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000
FXUS62 KCHS 260818
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
418 AM EDT Mon Jul 26 2021

.SYNOPSIS...
Weak low pressure will move onshore across southeast Georgia
today. A trough will then bring unsettled weather into midweek.
Conditions could improve later in the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The persistent but weak low, which is Invest 90L and we
have been talking about for several days, is drawing closer to
the northeast Florida and southeast Georgia coasts this morning.
The cyclonic flow around this system and its associated
frictional convergence has been generating some heavy rains
across parts of northeast Florida. Although the feature is
moving across the warmer Gulf Stream waters, the proximity to
land will prevent it from becoming anything more than a tropical
depression, if even that.

Closer to the immediate area, an inverted trough north of the
low is across the adjacent Atlantic waters and will produce
isolated to scattered showers as we start the day. As low level
convergence and moisture increase, we will begin to experience
some of those showers move ashore of the coastal corridor early
on.

As the surface low moves onshore of southeast Georgia, most
likely in the region between Savannah and Saint Simons Island by
early afternoon, it will have much greater CAPE and instability
to work with than recent days. Also the moisture significantly
greater, with a PWat that is near or above the 90th percentile
for late July. With dew points that are at least in the lower
and middle 70s, the humidity has returned and so will a better
coverage of convection than the past couple of days.

We show PoP as high as 30-50% from the Charleston region south
to Beaufort, Savannah and the Altamaha River counties, with the
best coverage south of Savannah. Far north and northwest zones
will have the lowest chances, at just 20%. However, with the sea
breeze merges with the inverted trough that moves onshore,
coverage might need to be increased a bit higher.

Given the abundance of moisture and an overall sluggish storm
motion, there is the potential for some locally heavy rainfall,
especially from about Beaufort south to the Altamaha River
basin. WPC has included parts of the forecast area in a Marginal
Risk for excessive rainfall as a result.

We`re not particularly thrilled with any severe potential, but
the HREF does indicate stronger updrafts where meso-scale
boundary interactions occur. So the risk is non-zero.

The typical summertime humidity has returned for today after a
nice respite the past couple of days. Temps though will be held
a bit below climo given increasing cloud cover and the greater
rain chances. We`re expecting mid and upper 80s over the coastal
counties, upper 80s to lower 90s inland.

Tonight: The remnants of the surface low will dissipate over
southeast Georgia, leaving us within a southeast to south flow
around the far western portion of the sub-tropical Atlantic
ridge. Diurnally driven convection will fade early on, before
some nocturnal convection that forms over the ocean makes a run
for the shore overnight. We have PoPs in the 20/30 percent range
as a result. Given the southeast-south synoptic flow, plus
elevated dew points, it`ll be a warm and sticky night. It
conceivably could turn out to be the warmest night of the month
so far for many communities. Lows will generally be down only
into the mid and upper 70s. There will be areas of low stratus
and maybe some fog west of I-95 late.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
The mid-levels will consist of a long-wave trough over the
Northeastern U.S. and a strong and broad ridge to our west. Both the
trough and the ridge will strengthen/amplify with time. The surface
pattern will feature high pressure in the Atlantic and troughing
inland.

Tuesday and Wednesday: Lots of moisture will be in place across our
area. PWATs are forecasted to exceed 2.25", which is above normal
per NAEFS and above the 90% for CHS sounding climatology.
Instability will also be the most we`ve had in recent days.
Models indicated MLCAPEs should easily exceed 1,500 J/kg with steep
lapse rates and DCAPEs approaching 1,000 J/kg. Convection should
start in the morning, then increase in both coverage and intensity
into the afternoon, aided by the inland moving sea breeze and
mesoscale boundary interactions. The highest POPs are far inland
late in the afternoon, but the timing/location will need to be
adjusted with future forecasts. Given the aforementioned conditions,
a few pulse severe thunderstorms with damaging winds are possible
both days. Additionally, there won`t be much shear and steering
winds should light. This means these storms may not move too fast.
With the abundant moisture, locally heavy rainfall in slow- moving
and/or backbuilding storms could lead to flooding, especially in low-
 lying and poorly drained areas. The worst convection should
dissipate in the evening. But showers/thunderstorms are expected to
persist Tuesday night. Drier air should move in from the north
Wednesday night, which will help dissipate the storms more quickly.
Temperatures will be near normal.

Thursday: Drier air moving in from the north could limit the risk of
convection to only our far southern GA counties. But the details
will need to be ironed out in future forecasts. Temperatures could
be slightly above normal, especially inland.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
The models have a mid-level trough prevailing to our north and a
strong ridge to our west. The surface pattern will feature high
pressure in the Atlantic and an occasional trough inland. The NBM
has below normal POPs through Friday with temperatures a few degrees
above normal. Diurnal convection and normal temperatures return for
the weekend.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
The weak surface ridge will give way to an inverted trough, as
an area of low pressure reaches the Southeast U.S. coast by
18Z.

The approach of the weak surface low along with much greater
moisture and modest instability will allow for scattered SHRA
and a few TSRA around the area throughout the day. Until trends
become better defined, we have just VCSH and VFR ceilings to
occur. However, brief and sporadic flight restrictions are
possible.

Extended Aviation Outlook: Flight restrictions are possible in
showers and thunderstorms Tuesday and Wednesday.

&&

.MARINE...
Today: The weak surface low that is just offshore northeast
Florida early this morning will move onshore of southeast
Georgia by early afternoon. Additionally, an inverted trough
that is across the local waters will shift onshore this
afternoon.

Our wind and seas forecast is with the understanding that the
surface low doesn`t strengthen as it moves over the Gulf Stream.
Because of this we have NE and E winds that turn to the E and SE
this afternoon. Speeds will mainly be at or below 15 kt, except
higher in and near widely scattered thunderstorms. Some of these
storms could be on the strong side. Seas will be up to 4 or 5
feet this morning, falling about a foot or so this afternoon.

Tonight: The marine community will be situated on the western
periphery of the sub-tropical ridge, producing SE and S winds of
15 kt or less. Seas will hold around 3 or 4 feet, with widely
scattered thunderstorms to occur.

Extended Marine: High pressure will prevail in the Atlantic
with occasional troughing inland. Winds will generally be from
the south/southwest. Each afternoon, expect slight backing of
the winds and higher gusts associated with the sea breeze along
the land/sea interface and in the Charleston Harbor. Each night,
expect slight veering of the winds. Winds could increase
towards the end of the week, but at this time no marine
headlines are anticipated. There may be an increased risk of
waterspouts Wednesday and Thursday mornings.

Waterspouts: Our local Waterspout Index is pointing to at least
Moderate Risk for today. That along with the SPC Non-supercell
Tornado Parameter that is as high as 10-12 units near the
surface low, suggests that mariners need to be aware of
waterspouts occurring today. We will add mention to the
Hazardous Weather Outlook and consider issuing a Marine Weather
Statement.

Rip Currents: Although conditions are marginal for rip currents,
our local Rip Current Calculator is supportive of a Moderate
Risk of rip currents at the beaches today.

&&

.TIDES/COASTAL FLOODING...
In all likelihood we have seen the last of any salt water
flooding with the recent series of high astronomical tides. But
since there remain large positive anomalies, we will have to
keep a close watch on the late evening high tide in downtown
Charleston for any possible Coastal Flood Advisory.

&&

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

&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...