957
FXUS62 KCHS 210749
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
349 AM EDT Sun Jul 21 2024

.SYNOPSIS...
High pressure will remain over the western Atlantic while a
surface trough lingers inland.

&&

.NEAR TERM /THROUGH TONIGHT/...
Early this morning: Areas of low stratus and some fog will
occur, with the best chance for fog to occur near and west of
I-95 where the lowest condensation pressure deficits will occur.

For today: The large scale pattern aloft remains essentially
the same from recent days, with the local area within a
southwest flow between sub-tropical ridging from the Atlantic
into the Gulf of Mexico, and a broad trough persisting in the
central states and Mississippi Valley. It isn`t much different
either at the surface, with the typical Bermuda High still in
place, with a trough wavering inland. What is different today
when compared to yesterday is that the shear is a bit less, and
the right entrance region of the upper jet might be a bit
further away. However, not much else has changed, with
considerable moisture (Pwat 2-2.25 inches), MLCAPE is modest at
1500-2500 J/kg, and there is forcing from the sea breeze, the
inland trough, and subtle mid level short wave passing through
during the peak heating of the afternoon. Thus we still show at
least scattered to numerous showers and t-storms in the
afternoon, when the cap breaks and we reach our convective
temperatures. Depending upon how much boundary interactions
occur, we could require categorical PoPs in some areas.

The severe potential is less than it was Saturday with less
shear, DCAPE values forecast at just 600-800 J/kg, and poor mid
level lapse rates. But where updrafts become enhanced by
boundary collisions/mergers, a few strong or marginally severe
winds can occur.

Steering flow is west to east, but becomes more erratic late in
the day. This could potentially lead to some localized
flooding, with the best chances for any Flood Advisories to
occur in the vicinity of I-95 and westward. As of this time the
risk for any flooding concerns in the Charleston metro and the
Beaufort region is very low, with better probabilities in the
Savannah area.

The areas of low stratus this morning will cut down on
insolation, but we still should be able to get into the upper
80s and lower 90s before convection develops after 1 or 2 pm.
Temperatures will then drop down into the 70s when the rains
occur, and we have attempted to show this in the grids. Max heat
indices will reach 102-107F degrees, which is not enough for
any Heat Advisories.

Tonight: The subtle mid level short wave lifts northeast early
on, and with the loss of heating, we look for convection to
steadily wane early on. However, low level convergence from off
the ocean will allow for at least a slight chance/chance of
showers and a few t-storms to attempt to move onshore during the
overnight. Guidance is again indicative of some low stratus and
fog, but we`ll defer any mention in the forecast to later
shifts. Climatology and persistence is the best forecast for low
temperatures down in the lower and middle 70s.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
The mid-levels will consist of troughing stretching from the Great
Lakes region into the Southern Plains. Meanwhile, High pressure will
be over the western Atlantic. Our area will initially be sandwiched
between these to features, followed by the High gradually building
closer to our area into Wednesday. At the surface, the pattern will
be rather consistent with High pressure over the western Atlantic
and a trough over the Southeast U.S. This pattern will cause deep
moisture to be across the region, with PWATs ranging from 2-2.25".
High temperatures are forecasted to be in the lower to maybe middle
90s each day across most of our area, except cooler at the beaches.
The combination of this heat and moisture will generate plenty of
instability, with models in good agreement pointing towards
scattered to numerous convection each afternoon and evening,
followed by convection trending lower over land areas overnight.
Typical for this time of year, a strong to marginally severe
thunderstorm cannot be ruled out each day, with the main hazard
being damaging winds. Locally heavy rainfall is also a concern,
especially in areas where there is storm training. Heat indices will
remain above 100 degrees each day. Though, they could make a run for
108 degrees along our coastal counties on Wednesday, before
convection initiates. Overnight lows will remain mild, generally in
the 70s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The offshore ridge looks to expand westward across the area midweek,
before likely weakening late week as a trough passes by to the
north. A summertime pattern will persist with High pressure in the
western Atlantic and troughing along or near the Southeast U.S. This
synoptic flow will keep abundant moisture across the region. Expect
diurnally driven convection with temperatures near normal.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
KCHS/KJZI: Lingering light rains will end near the start of the
06Z TAF cycle, followed by some low stratus moving in,
dependent upon the amount of clearing. We`ll keep watch for
IFR/MVFR ceilings through 15z as a result. Convection that
forms on the sea breeze in the afternoon looks to be inland from
KJZI, so no mention in the forecast. But it will be close by
KCHS where we include VCTS for a few hours between 19Z and 23Z.

KSAV: This terminal has a decent chance of experiencing low
stratus and maybe light fog for a few hours this morning. We
show MVFR ceilings as a result from 10-14Z. Convection will form
along the sea breeze boundary this afternoon, with
probabilities high enough to include mot only VCTS from 18-24Z,
but a TEMPO for TSRA, MVFR conditions, and some gusty winds from
19-23Z.

Extended Aviation Outlook: Convection could produce brief flight
restrictions, especially each afternoon/evening.

&&

.MARINE...
Today and tonight: Little to no change in the synoptic pattern,
featuring an inland trough and high pressure over and near the
local waters. SW and W winds this morning will back to the S and
SW this afternoon with sea breeze circulations, then gradually
clocking around further to the SW tonight with some land breeze
influences. Outside of convection, winds will be mostly 15 kt or
less (maybe a little higher with low level jetting tonight),
with seas no more than 3 or 4 feet.

Extended Marine: The synoptic pattern will consist of High pressure
in the western Atlantic and a surface trough across the Southeast
U.S. Each day, expect some backing of the winds. The strongest gusts
will be along the land/sea interface and across the Charleston
Harbor due to the afternoon sea breeze. Each night, expect some
veering of the winds. Stronger winds are possible near the coast due
to the formation of the nocturnal jet. Seas will average 2-4 ft.

Rip Currents: A low-end moderate risk of rip currents remains in
effect today due to the 2-3 ft swells with a period of 8-9
seconds, combined with a greater than normal tidal range and the
developing sea breeze.

The combination of a 2 ft swell every 9 seconds and
gusty onshore winds with the afternoon sea breeze could support an
enhanced risk of rip currents on Monday. Internal calculations and
RCMOS point towards it being a borderline Low/Moderate Risk. We
opted to maintain the Low Risk, but this may need to be raised if
any of the above parameters along with the longshore current are
stronger than anticipated.

&&

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

&&

$$

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