Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 270817
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
417 AM EDT Tue Jul 27 2021

.SYNOPSIS...
Weak low pressure will drift north along the Georgia and South
Carolina coasts today, resulting in unsettled conditions. A
trough will then bring more unsettled weather on Wednesday.
Drier conditions are expected Thursday and Friday, followed by
a return to a wet weather pattern next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The local area will be situated near the southern apex of
a mid and upper level trough, which will be nestled between one
anticyclone north of the Bahamas with a second and more intense
anticyclone centered in the Central Plains. Meanwhile at the
surface, the remnants of the tropical low pressure system is
located near Jesup and Hinesville in Georgia, and will drift
north and northeast through the day. An associated trough will
extend from that low across the immediate area, and these
systems will be the main impetus for convection.

Moisture will be abnormally high, some 2 to 3 standard
deviations above climo, with PWat as great as 2.25 to 2.50
inches. Combine this with MLCAPE reaching as much as 2000-2500
J/kg and weak short waves to ripple through, we are looking at
much greater coverage of showers and thunderstorms that in
recent days.

Convection will initially be located over the coastal counties
as we begin the day, where we find the best overlap of moisture
convergence, modest mid level lapse rates and some upper
difluence. Given relatively low LFC`s and little to no cap,
activity will then increase in both strength and areal coverage
throughout the day due to diurnal heating and as a result of
various meso-scale boundaries that occur. Guidance does give
mixed results on where the most numerous convection will occur,
but the consensus is eastern sections this afternoon. We are
showing PoPs as much as 40-50% this morning, then up to 60-70%
this afternoon.

Heavy rains will likely occur in many of the showers and
thunderstorms with the atmosphere so saturated, a sluggish
storm motion, and deep, warm cloud layers that are as much as
12-15K feet in depth. WPC has much of our counties within a
Slight Risk of excessive rainfall, which seems reasonable and
implied by the HREF probabilistic guidance that shows low to
moderate chances of 12 hour rains exceeding 3 inches through 00Z
Wednesday.

Given considerable cloud cover, any severe potential appears
low. However with DCAPE as much as 600-800 J/kg, some water
loading storms could generate wet downbursts with strong to
perhaps marginally severe winds.

Temps are somewhat difficult to forecast today given the high
concentration of convective rains. But since we are already
starting off far above climo, it won`t take much to reach the
mid and upper 80s, with even 90-91F near and west of US-301.

Tonight: The weak surface low is still meandering across the
area, but does look to start pushing toward the coast of South
Carolina overnight as a decent short wave moves in aloft. While
we do lose some of the potency to CAPE and instability after
dark and since the atmosphere was so worked over from during
the daytime hours, we still anticipate convection to persist
through the night. Boundary interactions, the aforementioned
short wave and the approach of a stationary front form the
northwest, will allow for at least scattered convection to
persist. There are signals of additional heavy rains as shown by
the HREF probabilistic forecast, with much of that risk to be
determined by where the surface low moves through. Temps will
end up near or slightly above climo given the continued
saturated environment.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
The mid-levels will consist of a long-wave trough to our north and a
strong and broad ridge to our west. The surface pattern will feature
high pressure in the Atlantic.

Wednesday: There will be a lot of moisture across our area in the
morning. PWATs are forecasted to exceed 2.25" at that time, which is
above normal per NAEFS and above the 90% mark for CHS sounding
climatology. There will also be a decent amount of instability
across our area by the afternoon. Models indicate MLCAPEs should
easily exceed 1,500 J/kg with steep lapse rates. Of bigger concern
could be a large area of DCAPEs exceeding 1,000 J/kg moving in from
the north/northwest with time. Given all of the moisture, models
anticipate convection should be ongoing in the morning, then
increase in both coverage and intensity into the afternoon, aided by
a weak inland moving sea breeze and mesoscale boundary interactions.
The highest POPs are across our GA counties late in the afternoon,
but we anticipate the timing/location will need to be adjusted with
future forecasts. Given the setup, a few pulse severe thunderstorms
with damaging winds are possible. Additionally, there won`t be much
shear and mid/upper level steering winds will be light, which will
lead to slow storm motions. With the abundant moisture, locally
heavy rainfall in these slow-moving and/or backbuilding storms could
lead to flooding, especially in low- lying and poorly drained areas.
The convection will dissipate in the evening as drier air moves in
from the north, with the night being dry. Temperatures will be near
normal.

Thursday and Friday: Models are in good agreement, bringing drier
air from the north into our area. The drier air and decent
subsidence should yield rain-free conditions with just some fair
weather clouds each afternoon. The more interesting thing to note
will be the temperatures. Thickness values and 850 mb temperatures
will yield above normal temperatures both days. Combined with dew
points well into the 70s, heat indices will rise to ~105 degrees on
Thursday. Friday could be our hottest day so far this summer. A few
spots far inland could hit the 100 degree mark for temperatures. For
heat indices, they`re expected to top out near 110 degrees. Heat
Advisories may be needed for at least our metro areas on Friday.
Lows will be steamy as well, with some locations not falling too far
below the 80 degree mark.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
The models have a mid-level trough prevailing to our north through
Saturday while a strong ridge is far to our west. The trough will
amplify/strengthen across the East Coast early next week. The WPC
surface pattern has high pressure in the Atlantic. A cold front will
approach from the north/northwest this weekend, possibly remaining
just to our north. Likewise, the NBM has increased diurnal
convection through Monday. There is a small potential for Heat
Advisories across parts of our area on Saturday, followed by
temperatures gradually dropping into Monday.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
An unsettled pattern will prevail with the 06Z TAFs, courtesy of
overly abundant moisture, sufficient instability and lift
generated from weak low pressure over southeast Georgia, an
associated trough and various meso-scale boundaries.

The first part of the day will produce MVFR ceilings impact
KSAV, due to the proximity of that terminal to the surface low.
Since there are already IFR ceilings not far from KSAV, it is
certainly possible that even those lower ceilings could move in
for a few hours.

Otherwise, scattered SHRA and isolated TSRA pre-dawn will
increase in coverage during the daytime hours, leading to
occasional SHRA/TSRA impacting not only KSAV, but also KCHS and
KJZI. Temporary flight restrictions and gusty winds will result,
with the best chances currently looking to occur during the
late morning through the mid to late afternoon.

There will be a lull in convection during the late afternoon and
evening, but with the approach of a short wave Tuesday night,
additional convection can occur, especially at KCHS and KJZI.

Extended Aviation Outlook: Flight restrictions are possible in
afternoon/evening showers and thunderstorms Wednesday, then again on
Saturday.

&&

.MARINE...
Today and tonight: A weak area of low pressure will meander over
Georgia and South Carolina, perhaps reaching the coast of South
Carolina by daybreak Wednesday. This feature is having a
difficult time moving as it`s blocked by strong sub-tropical
ridging in the Atlantic. However, the gradient between the low
and the offshore high will be enough to produce mostly southerly
winds as high as 15 or 20 kt, before they drop off this
afternoon and tonight as both a slackening gradient forms and
the synoptic flow is altered by convection. Seas will average 3
or 4 feet throughout today, then 2 to 4 feet tonight.

Mariners should be remain alert and keep a watch on radar both
today and tonight, as there is the potential for some stronger
thunderstorms. This includes not only strong winds, but also the
potential for isolated waterspouts given favorable conditions on
the local Waterspout Index and the SPC Non-supercell Tornado
Parameter.

Extended Marine: High pressure will prevail in the Atlantic with
occasional troughing inland through at least Thursday. Winds will
generally be from the south/southwest. 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. Winds should increase
late Friday as a front approaches from the north/northwest. However,
no marine headlines are anticipated. The front should remain to our
north into the weekend.

Rip Currents: Small swells, onshore flow and cuts in the sandbar
from rip currents yesterday will result in a Moderate Risk of
rip currents at all beaches today.

&&

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

&&

$$

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


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