417
FXUS62 KCHS 181138
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
738 AM EDT Wed Sep 18 2024

.SYNOPSIS...
Low pressure across the Carolinas will weaken through mid week.
High pressure will then develop inland late this week, and
persist through the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Early morning update: Lower level theta-e gradient is surging
northward through southern/southeast Georgia, evidence by a
disconnected line of showers and some thunderstorms advancing
northward. Moisture gradient is expected to press into the
Georgia counties over the next few hours and could bring some
showers into the area. Going forecast already has that covered
with just some cosmetic tweaks made.

Meanwhile, additional showers are rotating through the Midlands
and eastern Georgia region along the eastern periphery of the
closed circulation. Radar trends suggest some of that activity
may spread into the western reaches of the forecast area within
an hour or two. That has been added to the forecast.

Previous discussion...
Well defined circulation center is located over the western
Carolinas this morning with some semblance of a surface trough
stretching through the central Carolinas into central Georgia.
Second stationary surface boundary is draped across the northern
Gulf and northern Florida region and into the Atlantic. Small
cluster of showers and thunderstorms have developed along the
southeast Georgia and northeast Florida coast during the
overnight hours where there is some modest instability and low
level convergence along the coast. Some remnant
showers/sprinkles from that activity are skirting up through the
far southern part of the forecast area into the Atlantic.

Today: Circulation center will be drifting eastward back toward
the coast through the course of the day while deep layer southerly
flow along the southern flank of the circulation allows higher
dewpoint air to creep back into the region. With heating,
forecast soundings and high-res guidance depict 500-1000 J/Kg
MLCAPE development this afternoon and possibly a bit higher
along the coast. With subtle mid-level height falls/ increasing
QG-forcing for ascent along the eastern flank of the circulation,
scattered to numerous showers and some thunderstorms are a good
bet, particularly during the afternoon hours. Pops have been
increased with this forecast cycle. Otherwise, a warmer day is
on tap with highs reaching the middle to upper 80s.

Tonight: Circulation center will be drifting northward to the
mid-Atlantic region with the stronger forcing for ascent
shifting offshore. With loss of heating, precip coverage will
diminish, although given the larger scale forcing, shower
activity might continue to percolate through the evening hours
and possibly into the overnight. Lows tonight will deep back
into the 60s.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY/...
Thursday: A vertically stacked low pressure will be located across
the eastern Mid-Atlantic Thursday morning and be slowly exiting the
coast. At the surface a remnant occluded front will be sagging south
into the Tri-County with a pronounced dry conveyor belt located over
coastal GA (as seen on RH fields). Due to this, have raised PoPs
across the TriCounty (30 - 40 %), while also keeping GA dry as 500/
300 MB RH is forecast to be only around 10%. Forecast soundings for
SC do show some instability in place (MLCAPE around 300 J/kg), but a
mid-level inversion does exist. As such, expect most of the coverage
to be shower based, with a few rumbles of thunder possible. Expect
high temperatures in the mid 80 across SC and upper 80s over GA.
This is thanks to a gradient of 1000/850 mb thicknesses with higher
values over GA (ahead of the occlusion) and slightly lower values
over SC. Low temperatures Thursday night will be in the upper 60s to
near 70.

Friday: The area of low pressure from Thursday will be just off the
coast of New England Friday with SC and GA falling on the convergent
side of the trough axis. This coupled with decreasing PWATs (down to
around 1.45") will keep the day precipitation free with surface high
pressure nudging in from the north. Expect high temperatures mostly
in the mid 80s with low temperatures Friday night in the mid
60s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Global ensemble guidance is in very good agreement that a trough
axis will remain centered along and south of the Canadian Maritimes
this weekend with a slow translation of the wave axis east Monday
into Tuesday. This will allow for dry conditions Saturday and Sunday
with slightly above normal temperatures as surface high pressure
remains in control.

Monday into Tuesday, the long wave trough axis will move east of the
area with mid-level heights rapidly rising across the region. As
this occurs, the humidity will return to the region as well as the
chance of afternoon showers and thunderstorms (starting on
Tuesday).

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Overall VFR conditions are anticipated at the terminals through
12Z Thursday. However, a decent coverage of showers and some
thunderstorms is expected this afternoon into this evening and
likely to impact the terminals after 18Z. SHRA has been
introduced to all three terminals after 18Z. Thunder is a
possibility, although probabilities are lower at any
particularly location and have not been included in the terminal
forecasts at this time. Expect later amendments as convective
trends become clear. Shower coverage will slowly diminish this
evening into the early overnight hours.

Southerly winds under 10 knots prevail today and this evening.
Light northerly winds develop later overnight behind a weak
boundary that will slip through the region.


Extended Aviation Outlook:
Thursday: Morning MVFR cigs possible with prevailing VFR in the
afternoon. A few afternoon showers and thunderstorms are
possible which could temporarily restrict cigs and vsbys. A weak
boundary will cross the terminals Thursday turning the winds
from the northeast.

Friday through Sunday: VFR. No precipitation is expected.

&&

.MARINE...
Southerly winds around 10 knots will prevail across the coastal
waters through tonight with seas running 2 to 4 feet.

Thursday: Another round of morning showers and thunderstorms are
possible with a weak boundary crossing the waters Thursday. Winds
will turn from the northeast 5 to 10 kt. Seas 2 to 4 ft.

Friday: Northeast winds will slowly start to increase in speed 10 to
15 kt as surface high pressure builds in from the north. Wind gusts
of 15 to 20 kt possible, or below Small Craft Advisory criteria.
Seas 3 to 4 ft.

Saturday and Sunday: Northeast winds 10 to 20 kt with gusts 15 to 25
kt. Seas around 4 ft. A Small Craft Advisory might be required as
the pressure gradient tightens from building high pressure.

Rip Currents: The combination of a lingering swell around 2 ft
at 9 seconds and high astronomical tides from the full moon will
lead to a Moderate Risk of rip currents for all of our beaches
today.

&&

.TIDES/COASTAL FLOODING...
Positive tidal anomalies along with high astronomical tides from the
full moon will lead to elevated tides for the rest of this week and
through the weekend. Coastal Flood Advisories will probably be
needed for both the morning and evening high tides through Friday
for both Charleston Harbor and Fort Pulaski.

Saturday and Sunday, the astronomical high tides start to lower but
the morning high tides will likely be elevated enough to still
require Coastal Flood Advisories (especially for Charleston and
coastal Colleton Counties). More uncertainty exists for Fort
Pulaski.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Coastal Flood Advisory until 11 AM EDT this morning for SCZ049-
     050.
MARINE...None.

&&

$$

NEAR TERM...Adam
SHORT TERM...Haines
LONG TERM...Haines
AVIATION...Adam/Haines
MARINE...Adam/Haines