the two Georgia marine legs. The highest seas will
occur early this evening, 4-6 ft nearshore and 6-8 ft over the
Georgia offshore waters.

Saturday through Wednesday: South to southwest flow mainly in the
10-15 knot range should prevail across the local waters through
the weekend and into the middle of next week. The exception
will be Sunday night into Monday when winds are expected to
increase a bit as the gradient tightens, with 15-20 knots more
common. Seas should average 2-4 feet through the period, though
perhaps up to 5 feet at times in the outer waters as winds surge
a bit.

Rip Currents: Onshore winds and lingering swell energy will help
to keep the rip risk elevated Saturday and again Sunday. For
Saturday, depending on the peak winds and the amount of swell
that arrives at the beaches, the local rip current calculator
results in a borderline high risk. Current approach is to
maintain the moderate risk for Saturday and allow the overnight
shift to re-evaluate.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
GA...None.
SC...None.
MARINE...Small Craft Advisory until 4 AM EDT early this morning for
     AMZ374.
-- End Changed Discussion --

&&

$$

NEAR TERM...NED
SHORT TERM...
LONG TERM...
AVIATION...NED
MARINE...