Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 200858
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
458 AM EDT Fri Apr 20 2018

.SYNOPSIS...
Cool and dry high pressure will build into the region today
through Saturday. A low pressure system will approach the area
late Sunday and will then slowly move up the coast through the
middle of next week. Weak high pressure will return late next
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Satellite shows that the ongoing altocumulus clouds have
expanded some in coverage from earlier, so we have adjusted the
sky cover forecast to show more clouds early this morning across
the central and southern coastal counties.

Previous discussion...
Early this morning: There is quite a bit of stratocumulus and/or
altocumulus near or just offshore the coastal areas, near the
SE fringes of an expanding continental high pressure that is
building in from the NW. Isentropic ascent and some convergence
within a NE flow in the boundary layer will maintain coverage of
these clouds prior to daybreak, before the large scale
subsidence around the building high strengthens considerably.
It`s a much cooler start to the day when compared to 24 hours
ago, as much as 15-20F lower than early Thursday.

For Today: The broad east coast trough will pull offshore, while
strong Canadian high pressure is firmly in control at the
surface, covering much of the eastern half of the country. The
lack of moisture and a pronounced subsidence cap around 4-5K ft
will extend our rainfree pattern to 5 straight days, maintaining
the Severe Drought (D2) across the forecast area. the majority
of us will be mostly sunny or sunny throughout the day, with
the exception over McIntosh County where some of the early
morning stratocumulus could move back onshore as the low level
flow veers to the E-NE.

We stayed close to the low level thickness forecast since there
is plenty of strong spring insolation to negate the cooler
start. This is supportive of highs in the upper 60s or lower 70s
away from the coast, where onshore flow will limit highs to the
lower and middle 60s.

Steady isallobaric pressure rises and cold advection will wane
later this morning, and this will allow for winds to come down
the second half of the day. The exception will be across
coastal Georgia where there is still a little bit of a gradient
that persist. NE winds will be as high as 15-25 mph this
morning, with gusts up near 30 mph along the coastal corridor.
Winds then veer a little more easterly at no more than 10-15
mph this afternoon, except 15-20 mph over the coastal sections,
highest over the Georgia zones.

Lake Winds: Steep low level lapse rates with cold advection atop
the warmer waters of Lake Moultrie will produce NE winds as high
as 20-25 kt this morning. Thus a continuation of the ongoing
Lake Wind Advisory through 11 am.

Tonight: Mid level ridging will inch closer from the west, with
strong surface high pressure persisting overhead. It looks like
the bulk of any marine-induced stratocumulus will stay in the
ocean or to the south of the Altamaha, leaving us with mostly
clear or clear skies. Winds will decouple in many communities
inland from US-17, and with the low dew points this allows for
decent radiational cooling to occur. It`s across these locations
where our coolest temps in the lower or middle 40s will occur.
Closer to the coast there is enough mixing to hold temps to
about 50-55F.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
Surface high pressure centered to our north will remain in control
of the local weather on Saturday, allowing for quiet and dry
conditions. Mostly sunny skies are expected for much of the day,
with the exception of far southeast Georgia where an increase in
cloud cover is expected. Cool northeast flow will keep temperatures
below mid-April normals. Highs will top out in low to mid 70s
inland, and only in the upper 60s along the coast. Lows Saturday
night will mainly be in the low to mid 50s.

On Sunday, an area of low pressure will organize upstream as a
closed upper low moves into the Gulf Coast states. Isentropic ascent
will begin to spread into the area from south to north, although
will initially only result in thickening of cloud cover and maybe a
few showers. Better forcing and moisture (noted by PWats of 1.5+
inch) arrives late Sunday night into Monday as the surface low moves
closer and a nice slug of upper divergence shifts over the area.
More widespread showers are expected with the current forecast
featuring 70-80% PoPs. Rainfall totals on average will be 1-1.5
inches, which will be welcomed given the current drought status. A
couple models do indicate a small pocket of instability across
southeast Georgia, so a rumble of thunder or two can`t be ruled out.
Much will be dependent on the track of the low. Temperatures will be
in the low to mid 70s. Lows Sunday night will be a bit more mild in
the mid to upper 50s.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
A closed upper low over the Southeast on Tuesday will get absorbed
by a stronger shortwave dropping out of the Midwest during the
middle of next week. Concurrently at the surface, an area of low
pressure will lift northeast out of southern Georgia Monday night
and then move slowly up the coast. Weak high pressure will return
late week as the surface low continues to move away from the area
into the northeast U.S. Main chances for rain will come early in the
period as the low drifts directly over the area. Temperatures will
be near normal.

&&

.AVIATION /09Z FRIDAY THROUGH TUESDAY/...
VFR. Gusty winds will continue at times for both KCHS and KSAV
into this morning, due to large pressure rises in tandem with a
continued influx of cooler air arriving courtesy of continental
high pressure.

Extended Aviation Outlook: VFR expected through early Sunday.
Flight restrictions are likely later Sunday into Tuesday as low
pressure impacts the area with rain and breezy conditions.

&&

.MARINE...
Today: Strong continental high pressure will generate about a
1-2 mb rise in pressures this morning. Combined with cold air
advection (CAA), this will easily allow for plenty of mixing of
the 30 kt of geostrophic winds to occur. And as a result we
have Small Craft Advisory (SCA) flags flying for all marine
waters. It`ll take time for the elevated E-NE winds (20-30 kt)
and seas (3-6 ft nearshore waters and 6-9 ft further out) to
come down, so the ongoing advisories will continue at least
through late morning or this afternoon.

Tonight: The waters will stay along the SE portions of robust
Canadian high pressure centered near the Great Lakes, keeping a
modest E-NE gradient in place. Current thinking is that the only
advisories still in effect will be on the Georgia waters, but
AMZ354 should end by 7 pm. With that gradient persisting, winds
will still be as high as 15 or 20 kt, a little less in
Charleston Harbor. The onshore fetch holds seas up near 3-4 ft
within 20 nm and 4-6 ft further offshore.

Saturday through Wednesday: High pressure on Saturday will lose
influence early next week as a low pressure system moves across the
area. Marine conditions will deteriorate as the pressure gradient
tightens, with Small Craft Advisories likely for most, if not all,
waters beginning as early as Sunday evening. There are some
indications that a few gusts could approach gale force Monday
afternoon, especially across Charleston county waters and outer GA
waters, but much depends on the track of the surface low. Will
continue to monitor trends. Winds will vastly improve on Tuesday
as the low begins to lift northeast, but seas will likely stay
elevated into Wednesday.

Rip Currents: A tough forecast in the surf zone today, with some
concern about how much swell energy actually reaches the beaches
and exactly how strong winds remain into the afternoon. To mesh
better with WFO JAX (who has a High Risk) we are going with a
Moderate Risk of Rip Currents at the Georgia beaches and the
beaches south of Charleston County. It is these locations where
the higher onshore winds will persist longer than further north.

A prolonged stretch of moderate to perhaps strong onshore winds
and larger surf, first due to high pressure, then as low pressure
passes nearby early next week, will lead to an enhanced risk of
rip currents at the beaches.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Lake Wind Advisory until 11 AM EDT this morning for SCZ045.
MARINE...Small Craft Advisory until 4 PM EDT this afternoon for AMZ352.
     Small Craft Advisory until 2 PM EDT this afternoon for AMZ350.
     Small Craft Advisory until 7 PM EDT this evening for AMZ354.
     Small Craft Advisory until 2 AM EDT Sunday for AMZ374.
     Small Craft Advisory until 11 AM EDT this morning for AMZ330.

&&

$$
NEAR TERM...
SHORT TERM...ECT
LONG TERM...ECT
AVIATION...
MARINE...


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