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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KCHS 271434
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1034 AM EDT Wed Sep 27 2023

.SYNOPSIS...
A wedge of high pressure will build into the region today and
holds into early next week, while a broad trough of low
pressure persists well offshore.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Strong mid-latitude blocking remains across eastern Canada and
the Great Lakes with a closed cut-off low centered near Chicago.
West-southwesterly flow aloft along the periphery of the low is
in place across the south/southeast states with an upper jet
axis that extends off the mid-Atlantic coast. Strongest larger
scale forcing for ascent and associated precip has moved well
out into the Atlantic at this juncture.

But the wedge is in with surface high pressure and cooler air
wedging down through the Carolinas into Georgia behind a
boundary that stretches from northern Florida northeastward into
the Atlantic. Instability and baroclinically driving bands of
showers are fairly widespread in the Atlantic waters...some of
which have been streaming into inland portions of southeast South
Carolina and southeast Georgia.

Pattern changes very little over the next few days with cold
air damming/wedging holding tight...at least through Thursday.
Ongoing post-frontal f-gen forcing in the Atlantic is forecast
to work further offshore through this afternoon...suggesting our
overall shower threat will gradually diminish as we go through
the afternoon and especially tonight.

Bigger forecast challenge revolves around the low cloud cover
and impact on temperatures. HRRR forecast soundings do suggest
that ceilings will nudge upward through the afternoon with
perhaps some breaks in the low cloud cover. But overall it will
remain cloudy and I`ve lowered forecast max temps into the lower
to middle 70 range...around 10 degrees below normal for late
September. That said, any amount of sunshine this afternoon will
bust our temp forecast.

Cloudy skies persist through tonight with lows in the lower to
middle 60s.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
A closed mid level low near the Lower Great Lakes Thursday will
shift to the the vicinity of Lake Erie and Lake Ontario Friday, then
off Long Island and New England as it deepens Saturday. On Thursday,
the local area will lie within a west-southwest flow between the low
and a large mid level ridge from the Bahamas into the Atlantic. The
flow aloft on Friday and Saturday is near the southern apex of the
trough, while a weak closed low tries to form in the northeast Gulf
of Mexico. At the surface a strong inland wedge holds firm Thursday,
weakens some on Friday, then intensifies again later Saturday. A
broad trough will also exist well offshore during this time, with
maybe waves of low pressure to form on that feature. There is not
much in the way of forcing for ascent, as we`ll have to rely on low
level convergence to generate any showers. The better probabilities
(20-30%) will occur from Hilton Head south to McIntosh County, where
the better trajectories from off the ocean occur. There is too much
dry air further north and west where we don`t see much chance
anywhere else in the forecast counties. Not very excited about the
chance for t-storms, but there might be just enough instability to
produce a few stray, diurnal t-storms. daytime highs will run near
or slightly below climo, while night time lows will be several
degrees below typical late September norms.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
Another Rex Block looks to develop over the CONUS, with a large
trough in the Atlantic, strong ridging covering much of the area
east of the Rockies, and a deep cyclone out west. At the surface
there is still wedging inland, with a broad trough far offshore.
rain chances are no higher than 20-30% over the coastal corridor,
and most especially southeast Georgia. Not much chance for anything
further inland. Temps will average close to normal during each day,
but below climo at night with light winds and some radiational
cooling effects.

&&

.AVIATION /13Z WEDNESDAY THROUGH SUNDAY/...
Prior to the 12Z TAFs, the backdoor cold front will begin the
day over the Savannah River, pushing to the SW. North of the
front, building CAD should support IFR ceilings across KCHS and
KJZI during the initial daylight hours, then MVFR ceilings will
likely linger into the afternoon hours. At KSAV, IFR ceilings
are forecast to develop between 12-14Z, highlighted with a
TEMPO. MOS and forecast soundings indicates that MVFR ceilings
will into this afternoon.

Deepening mixing and an increased pressure gradient should
result in NE winds strengthening to 10-15 kts with gusts around
20 kts. The gusty NE winds and at least MVFR ceilings are
expected to continue until late this afternoon. This evening,
conditions should favor steady NE winds around 10 kts and VFR
conditions.

The CAD should reinforce across the region tonight. Conditions
should feature steady NE winds around 10 kts and MVFR ceilings
developing between 5-8Z.

Extended Aviation Outlook: Perhaps occasional flight restrictions in
low ceilings late each night and through mid morning late in the
week. Otherwise VFR will prevail.

&&

.MARINE...
Today, a backdoor cold front will push across the coastal waters of
SC and GA today. In the wake of the front today, northeast winds
will strengthen to around 25 kts with gust around 30 kts outside the
CHS Harbor. Seas will begin the day between 3-5 ft, building to 4-7
ft by late this afternoon. The wind and sea conditions will remain
nearly unchanged through tonight. Small Craft Advisories have been
issued for all marine zones outside the CHS Harbor.

Thursday through Monday: The marine area will remain situated
between an inland area of high pressure, while a broad trough
persist far offshore. There will be fluctuations in the strength of
the inland high and the position of the trough, which will have
impacts on the strength of the NE`erly winds and the resulting
seas. For now we`re expecting Small Craft Advisories for all waters
outside Charleston Harbor into Thursday, with the outer Georgia
waters to continue into next week. Even if the advisories come to an
end on the waters, it might only be temporary, as there is a decent
pinching from time to time through the forecast period.

Rip Currents: Gusty NE winds will create a strong longshore current
with the surf zone of all beaches today. Breakers are expected to
peak between 2-4 ft today, with periods around 7 seconds. In
addition, experimental Rip Current MOS peaks in the high category
for all beaches. Based on local risk guidelines, a moderate risk for
rip currents is forecast for all beaches today.

Moderate northeasterly winds, a small amount of swell
energy, and the Perigean Spring Tides will lead to an enhanced risk
of rip currents late this week. The risk could very well remain
enhanced even into early next week with similar conditions.

&&

.TIDES/COASTAL FLOODING...
Gusty northeast winds should persist throughout the high tide
cycle this afternoon into early this evening. The gusty winds
should increase tidal departures, resulting in at least moderate
coastal flooding along the coast of Charleston and coastal
Colleton Counties. Tide guidance would indicate that tide levels
for the 7:11 PM could reach or exceed 8 ft MLLW. However, this
would require a tidal departure of 1.22 ft, which would be a
significant increase from expected 0.85 ft departure this
morning. The potential for major coastal flooding will be
addressed in the HWO.

Several days of at least moderate northeasterly winds, plus tides
that are already elevated due to the Perigean Spring Tides, will
result in a potential several day period of coastal flooding during
each of the two daily high tides. Moderate to perhaps major coastal
flooding can occur in Charleston and coastal Colleton, with minor to
moderate from Beaufort to coastal McIntosh, including the
Tybee/Savannah area. There is even the risk for minor flooding in
Tidal Berkeley (a somewhat rare occurrence). With such a prolonged
period of elevated tides, areas of beach erosion will also
occur.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until noon EDT Thursday for AMZ350-352.
     Small Craft Advisory until 5 PM EDT Thursday for AMZ354.
     Small Craft Advisory until 6 AM EDT Tuesday for AMZ374.

&&

$$

NEAR TERM...Adam/NED
AVIATION...NED
MARINE...NED