557
FXUS62 KCHS 010239
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1039 PM EDT Sun Jun 30 2024

.SYNOPSIS...
A cold front will impact the area Monday into Tuesday before
dissipating offshore. High pressure will return for the latter
half of the week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
The strong ridging aloft has weakened enough with a short wave
moving through, and that along with the associated slightly
lower heights, and an approaching cold front has allowed for
convection to occur.

That convection has congealed into cold pool clusters, which
continues to ride south and southeast along a moisture axis and
also where we find higher Theta-e air, plus the better MLCAPE.
We show 50-70% PoPs as a result from northern Colleton, Hampton,
and Allendale, into the interior counties of Georgia. Rainfall
rates of at least 2-3 inches an hour in some areas will produce
some minor flooding concerns, as PWat is 2.4 to 2.6 inches. The
DCAPE is less than earlier, down to just 500-600 J/kg. So there
is only a small chance of isolated gusts in excess of 40 mph in
the stronger storms. Lightning networks are not showing any
diminishing trend as of late Sunday, and with echo tops in
excess of 45 or 50K feet, and cloud top temperatures as cold as
minus 70-75C, it could be awhile before lightning diminishes.

The HRRR has been doing an excellent job of depicting the more
southward movement to convection, showing it mostly avoiding the
Charleston Tri-County district. It also has greater Georgia of
the convection making it to the Savannah area, where we have 40%
PoPs occurring after midnight.

The bulk of the convection will be over with after about 3 am,
although given lingering boundaries there could still be some
isolated activity after that time.

It`ll be another warm and muggy night even with the storms,
since dew points are still so high. In general low temperatures
will be just in the mid and upper 70s, with even some coastal
sites not dropping below 80F degrees. The temperature at KCHS
for June 30 could be a record high minimum if it doesn`t rain
between now and the end of the day (See CLIMATE section below).

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: Models continue to advertise a heavy rain/flooding
signal along parts of the Georgia and lower/middle South
Carolina coast for Monday. Subtropical ridging aloft will
quickly give way to broad troughing along much of the Eastern
Seaboard during the day as pronounced shortwave energy embedded
in the northwest flow aloft approaches the Carolina coast. This
will help push a slow moving cold front south into southern
South Carolina, propagating south of the Santee River by early
afternoon. Modified soundings ahead of the front show solid
mixed-layer instability (MLCAPE 1500-2000 J/kg with K-indices
pushing 40C) developing by late morning embedded within a weak
shear environment (0-6km bulk shear averaging ~15 kt). Moisture
is also abundant with surface dewpoints progged to hold in the
mid- upper 70s with PWATs nearing 2.50" at times (daily max for
KCHS for July 1 is just over 2.30" per SPC sounding
climatology).

Expect convection to develop near the Santee River in the
vicinity of the front by late morning, then slowly spread south
through the rest of southern South Carolina into Southeast
Georgia into the afternoon/evening hours in tandem with the
front. Given the degree of instability and moisture coupled with
surface moisture convergence values surpassing 50 g/kg/12hr at
times near the front and some forcing aloft with approaching
shortwave energy, the setup looks favorable of periods of
extremely heavy rainfall with hourly rainfall rates potentially
exceeding 3 in/hr. This is reinforced by the 30/12z HREF QPF
progs showing an ~90% chance for 3+ inches to fall in 3 hours
clustered in the Savannah-Charleston corridor. There are even
some signals that a few spots could see localized amounts in
excess of 6 inches (noted 12-hr probs of >5" averaging 70-75%
over central Charleston County, including Downtown Charleston).
Although antecedent conditions are fairly dry with D0 and D1
drought conditions in place, there is an increasing concern that
pockets of significant flash flooding could occur given the
intense hourly rainfall rates expected with the slow moving
convection, especially in urban areas such as Beaufort and both
the Charleston and Savannah Metro Areas. Complicating matters is
heavy rains could be falling at the afternoon high tide which
could enhanced the flood risk, especially in Downtown Charleston
and nearby communities. A Flood Watch has been issued from Noon
until 9 PM Monday for the South Carolina and Georgia coastal
counties, including Effingham, Hampton, Inland Colleton,
Dorchester and Inland Berkeley. Storm total QPF Monday into
Monday night will run 2-4" with localized amounts in excess of
6". Categorical pops 80-100% look reasonable, which is a bit
higher than the various 12z statistical guidance packages, but
closer to the 30/13z NBM. Highs will warm into the upper
80s/lower 90s, warmest across Southeast Georgia.

Convection will slowly wane through night, although elevated
pops will hold through daybreak Tuesday as the front moves south
and northeast onshore winds will keep moisture in place. This
will be especially true for coastal Georgia. Lows will range
from the upper 60s/lower 70s well inland to the near 80 along
the Georgia and far southern South Carolina beaches.

Tuesday and Wednesday: The front will slowly dissipate to the
south and offshore through the period as subtropical ridging
aloft builds back to the east. The pattern will return to more
typical summer time convective regime with onshore winds
supporting higher low- level moisture values across Southeast
Georgia. Scattered to numerous shower/tstms are expected each
afternoon and evening with activity diminishing during the
overnight hours. Highs will only warm to the mid-upper 80s
Tuesday with upper 80s/lower 90s for Wednesday. Lows Tuesday
night will range from the lower 70s well inland to the upper
70s/near 80 at the beaches.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
An impressive upper-level anticyclone will setup across the
Deep South and Southeast U.S. mid-late week. The risk for
showers/tstms will generally average below climatological
normals as a result. The heat will also steadily build with
highs reaching the upper 90s/near 900 inland Friday into the
weekend. Heat indices will push the 108 Heat Advisory criteria
during this time as well.

&&

.AVIATION /03Z MONDAY THROUGH FRIDAY/...
KCHS/KJZI/KSAV: Upstream convection associated with a short wave
and cold front will continue to approach the terminals during
the late evening and first part of the post-midnight period.
Recent guidance and trends suggest that this activity will fade
before getting to KCHS and KJZI. We`ll show VCTS from 04Z to 06Z
since there does remain various boundaries from convection to
form along. KSAV has a better chance of convection from 04Z to
07Z. We might need to include a period of TSRA with gusty winds
and flight restrictions if the upstream activity holds together.

The next chance for convection and prolonged flight restrictions
will occur Monday afternoon and evening as a cold front stalls
nearby. Considerable moisture, low level convergence, and
instability will favor several hours of at least MVFR weather
with moderate to heavy rains in SHRA and TSRA. Some potential
for IFR at times, and we can adjust in later TAF issuances if
necessary.

Extended Aviation Outlook: Brief flight restrictions are
possible in showers and thunderstorms, mainly in the afternoon
and evenings through the period.

&&

.MARINE...
Tonight: There remains a fairly tight gradient between an inland
cold front and Atlantic high pressure across Florida. This will
produce S and SW winds as high as 15 or even 20 kt, which in
turn allows for seas of at least 3 feet throughout. The inland
t-storms will weaken as they approach the coastal waters. But if
they hold together enough, a few of them could generate some
strong winds and lightning strikes on the waters south of
Edisto into the Georgia waters after 1 or 2 am.

Monday through Friday: A cold front will drop south through the
waters Monday with east/northwest winds prevailing all areas
Monday night. Widespread showers/tstms with heavy rain could
reduce vsbys to less than 1 NM at times and be accompanied by
gusty winds and cloud- to-ground lightning. There will be a bit
of a post-frontal surge Monday night into Tuesday with speeds
increasing to 15-20 kt over the South Santee to Edisto Beach
nearshore leg with 10-15 kt elsewhere. Northeast winds will
gradually veer more southerly for Thursday and Friday. Seas will
average 2-4 ft through the period, except up to 5 ft over
portions of the South Carolina nearshore waters in the northeast
flow behind the cold front.

&&

.CLIMATE...
Record High Minimum Temperature for June 30:
KCHS: 78F set in 1977. (Low so far today 79F)

Daily Rainfall Records for July 1:
KCHS: 2.66 set in 1948
KCXM: 1.64 set in 1966
KSAV: 2.76 set in 1871

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch from Monday afternoon through Monday evening for
     GAZ101-116>119-137>141.
SC...Flood Watch from Monday afternoon through Monday evening for
     SCZ042>045-047>052.
MARINE...None.

&&

$$

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