Area Forecast Discussion
Issued by NWS Charleston, SC

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132
FXUS62 KCHS 280530
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
130 AM EDT SAT MAY 28 2016

.SYNOPSIS...
Atlantic high pressure will prevail tonight. Tropical Depression Two
has developed over the southwest Atlantic and will approach the
Southeast coast late Saturday and Saturday Night, before moving
onshore over or near the area Sunday. The low could affect the
region into early next week before eventually lifting away to the
northeast.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
On the midnight update, we increased clouds a bit quicker to the
east of i-95 as isentropic ascent and moisture depth both
increase. We also kept temps warmer overnight along the SC coast
where clouds were present. Some light showers associated with the
far outer circulation of Tropical Depression 2 could approach the
lower South Carolina coast prior to sunrise, but with such dry air
in place, no meaningful accumulations are expected. Slight chance
pops for from Hilton Head to McClellanville look reasonable.

The 11 pm NHC forecast track has shifted the track about 20-30
miles east, thus keep the center of the tropical cyclone just
offshore Charleston Metro. The expected impacts have not changed
significantly. There is some concern that tropical storm force
winds, especially in gusts, could impact southern portions of
Dorchester County, but with the latest track farther east and a
bit weaker, the Tropical Storm Warning will not be expanded into
that zone with this forecast cycle.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH MONDAY/...
The forecast through the short term period continues to remain quite
complex and still somewhat uncertain, highly dependent upon the
evolution of Tropical Depression Two over the southwest Atlantic
between Bermuda and the Bahamas.

Overview: TD Two as of late Friday to the northeast of the Bahamas
has been encountering quite a bit of shear, on the magnitude of 20-
30 kt, also a fair amount of drier air is still evident over the
southeast and mid-Atlantic region. With time the shear is expected
to relax and the moisture is forecast to increase. These changes in
atmospheric conditions may become more favorable for the depression
to become a tropical area of low pressure as it approaches from the
southeast late Saturday and Saturday Night, moving onshore across
coastal South Carolina Sunday, then meandering around over or near
the forecast district into Monday as it encounters very weak
steering flow. The depression is forecast to move in tandem with mid
to upper level low, steered around sub-tropical ridging over the
western Atlantic. If it remains a weaker system it would be driven
more by the low level flow from the east-southeast, whereas if it
becomes a little more of an organized system it would be driven more
by the flow aloft which is a little more from the southeast-south.
The system will for a time move across the warmer Gulf Stream water
over the second half of the weekend, which could allow for some
strengthening. But it will eventually move across the "cooler" shelf
waters where SST`s are in the middle 70s (76-77F) which could allow
for some weakening prior to landfall on Sunday.

A worst case scenario shows landfall over Charleston and/or coastal
Colleton County. This is also the latest forecast from NHC.

Keep in mind that impacts on the local area will be very similar
whether or not the system gets a name (which would be Bonnie, since
there was already Alex in January). As of late Friday the main
weather hazards are rip currents (see below), maybe a little coastal
erosion, and locally heavy rains. The risk for tornadoes is
extremely low, pending how much shear occurs with the approach of
the tropical/sub-tropical system Saturday Night into Sunday.

Rainfall: We are forecasting 1-2 inches of rains across the
Charleston quad-County district, but with locally higher amounts if
the system stalls over or near the area. This will lead to at least
minor flooding concerns. We`re in a waning phase of the moon, so
tides aren`t extremely high. But if high tide occurs in sync with
moderate or heavy rains, then the risk for more significant
flooding would occur, especially in the Charleston metro. With
more subsidence and drier air on the west/southwest side of the
system, the rest of South Carolina and Georgia will generally have
less than an inch of rain, unless the track of the low is further
south.

Probability of Precip: Rain chances will start to ramp up from off
the Atlantic due to deeper tropical moisture and coastal
convergence, leading to Pops of40-60 percent over Charleston and
Berkeley County Saturday afternoon, 30-40 Pops for the rest of South
Carolina, and mainly 20 Pops over southeast Georgia. A similar
probability distribution will be required for Saturday Night,
although the expansion of 50% to 60% chances could expand a little
further west to include Dorchester and Colleton. Good chance to
occasional likely Pops will persist north/northwest zones to
isolated/low end chance Pop south/southeast into Monday as the low
mills around nearby. Adjustments will be required in future
updates/forecasts as trends become more defined.

Due to the small and likely compact nature to the system there will
likely only be some occasional convective bands that contain t-
storms, so we are showing no more than chance t-storms through the
period.

Temps: To be held to a small diurnal range north and northeast due
to an increase in both cloud cover and rain chances, and a bigger
range south/southwest due to better insolation during the day and
less cloud cover at night.

Winds: The risk for Tropical Storm force winds will be highest
Saturday Night into Sunday as the depression approaches the
coast. the areas most likely to experience Tropical Storm winds
with gusts up to 40 mph, will be coastal SC, including the
Charleston metro. Winds of this magnitude may bring down a few
trees and powerlines. Some power outages are possible.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Forecast for the work week continues to be quite uncertain as it
remains highly dependent on the evolution of a low pressure system
that will impact the Southeast. A couple models are more progressive
in lifting the low northeast away from the area quickly leaving a
rather benign weather pattern mid week, while others want to keep it
spinning in the vicinity before eventually dissipating. Have
maintained rain chances in the forecast through the period.

&&

.AVIATION /05Z SATURDAY THROUGH WEDNESDAY/...
KCHS: VFR overnight. Could see cigs dropping to mvfr this
afternoon or evening, as Tropical Depression 2 approaches.
Showers associated with very out fringes of the system may develop
in the vicinity by mid to late morning, but greater chances still
appear to be in the late afternoon and evening hours. diurnal
enhancements including the sea breeze will result in some wind
gusts near 20 kt this afternoon. Lighter winds this evening as
the boundary layer decouples a bit.

KSAV: VFR conditions will likely persist. Deeper moisture is
forecast to remain north of the terminal. There may be a few
spotty light showers around the area this afternoon and evening
but potential no enough to add any vchs to the KSAV terminal
forecast.

EXTENDED AVIATION OUTLOOK...Chances for at least periodic flight
restrictions this weekend into early next week, especially at KCHS
as the tropical system approaches from the southeast and moves
into the immediate vicinity on Sunday.

&&

.MARINE...
Overnight: Tropical Depression 2 northeast of the Bahamas and
sprawling high pressure from Bermuda to the mid- Atlantic states
will maintain east to northeast winds through tonight. The
gradient tightens up later tonight as the low moves closer with
wind speeds increasing to 10-15 kt late. Seas will be on a slow
uptick, building to 3 to 4 ft tonight.

Saturday through Wednesday: The marine forecast is moderate
confidence, and it is highly dependent on evolution of the
depression over the Southwest Atlantic. Winds and seas will slowly
begin to climb on Saturday, but will remain below any Tropical Storm
conditions. Then conditions will deteriorate Saturday Night, with
the worst conditions to persist into Sunday as the low approaches
the Charleston County coast, before eventually moving onshore.
Tropical Storm Conditions are expected during this time with winds
as high as near 35 or 40 knots.

As a result of these conditions we have added a Tropical Storm
Warning for all Atlantic waters, including Charleston Harbor, except
for AMZ354. Winds and seas over those waters could at least approach
Advisory thresholds, if not marginal Tropical Storm conditions. For
now there is enough uncertainty to not include any headlines for the
0-20 nm waters off the GA coast.

For early next week the system could either lift northeast or remain
spinning in the vicinity. Given this uncertainty, significant
changes should be expected in future forecast packages.

Rip Currents: Given that it`s the Memorial Day weekend, with the
approach of Tropical Depression Two and small swells, we have gone
with a moderate risk of rip currents at area beaches for Saturday.
An elevated risk may again occur Sunday due to the proximity of the
system and small swells.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Tropical Storm Warning for SCZ045-047>052.
MARINE...Tropical Storm Warning for AMZ330-350-352-374.
&&

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



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