516
FXUS62 KCHS 302329
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
729 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/...
This evening and tonight: Strong ridging aloft has kept
convection to a minimum in our area so far, but that will
change. Upstream convection is starting to congeal as cold
pooling occurs. Given favorable instability and MLCAPE, but
plenty of moisture, we do need to keep watch for a strong or
marginally severe storm as it gets into our northwest tier of
counties by 9 pm. DCAPE might still be strong enough at 800-900
J/kg to generate some 45-55 mph winds if it holds together.

Latest Hi-Res guidance shows that the activity will start to
weaken and diminish in coverage as it gets further along into
the forecast zones. However, given the approach of a short wave
and associated height falls aloft, this remains to be seen. We
delayed the coverage of convection a bit from the previous
forecast based on latest trends. But still holding onto
scattered to numerous PoPs, highest north and northwest. The
greatest coverage looks to be 9 pm to 1 am in those areas.

By the early morning hours, convection should steadily
dissipate and the late night hours should mostly be rain-free.
Expect a muggy summer night with lows in the mid to upper 70s
away from the coast.

&&

.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 /00Z MONDAY THROUGH FRIDAY/...
KCHS/KJZI/KSAV: Upstream convection associated with a short wave
and cold front will approach the terminals during the late
evening and first part of the post-midnight period. Recent
guidance suggests that this activity will fade before getting
to any of the sites. But we`re not entirely convinced that will
happen with the energy aloft, plenty of moisture, and decent
instability. We`ll show VCTS at all sites from 03Z to 06Z, and
amend if needed.

The greater 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...
This evening and tonight: Typical diurnal sea breeze surging
will continue along the coast into the evening hours, with gusts
up to around 20 knots at times along and near the coast. During
the early morning hours, modest nocturnal surging will top out
around 15 knots, with a few gusts to 20 knots possible. Then
closer to sunrise Monday, winds should mostly be in the 10-15
knot range out of the southwest. Seas should average 2-3 feet.

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...