National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS62 KCHS 300806
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
406 AM EDT Fri Jul 30 2021

...DANGEROUS HEAT THROUGH THE WEEKEND.

.SYNOPSIS...
High pressure will extend across the region today. A cold front
will stall over or just north of the area Saturday through the
middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The region will remain along the far southeast periphery
of a pronounced mid-level anticyclone centered across the
Central Plains through tonight. A modest northwest flow aloft
across the Southern Appalachians will help anchor a pronounced
lee-side trough to the west today. Expect another rare, mostly
rain-free summer day to prevail although there are signals that
there may be a somewhat favorable juxtaposition of modest H8
theta-e pooling with some surface based instability to yield one
or two tstms across interior Southeast Georgia for about a 2-3
hour period this afternoon. Some of the CAMs certainly support
this scenario, including the 30/00z H3R, but there is some
uncertainty on exactly how much mixing out of dewpoints will
occur in this region. If the mixing ends up being much deeper
than expected, considerably more CIN will remain in place with
weak capping noted around 800 hPa. No mentionable pops will be
maintained with this forecast cycle, but slight chance pops may
need to be introduced at some point today once dewpoint trends
across interior Southeast Georgia become more apparent.

It will be a hot and humid day for the region. Although H8
temperatures will remain nearly identical to yesterday, low-
level thicknesses are progged to rise about 10-20 meters across
the board in the northwest flow aloft. This coupled with a
pinned or somewhat delayed resultant sea breeze could support
highs in the mid-upper 90s inland from the beaches with a few
spots potentially making a run for the century mark. Highs were
nudged down about 1-2 degrees from earlier forecasts.

The main forecast challenge for this afternoon centers on
hourly dewpoints and how the resulting heat indices will evolve
through the day. Observation sets along the Intracoastal down to
the KSSI ASOS showed a typical pooling of dewpoints around 80
for several hours yesterday afternoon as the sea breeze matured.
Local analogs to previous mid-summer heat events show something
similar happening again today with dewpoints maxing out around
80-81F in the vicinity of the sea breeze for about a 3-5 hour
period along the coast. This matches trends noted in the H3R and
RAP with both show about a 25-35 NM wide band of 80+F dewpoints
developing by 21z across the coastal counties. While the band
will likely be sharper than this, the trend is certainly there
for a ribbon of 80F dewpoints to advect west and northwest
through the afternoon and intersect higher temperatures that
will be present just inland from the coast. Expect a rather
large area of heat indices of 113-117F to develop across the
coastal counties with the axis of highest values forming roughly
along an Awendaw/eastern portions of the Francis Marion
National Forest-Downtown Charleston-West Ashley/James Island-
Wiggins-Beaufort-Bluffton-eastern portions of Savannah
Metro/Wilmington Island-Halfmoon Landing-Sapelo Island line. In
fact, a narrow corridor of 115 heat indices could work as far
inland as southern parts of Dorchester County and across
northern/western parts of the Charleston Metro Area. The
Excessive Heat Watch will be upgraded to an Excessive Heat
Warning for all coastal zones including parts of Inland Berkeley
and Dorchester Counties and will include Inland Chatham County
as well to capture the entire Savannah Metro Area.

The dewpoint forecast is a bit more uncertain farther inland
where warmer temperatures and considerably more mixing will
occur. Some of the higher resolution output suggest dewpoints
could mix out in the mid-upper 60s across far interior Southeast
Georgia into portions of Hampton and Allendale and up into far
northern Colleton, Berkeley and Dorchester Counties adjacent to
the Midlands. This would yield heat index values more in the
103-108 range, or below the 110 Heat Advisory criteria. The GFS,
NAM, and EC statistical guidance packages are much higher with
dewpoints and could be picking up the potential latent heat of
evaporation implications due to lingering wet grounds across
the region from heavier rains earlier in the week. At this
point, its best to keep the Heat Advisory in place, especially
considering its really the first heat event of the summer.

Tonight: A broken band of showers/tstms moving across the
Midlands and Pee Dee could make a run for the Walterboro-Moncks
Corner corridor by mid/late evening as a cold front approaches
from the northwest. Instability should be on the decline by this
point with the decaying sea breeze circulation potentially
reaching the southern Midlands by then. A dry forecast will be
maintained for now. It will be a warm and humid night with lows
ranging from the mid 70s well inland to the lower 80s at the
beaches and Downtown Charleston. In fact, it is very possible
Waterfront Park could stay in the mid 80s through the night.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
Saturday: On Saturday morning, cloud cover should be somewhat
minimal as mid-level ridging lingers to the south and drier air
remains in the vicinity. This will lead to a rapid increase in
temperatures and another hot day is expected. By the
afternoon/evening, weak shortwave energy in combination with an
approaching cold front and increased moisture represented by
PWATs~2.0" will increase rain chances. Although, coverage will be
limited to isolated to scattered and mainly along boundary
interactions, including the sea breeze. Decent instability will be
over the region so a stronger thunderstorm or two cannot be ruled
out.

Aside from rain chances, the heat will continue to be a concern. As
dewpoints crawl into the mid to upper 70s and temperatures reach the
mid to upper 90s, heat indices are forecast to be in the 108-114F. A
Heat Advisory will likely be needed for most or all of the area.

Sunday and Monday: As a cold front stalls across the region, rain
chances will increase and a wet couple of days is expected. PWATs
are forecast to be around 2.0-2.5 inches as shortwave energy lingers
across the region. Higher rain coverage than previous days is
expected, especially along any boundaries that form, including the
sea breeze. Again, there will be sufficient instability so could see
a few stronger thunderstorms.

In regards to temperatures, Sunday could be another hot day.
Although, temperatures will greatly depend on how quickly cloud
cover moves into the region due to increased moisture. High temps
are currently forecast to reach the mid to upper 90s with slightly
cooler temperatures along the coast. Dewpoints should be in the mid
to upper 70s mainly along the coast and just slightly inland. A Heat
Advisory could be needed in the aforementioned areas as heat indices
are forecast to be in the 110-114F range. Monday is expected to be
cooler with high temperatures in the 80s to low 90s.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
An active pattern will prevail through the week as a cold front sets
up across the area or just in the vicinity while mid-level troughing
deepens. Showers can be expected each day due to shortwave energy,
boundary interactions and the sea breeze. Plentiful moisture
(PWATs~2.0 inches) will prevail across the region. Typical
summertime thunderstorms can also be expected. High temperatures
will be in the 80s.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
VFR.

Extended Aviation Outlook: Flight restrictions will be possible
at KCHS, KSAV and KJZI starting Saturday in showers/thunderstorms.
Although, better rain chances will start on Sunday and persist
through mid-week, likely bringing more flight restrictions.

&&

.MARINE...
Through Tonight: A delayed, but robust sea breeze will impact
with the Charleston Harbor this afternoon bringing southwest
winds 15-20 kt with gusts to 25 kt. A Small Craft Advisory has
been issued for the afternoon and early evening hours.
Otherwise, south to southwest winds will prevail over the waters
with 15-20 kt South Carolina waters and 15 kt for the Georgia
waters. These conditions will persist into tonight as a cold
front approaches from the northwest. Seas will average 2-4 ft.

Saturday through Wednesday: A cold front will position itself inland
as high pressure lingers across the waters. Depending on the
positioning of the front, the pressure gradient will become pinched
at times, increasing winds. Southwesterly winds will generally
prevail at 10-15 knots and seas 2-4 feet.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
     for GAZ087-088-099>101-114>116-137-138-140.
     Excessive Heat Warning from 11 AM this morning to 8 PM EDT
     this evening for GAZ117>119-139-141.
SC...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
     for SCZ040-042-043-047.
     Excessive Heat Warning from 11 AM this morning to 8 PM EDT
     this evening for SCZ044-045-048>052.
MARINE...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
     evening for AMZ330.

&&

$$