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000
FXUS62 KCHS 252202
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
602 PM EDT Mon Sep 25 2023

.SYNOPSIS...
Weak Atlantic high pressure will remain across the area through
tonight. Broad low pressure will form offshore by mid week,
bringing unsettled weather to our area through the end of the
week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Moisture will begin to return in earnest overnight as high
pressure shifts farther offshore and a large inverted trough
over the eastern Gulf of Mexico propagates to the north.
Guidance is similar in keeping much of the area dry overnight
despite increasing PWATs; however, lowering cloud decks and
possibly some shower activity will begin to move into the far
southern areas just before daybreak as 300-305K isentropic lift
intensifies. While some light returns yielding a few sprinkles
may show up on radar as far north as Savannah and Hilton Head by
sunrise, the risk for any measurable rainfall will likely be
confined to parts of Long, McIntosh and possibly Liberty
Counties where the lowest condensation pressure deficits (<30mb)
will be found. Any meaningful rainfall should hold off until
after daybreak. Pops were adjusted slightly to better reflect
this idea. Given the slightly faster increase in clouds
overnight, lows were nudged up across about 1-2 degs across the
far south and along the coastal corridor. Lows will range from
the mid 60s well inland to the mid-upper 70s at the beaches and
Downtown Charleston.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Pops were nudged up by about 10-20% for the parts of Southeast
Georgia into far southern South Carolina with the early evening
update based on 18z guidance and HREF output.

The mid-levels Tuesday morning will consist of High pressure
over the Bahamas and Low pressure over the Northern MS Valley.
The Low will slowly move towards the Great Lakes region through
Thursday, with weak troughing along its southern extent. This
will push the High further offshore, and cause heights to
gradually lower across our area. At the surface, weak Atlantic
High pressure Tuesday morning will quickly shift further
offshore. Broad Low pressure will form in its place just
offshore, then transition into a coastal trough by Wednesday.
It`ll hover in place into Thursday. This synoptic pattern will
usher deep moisture into the Southeast. PWATs should be around
2" across our southernmost counties during the short term, and
around 1.5" across our northernmost counties. For reference, the
90% PWAT for CHS is 1.9" per the SPC climatology page. Not
surprisingly, the synoptic models have decent shower coverage
spreading from south to north on Tuesday and Tuesday night and
persisting into Thursday. While there will be hourly
fluctuations in the POPs, the highest values should be across
our far southern GA counties and along our coastal counties.
Likewise, the same can be said of the QPF. But given the overall
weather setup, we`re not expecting flood advisories due to
rainfall. Also, there isn`t much instability, we only have a
slight chance of thunder. Temperatures will go through quite a
range, with highs Tuesday in the mid to upper 80s, then dropping
to the mid 60s to lower 70s Tuesday night. Wednesday, highs
will be much cooler, struggling to make 80 degrees across most
locations. Expect the low/mid 60s to around 70 degrees Wednesday
night. Highs will only be in the upper 70s on Thursday.
Finally, northeast winds should be gusty along the immediate
coast Wednesday and Thursday.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
The upper Low/trough over the Great Lakes Region Thursday night
will shift offshore into the weekend. Replacing it will be
ridging building in from the west early next week. At the
surface, the inland wedge pattern will persist through the
weekend with a trough just offshore. Unsettled weather will
persist, mainly along the coast or just offshore. Temperatures
will be near normal.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
26/00z TAF discussion:
KCHS/KJZI: VFR for much of the period. Light rain could move in
from the south during the late morning into the afternoon
hours, but coverage looks to remain fairly light at this time.
Where the corridor of rain sets up is still a bit uncertain this
far to the north, so VCSH was maintained for both terminals
from 20z on for the 00z TAF cycle.

KSAV: VFR tonight will give way to lower cloud decks and rain
spreading from the south as daybreak approaches. Current data
suggest cigs will remain VFR even as rain moves in, but would
not be surprised to see cigs drop to MVFR at times, especially
in the afternoon. Rain intensity looks to remain fairly light as
a large area of mostly light, stratiform rain moves north out
of far Southeast Georgia. Vsbys were capped at 6SM in -SHRA from
17z on for now to trend, but MVFR vsbys may need to be
introduced with the 06z TAF cycle.

Extended Aviation Outlook: Brief flight restrictions are
expected across our GA counties Tuesday, then our entire area
Tuesday night through Thursday. Conditions improve Thursday
night, with lower probabilities of flight restrictions Friday
and Saturday. Gusty afternoon winds around 20 kt will impact
the terminals Wednesday through Saturday.

&&

.MARINE...
Tonight: The Atlantic ridge will weaken and push eastward, with
southerly winds of 5-10 knots and seas 2-4 feet. Could see
isolated to scattered showers and thunderstorms return across
the extreme southern waters of both Georgia marine zones.

Tuesday through Saturday: Weak Atlantic High pressure will
quickly shift further offshore Tuesday morning. Broad Low
pressure will form in its place just offshore, then transition
into a coastal trough by mid week. It`ll hover in place into
Thursday, then slowly shift further offshore into the weekend.
This synoptic pattern will cause the surface pressure gradient
to become elevated, leading to gusty northeasterly winds. Wind
gusts will reach 25 kt or maybe even 30 kt at times across the
waters, so Small Craft Advisories will be needed. Though, when
they go up for each zone and how long they stay in effect is
still somewhat uncertain. Most likely, the GA waters beyond 20
nm will have the advisory up the longest. In response to these
winds, short- period seas will build, also reaching 6 ft or
higher at times. Seas will be the highest far offshore. While
winds may start to ease late this week, seas may take a little
bit longer to subside.

&&

.TIDES/COASTAL FLOODING...
The combination of developing, gusty northeasterly winds along
with increasing astronomical high tides will lead to coastal
flooding concerns from Tuesday onward. The highest probabilities
of impacts will be along along the southeast South Carolina
coast. Coastal Flood Advisories will eventually be needed for
the evening high tides and possibly the morning high tides as
well. Levels could approach Coastal Flood Warning levels by the
mid-week evening high tides.

&&

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

&&

$$