692 
FXUS62 KCHS 301210
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
710 AM EST Fri Dec 30 2022

.SYNOPSIS...
High pressure will prevail today. A cold front will move
through Saturday night, followed by high pressure through early
next week. Unsettled weather could return for the middle of
next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The mid and upper levels remain highly amplified, 
showing ridging off the eastern seaboard, while a trough moves 
through the Southern Plains and northern Mexico. Embedded short 
wave energy within the trough will cause weak low pressure to 
form across southern Louisiana, while strong Atlantic high 
pressure holds firm to the north of Bermuda. While the local 
area is generally within the anticyclonic flow around the high, 
it is interrupted by an inverted trough across the nearby 
coastal waters. This trough will attempt to push toward the 
coast through the day, but looks to really struggle to cross the
cooler shelf waters.

Isentropic ascent, low level moisture convergence due to the 
proximity to the trough and a mid level perturbation that moves
through will be enough to generate at least isolated to maybe 
scattered showers. Amounts will be rather paltry given large 
condensation pressure deficits and considerable dry air above 
700 or 750 mb.

Recent satellite images, simulated satellite products and 
cross-sections indicate that stratocumulus will continue to 
prevail from off the ocean within a moist southeast to south low
level flow. The extent of the clouds (which as of this time 
looks to average about 60-80 percent sky cover) will have a 
bearing on temperatures. Given warm advection and at least 
partial sunshine developing, and utilizing a mix of the various
MOS guidance (except for the cooler NAM) along with the partial
thickness forecast, we arrive at max temperatures mostly 70-75F
degrees. Nearby ocean temperatures that are down near 50F 
degrees will hold the barrier islands down much cooler with a 
developing northeast-east sea breeze. 

Tonight: High pressure will retreat further east in the 
Atlantic, as a warm front develops across northern Florida, 
stemming from the surface low moving into Mississippi. As the 
warm front lifts north overnight, a southerly flow will allow 
deep subtropical moisture to spread into the area. This is
evidenced by PWat climbing to near or in excess of 1.5 inches,
or just shy of how high it has gotten on record for this time of
year. Despite this, the better forcing stays off to the west 
and northwest of the local area. This keeps the higher
probabilities of showers to our west through 6 AM Saturday. We
show some showers moving in from the west after midnight, with
coverage peaking at 20-40 percent by morning. An upstream band
of convection will move quickly east and move into western
Georgia by midnight. However, the band will weaken as it 
continues to draw closer, moving into a more stable environment.

The SREF and to some extent the HREF do show fog overnight. But
given considerable cloud cover, 925 mb winds climbing to 15 or 
20 kt and unfavorable condensation pressure deficits, we opted 
not to carry any fog in the forecast. Min temperatures will be 
as much as near 15F degrees above climo.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
Saturday: A mid-level trough will progress eastward through the Deep 
South, likely passing to the north of the forecast area in the 
evening hours. At the surface a cold front will sweep through the 
southeastern states, pushing offshore in the overnight hours. 
Rainfall is expected ahead of the cold front, with showers and 
possibly a thunderstorm likely by the afternoon. With shear values 
on the order of 40 to 50 knots and ML CAPE forecast to reach to 
around 300 J/kg thunderstorms are certainly possible. High 
temperatures are forecast to reach into the low to mid 70s. Even 
with the cold front passing through overnight temperatures are not 
expected to be chilly. The current forecast features overnight lows 
in the mid to upper 50s. 

Sunday and Monday: Zonal flow to slight ridging will develop in the 
upper levels while high pressure builds in at the surface. 
Subsidence from the building high pressure will result in a dry 
forecast through the period. Downsloping flow in the upper levels 
will likely result in high temperatures several degrees above 
normal. Both days feature high temperatures in the low to mid 70s. 
Overnight lows Sunday should only dip down into the upper 40s 
inland, with low 50s elsewhere.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
Broad ridging will prevail aloft through midweek while a trough over 
the Midwest progresses northeastward. A low pressure system is 
forecast to form associated with this feature passing over the Great 
Lakes late week. A cold front is expected to push through the 
forecast area late Tuesday or Wednesday bringing the region it's 
next shot of rainfall. Above normal temperatures will likely return 
to near normal after the front moves through mid-week.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
A nearby trough in the Atlantic and a moist onshore flow will 
produce VFR or possibly brief MVFR ceilings at the terminals 
with the 12Z TAFs, along with a small potential for showers 
into late morning and early afternoon. 

There might be some fog concerns late tonight, either from
stratus build down and/or warm and humid air moving in from the
south late. 

Extended Aviation Outlook: Flight restrictions are possible 
Saturday with showers/thunderstorms. Otherwise, prevailing VFR.

&&

.MARINE...
Today: A large and robust area of high pressure centered north
of Bermuda will move little, while an inverted trough likely
gets hung up near the cooler shelf waters of the local marine
area. With sea breeze influences and the clockwise flow around
the high, winds will be mostly NE or E at around 10 kt. Seas
will average 2-4 feet.

Tonight: The coastal trough looks to become a part of a warm
front that lifts into the area from the southwest late at night.
This happens in response to low pressure lifting from Louisiana
into southern Mississippi and Alabama, while the Atlantic high
pulls further east. Winds look to be easterly around 10 kt or
so, with seas of 3-4 feet. There are indications of some fog
trying to form after midnight, maybe advecting in from south of
the area. This depends on the northward extent of the warm
front, so for now we did not include in the forecast.  

Saturday through Wednesday: Southerly winds across the marine waters 
Saturday morning will shift to the SW as a cold front pushes through 
the region. A surge in wind speeds is expected, with 15-18 knots and 
gusts to around 20 knots. Seas are forecast to build, with 6 ft 
waves possibly impacting the offshore Charleston waters and the 20 - 
60 nm offshore GA waters. A short-lived Small Craft Advisory may be 
needed. Winds are expected to ease Sunday as high pressure builds 
into the region, with winds generally less than 10 knots and seas 
averaging 3 to 4 feet. Another surge in winds and seas is expected 
midweek as another cold front passes through. Wind gusts could 
approach 25 knots and seas could build to 6 ft. Small Craft 
Advisories may be needed for all marine zones.

&&

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

&&

$$

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