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000
FXUS62 KCHS 281432
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1032 AM EDT Wed Jul 28 2021

.SYNOPSIS...
Weak low pressure will weaken over the area today. Dry and hot
conditions are expected Thursday and Friday. A cold front will
remain over or just north of the area this weekend into early
next week, resulting in a wet weather pattern.

&&

.NEAR TERM /THROUGH TONIGHT/...
Radar imagery late this morning showed a large area of showers
with embedded thunderstorms over much of southeast South
Carolina. This slow moving activity has produced rainfall
amounts of over 6 inches, mostly over Colleton county and parts
of Dorchester county. Later this afternoon, coverage should
become more isolated to scattered, especially in the northern
zones as drier air begins to move in. With the late morning
update, made adjustments to PoPs and temperatures based on
latest trends.

Today: Weak low pressure will gradually weaken as the H5 trough
axis gradually shifts offshore. As the low meanders to the
northeast, a weak surface trough will drop south across
Southeast South Carolina and Southeast Georgia later this
afternoon. The atmosphere ahead of the surface trough will
remain very moist and unstable characterized by MLCAPE 2500-3000
J/kg with lifted indices -5 to -7C. As low-level convergence
builds in the vicinity of the surface trough with a possible
interaction with a very weak sea breeze circulation across the
coastal counties, expect another round of scattered showers/tstms
to impact the local forecast area. The greatest coverage looks
to occur over the South Carolina coastal counties and areas
along/south of the I-16 corridor in Southeast Georgia where the
better juxtaposition of low-level convergence and instability
looks to occur. This includes both the Charleston and Savannah
Metro Areas. Afternoon pops 30-50% will be highlighted with the
highest values pinned to the corridors noted above. Convection
will slowly wane across the South Carolina counties by mid-late
afternoon as the sea breeze pushes south and drier air residing
over the Tennessee Valley and Mid-Atlantic region advects to
the south.

Modified soundings look quite moist and typical of deep summer.
Although both surface-based and mixed-layer instability values
are high, but the lack of any mid-level dry air and a chaotic
steering flow both suggest the risk for severe weather is low.
While an isolated wet microburst could still occur where
updrafts become locally enhanced by any mesoscale boundary
interactions, the main convective hazards look to be frequent
cloud-to-ground/water lightning and minor flooding for low-
lying and poor drainage areas. Locally excessive 1-hr and 3-hr
rainfall rates within a plume of PWATs >2.25" could yield a
quick 2-4 inches where slow moving tstms occur. Highs will range
from the upper 80s/near 90 inland to the mid 80s at the
beaches. Dewpoints in the mid-upper 70s will yield heat indices
100-105 for most areas this afternoon.

Tonight: Lingering convection along the Georgia coast early in
the evening will quickly diminish as sunset approaches and
subsidence spreads in as the upper trough shifts farther
offshore. Dry conditions will prevail overnight. The boundary
layer is expected to decouple quickly after sunset. The
combination of lingering wet grounds, clear skies and extensive
dry air aloft could support some degree of fog, mainly inland
from the coast. Guidance appears mixed on how widespread/dense
the fog will become, likely because any fog that forms should
somewhat shallow in nature. Patchy fog will be introduced across
the interior. Lows will range from the lower-mid 70s inland to
around 80 at the beaches and Downtown Charleston.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
he mid-levels will consist of a long-wave trough to our north and a
strong and broad ridge to our west. The trough will
strengthen/amplify with time, pushing the ridge further to the west.

Thursday and Friday: High pressure will prevail. Models are in great
agreement, bringing drier air from the north into our area. The
drier air and decent subsidence will yield rain-free conditions with
just some fair weather clouds each afternoon. A cold front will
approach from the north/northwest late Friday. But it`s not expected
to reach our area or bring any convection.

The main weather story both days will be the heat and humidity.
Thickness values and 850 mb temperatures will yield temperatures
well above normal both days. Dew points will also rise to at least
the mid 70s each afternoon. This will result in the highest heat
indices so far this summer.

Thursday`s high temperatures are forecasted to be in the mid to
upper 90s, slightly lower at the beaches. Heat indices will top out
in the 105-109 degree range. These values would be even higher if
not for the inland moving sea breeze in the afternoon.

Friday high temperatures could be the hottest of the summer.
Most locations will be in the upper 90s with several spots inland
expected to hit the 100 degree mark. Even the beaches won`t be much
lower, generally being in the lower 90s. Heat indices could be
rather interesting. 850 mb winds will be from the northwest, which
could mix down some drier air, mainly for our far inland counties.
The sea breeze will also be pinned to the coast, which will yield
the high temperatures. But the pinned sea breeze will also cause
even higher dew points (upper 70s to near 80 degrees) to pool along
our coastal counties. This would yield heat indices above 110
degrees for our coastal counties and the CHS and SAV metro areas.
Heat Advisories would be needed for these locations. Farther inland,
if the dry air mixes in both the dew points and heat indices would
be a few degrees lower, which would be just short of Advisory
criteria.

Lows both nights will be steamy as well, with some locations not
falling too far below the 80 degree mark. Temperature records for
the hottest nights could be in jeopardy.

Saturday: Most of the models have dry conditions persisting across
our area. Meanwhile, the NBM has convection in the afternoon. To
stay consistent with our neighboring offices, we went with the NBM.
Temperatures and dew points look to be similar to Friday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
The models have a mid-level trough amplifying/strengthening across
the eastern half of the country next week. Likewise, the WPC surface
pattern has high pressure prevailing in the Atlantic with a front
just to our north. The resulting NBM has a wet pattern developing
next week, enhanced by diurnal convection. Temperatures should
gradually drop below normal into Tuesday.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
Near term concern is tstms potentially impacting KCHS and KJZI
through mid-morning. Radar shows scattered showers/tstms slowly
expanding ahead of weak low pressure between KNBC and KRBW. This
activity could briefly impact operations at both terminals as
brief periods of heavy rainfall could occur through 14-15z as
this activity pushes through. Later today, scattered
showers/tstms are expected to develop along a southward moving
surface trough. Activity will develop near KCHS and KJZI by 16z
and KSAV by 20z. It is unclear where direct impacts will occur
at any terminal, but chances are highest at KJZI per latest
high-res guidance. VCTS will be carried at KCHS and KJZI
16-19/20z and 20-00z at KSAV with a TEMPO for 4SM TSRA 16-19z at
KJZI. VFR will prevail for much of tonight, although shallow
ground fog could impact any of the three terminals prior to
daybreak. The risk for low clouds and more significant fog will
remain west of the terminals.

Extended Aviation Outlook: Flight restrictions are possible in
afternoon/evening showers and thunderstorms this weekend.

&&

.MARINE...
Through Tonight: Quiet marine conditions will prevail with winds
less than 10 kt and seas 2-3 ft.

Waterspouts: There is a moderate risk for waterspouts this
morning. The risk will be highest near any cloud lines or
lingering convective outflows over the coastal waters.

Thursday through Sunday: High pressure will prevail Thursday.
Winds are expected to increase Friday and Friday night as a
front approaches from the north/northwest. Gusts could approach
25 kt across most of our waters, which is just short of Small
Craft Advisory criteria. The front should remain over or just to
our north this weekend into early next week. Each afternoon,
expect slight backing of the winds and higher gusts associated
with the sea breeze along the land/sea interface and in the
Charleston Harbor. Each night, expect slight veering of the
winds.

&&

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

&&

$$

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