660
FXUS62 KCHS 301919
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
319 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: Radar imagery shows isolated and
mostly weak convection developing across the forecast area early
this afternoon. This general coverage and intensity should
continue for the afternoon hours. As we get into the evening,
and upstream shortwave aloft will approach and we should start
to see stronger convection develop within a lee trough across
the Midlands. Hi-res guidance continues to indicate the
potential for this activity to push in during the mid to late
evening hours, perhaps lingering into the early morning hours.
Though we will be past sunset, there should be sufficient
instability to support thunderstorms moving in, especially if
there is some organization along outflow boundaries. Being after
sunset will certainly help, but we can`t completely rule out
one or two strong to marginally severe storms. DCAPE values
should be upwards of 1,000 J/kg and if a cluster of storms can
get going along outflow boundaries it is possible we could see
some instances of strong, gusty winds. The overall severe threat
is low, but certainly needs to be watched.

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 /19Z SUNDAY THROUGH FRIDAY/...
The first issue for the terminals will be the potential for an
hour or two of thunderstorms nearby through about 20z. The best
chances will be at KCHS and KSAV, and any development should be
inland of KJZI. This activity should be short-lived but could
produce a brief period MVFR conditions if a shower or storm
develops right on top of the terminal.

The second issue will be later this evening for showers and
thunderstorms to track into the Charleston Tri-County region and
impact KCHS and KJZI. The main window of time for this would be
roughly from around 01-05z. These thunderstorms should be
weakening as they arrive, but could still produce a brief period
of flight restrictions late this evening. Can`t rule this out
at KSAV as well, but the chances appear to be lower.

Finally, the more confident concern for thunderstorms comes
near the end of the 18z TAF period. Model guidance is quite
consistent with thunderstorms along the coastal corridor, with
high chances for flight restrictions at KCHS, KJZI, and KSAV. We
have introduced TSRA starting at 16z at KCHS and KJZI, and at
17z at KSAV. This highlights the beginning of the time window
when thunderstorms could impact the terminals, and the highest
chances will be near the end and just beyond the 18z TAF window.

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...
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...Heat Advisory until 7 PM EDT this evening for GAZ087-088-
     099>101-114>119-137>141.
     Flood Watch from Monday afternoon through Monday evening for
     GAZ101-116>119-137>141.
SC...Heat Advisory until 7 PM EDT this evening for SCZ040-042>045-
     047>052.
     Flood Watch from Monday afternoon through Monday evening for
     SCZ042>045-047>052.
MARINE...None.

&&

$$

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