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000
FXUS62 KCHS 280130
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
930 PM EDT Tue Jul 27 2021

.SYNOPSIS...
Weak low pressure will drift north along the Georgia and South
Carolina coasts tonight into Wednesday, resulting in unsettled
conditions. Rain-free and hot conditions are expected Thursday
and Friday, followed by a return to a wetter weather pattern as
a cold front stalls over or just north of the region next
weekend into early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
As of 930 PM: KCLX detected an expanding area of showers and
thunderstorms across SE GA north to Beaufort Co and the adjacent
waters. Latest run of the HRRR indicates that convection will
gradually increase across the forecast area through the rest of
the evening and into the early morning hours. PoPs will be
increase through the late night hours. Hourly temperatures will
also be adjusted across the areas of greatest rainfall.

As of 735 PM: KCLX detected an increase in convection across SE
GA, showers and thunderstorms were nearly stationary. Late this
evening, convection should develop over the waters on the NE
side of the coastal low, centered over HHI. Some of this
activity may brush the coast late tonight. The forecast update
will feature slight adjustments to PoPs and Wx to reflect latest
radar and model trends.

Previous Discussion:
Weak low pressure near the southeast Georgia and South Carolina
coasts will slowly lift northward tonight. Aloft, a mid level
trough will extend south along the East coast, with perturbations
expected to pass over the area at times. Moisture will continue
to be anomalously high, as noted by precipitable water values
well over 2 inches. Given this and the aforementioned features,
showers and thunderstorms will be possible just about any time
tonight. Though, it appears there could be higher coverage near
the SC coast after midnight. HREF probabilistic guidance
continues to indicate the potential for locally heavy rainfall.

Otherwise, low temperatures will average in the mid 70s, except
higher near the immediate coast and beaches. There could be
some light fog around, especially in areas that received prior
rainfall.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Wednesday: The day begins with a challenging precipitation forecast
as weak surface low pressure continues to drift north along/near the
our coast and an upper trough axis amplifies over the region.
Similar to the Tuesday morning scenario, showers/ thunderstorms
could be ongoing especially along the coast and adjacent coastal
waters and more specifically along the SC coast/marine zones.
However, per latest high resolution model trends, there remains
uncertainty about whether best precipitation coverage will reside
over coastal counties or will be displaced farther north/east. Thus,
morning PoPs are capped around 50 percent across coastal counties,
especially in SC.

The Wednesday afternoon PoP forecast also remains challenging as the
uncertain morning convective regime will influence the
subsequent development/evolution of diurnal convection. In general,
a deep layered trough axis, ample moisture featuring PWAT values
2.25-2.50 inches and localized convergence along a weak sea breeze
and other complex mesoscale boundaries should support scattered to
numerous showers/thunderstorms Wednesday afternoon/evening. As
guidance depicts the axis of highest PWATs pushing south with time,
highest PoPs reside over southern counties.

The potential for locally heavy rainfall will continue Wednesday.
The overall severe threat will remain low, but an isolated/brief
severe thunderstorm with damaging wind gusts could occur Wednesday
afternoon/evening.

Outside showers/thunderstorms, high temperatures should average in
the upper 80s to lower 90s inland and in the middle to upper 80s on
the beaches.

Wednesday night: Guidance remains in excellent agreement that the
upper trough axis will push offshore, the inland upper ridge will
expand east, and a swath of drier air will spread into the region.
Thus, any residual evening convection should shift south/east of the
region, and the overnight hours should remain rain-free. Light winds
and boundary layer moisture could support some fog late night into
early Thursday morning. Temperatures should bottom out in the lower
to middle 70s inland and in the middle to upper 70s on the beach.

Thursday and Friday: The inland upper ridge will continue to expand
over the region, supporting rain-free conditions and the hottest
weather thus far this summer. Inland high temperatures should top
out in the middle to upper 90s Thursday and in the upper 90s to
around 100F Friday. Even on the beaches, high temperatures in the
upper 80s to lower 90s should prevail. These temperatures will
combine with dewpoints in the 70s, even around 80F close to the
coast, to support maximum heat index values around 110F in some
areas Thursday and and in the 110-115F range Friday. Heat Advisories
will likely be required, and an Excessive Heat Watch/Warning could
eventually be required for some areas Friday, especially for coastal
counties just inland from beaches. Low temperatures will average
75-80F most areas Thursday night.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Some locations will remain above 80F Friday night. Then, expect a
final day of hot temperatures featuring high temperatures 95-100F,
heat index values around 110F in some areas. Models feature more
moisture Saturday, so isolated.scattered thunderstorms could develop
especially from mid/late afternoon into the evening. Then, the upper
ridge will retreat west and will allow the east CONUS trough to
amplify south/into our region Sunday through at least early next
week. The upper trough will push a surface cold front toward our
area, but forecast uncertainty remains high regarding the progress
of this front. Most likely, the front will stall over or near our
region. Regardless of details, this scenario will bring an end to
above-normal temperatures and could even support below-normal
temperatures. Also, the arrival of increasing moisture/PWAT values
exceeding 2 inches should support a transition to above normal PoPs.
Of interest, recent operational runs of the GFS and European models
depict an unusually deep upper trough digging west of the region,
setting up a baroclinic zone and an extended period of wet weather
next week.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
The combination of deep moisture and a slow moving sfc low
should yield a period of MVFR ceilings between midnight and
mid-morning Wednesday across the terminals. Showers and
thunderstorms are expected to develop over the Atlantic on the
NE side of the coastal low. KJZI could be brushed by a shower or
thunderstorms late tonight. KSAV and KCHS could see a shower in
the vicinity late tonight. Winds will shift from NW during the
daylight hours on Wednesday.

Extended Aviation Outlook: Flight restrictions possible in
showers/thunderstorms Wednesday afternoon/evening and again in
patchy stratus/ground fog late Wednesday night/early Thursday. Then,
VFR expected until scattered thunderstorms develop next weekend.

&&

.MARINE...
Tonight: Weak low pressure near the southeast Georgia and South
Carolina coasts will slowly drift northward tonight. The
pressure gradient isn`t particularly tight, so winds should
average 15 knots or less. Seas will range from 2 to 4 feet
tonight.

Mariners should remain alert and keep a watch on radar through
tonight, as there remains some threat for isolated waterspouts
and perhaps gusty winds.

Wednesday: Thunderstorms could produce locally hazardous marine
conditions. Otherwise, a weak pressure pattern will
maintain winds/seas well below SCA thresholds through late week.
Next weekend, a tightening pressure gradient will support stronger
southwest winds at least 15-20 knots, especially at night,
accompanied by seas 3-5 feet. The probability for Small Craft
Advisory conditions will increase accordingly.

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. A low risk of rip currents is
forecast for Wednesday.

&&

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

&&

$$

NEAR TERM...NED
SHORT TERM...SPR
LONG TERM...SPR
AVIATION...NED
MARINE...ETM/SPR