Area Forecast Discussion
Issued by NWS Charleston, SC

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033
FXUS62 KCHS 251624
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1224 PM EDT Sun Jun 25 2017

.SYNOPSIS...
A cold front will gradually move into the area today, then linger
just offshore into Tuesday. High pressure will build in from
the northwest by mid week, before it moves into the Atlantic for
the second half of the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
There has been a quick onset to scattered showers and a few
t-storms near and west of US-301 across interior GA as a short
wave rides around the edge of the sub-tropical ridge and heads
E-NE. Expect an increase in coverage and intensity to showers
and t-storms through the afternoon, as we still have PoP`s as
high as 70-80%. And where we have 60% or greater we have
attempted to show rain-cooled conditions with the diurnal temp
curve.

A cold front will limp into the area from the NW this afternoon,
but struggling to do so as sub-tropical ridging persist across
Florida. There are little in the way of height falls until after
18, and it could take until mid or late afternoon before the
best forcing occurs due to the nearby proximity of the upper
level sub-tropical jet to our NW and associated upper difluence.
Coupled with considerable deep pooling of moisture throughout
the troposphere, we continue to forecast likely or definite
PoP`s in most places, except for good chance from Allendale to
Jenkins County. The highest chances look to be over the coastal
corridor, including Savannah, Beaufort and Charleston where the
W-SW storm motion bump up against the sea breeze boundary.

PWat is around 120-130% of normal and will support a heavy rain
threat. There is likely still enough storm motion to negate any
widespread flooding concerns, but minor to moderate flooding in
likely with rainfall rates of 1-2 inches an hour. We`re
fortunate that during any heavy rains this afternoon we`ll be in
the low tide cycle but Flood Advisories seem possible.

Any severe potential is extremely low, but given sufficient
CAPE in excess of 2500 J/kg, LI`s around -4 to -6C and 20-30 kt
of 0-6km bulk shear (mainly north) a few multi-cellular
clusters could form and produce 40 or 50 mph winds in wet
microbursts.

Before the onset of convection temperatures will top out in the
upper 80s and lower 90s most communities away from the beaches,
with even a few spots 92-94F near the I-95 corridor from
Savannah south.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
Tonight: Showers/tstms will gradually diminish overnight as
instability and upper level support wane. The stationary front
will continue to linger just inland from the coast through
daybreak with the leading edge of the lower dewpoints forecast
to remain just to the west and northwest across east-central
Georgia, the CSRA and Southern Midlands. 50-70% pops will be
held through the evening hours with pops lowering with time.
There will be a continued risk for at least minor flooding, but
a more substantial flood risk could develop for Downtown
Charleston if heavy rains fall during the elevated high tide
around 1008 pm. Lows will range from the lower 70s inland to the
mid-upper 70s at the beaches.

Monday and Tuesday: The main challenge in the short term is the
location of the cold front. All models agree that a long wave
trough will be over the eastern half of the country Monday and
Tuesday. They disagree on the location of the front. The front
will be wedged between Atlantic high pressure well offshore and
continental high pressure building in from the northwest. The
interaction between these features will drive the exact location
of the front, which will affect our local weather. The current
forecast has the front lingering just offshore Monday, then
shifting a bit further offshore Tuesday. Given the vicinity of
the front, chance POPs are in the forecast Monday, highest near
the coast tapering inland. Following a nocturnal decrease in
showers over land they should increase Tuesday afternoon, again
with the highest POPs being near the coast and tapering inland.
But with the front and it`s associated moisture and meager
instability further away from our area Tuesday, the overall
coverage of showers should be less. The severe threat will be
low both days because the best instability will be well
offshore, away from our area. Tuesday night shower activity will
decrease, especially as the front is pushed away by the
continental high building to the northwest. Cooler air
associated with the high will allow lows to drop into the mid
60s across far inland, the upper 60s most other locations, while
the immediate coast is in the lower 70s.

Wednesday: The center of high pressure will move from the Mid-
Atlantic States in the morning to offshore of the NC Outer Banks
by late in the day. This will allow southeast flow to develop
across our area late in the day. But beforehand, dew points will
be low for this time of year, generally the upper 50s far
inland to the mid/lower 60s elsewhere. This will make it feel
very comfortable outside, especially with mostly sunny skies and
high temperatures around 90 degrees.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The center of High pressure will move offshore of the NC Outer Banks
Wednesday night, and drift further east and out to sea through
Saturday. Southerly flow will develop across the Southeast during
this time period. Temperatures will trend back to normal and
moisture will increase, bringing a return to the typical summertime
shower/thunderstorm pattern. The risk of showers/storms appears to
increase each day into the weekend.

&&

.AVIATION /16Z SUNDAY THROUGH THURSDAY/...
The main concerns center on the timing of potential tstm
impacts at both KCHS and KSAV. Weak front is forecast to meander
into the region this morning and serve as the main focus for
shower/tstm to develop this afternoon. Confidence is increasing
that impacts will occur at both terminals, roughly in the mid-
late afternoon time frame. With increasing confidence based on
consistency in the various high resolution models, will show
prevailing TSRA at both sites with temporarily lower conditions.
Limited both cigs and vsbys to MVFR for now, but localized LIFR
and IFR conditions are possible in heavier convective elements.
The bulk of the activity should push east of the terminals by
00z.

Extended Aviation Outlook: Periods of flight restrictions are
expected Monday and maybe Tuesday due to a cold front lingering
offshore.

&&

.MARINE...
This afternoon: We`ll have quite a bit less wind than we
experienced last night and yesterday, as sea breeze
circulations back winds around to the S-SW at 15 kt or less.
Seas will average 2-4 ft. Mariners are advised that scattered to
numerous showers and t-storms will impact the waters the second
half of the afternoon into the evening, some of which will be
strong with heavy rains, reduced visibilities, lightning strikes
and higher winds and seas.

Tonight: South to southwest winds will diminishing to 5-10 kt,
except 10 kt over the Georgia offshore waters as a cold front
stalls just inland from the beaches. Numerous showers/tstms will
likely impact the waters with vsbys dropping to less than 1 nm
at times in locally heavy rainfall. Seas will average 1-2 ft
nearshore waters and 2-3 ft Georgia offshore waters.

Monday and Tuesday: The cold front should meander over or just E-SE
of the of the waters, held in position by Atlantic high pressure to
the E-SE and continental high pressure to the NW.
Models continues to show large variations over the location of the
front, so it`s hard to pin down the exact position. We went more
with a blend, weighted towards the GFS, which has the front further
offshore than the ECMWF. This gives is NE winds no more than 10 kt.
Seas will average 2-3 ft.

Wednesday and Thursday: Continental high pressure slides off the NC
coast, as its associated ridge axis orients itself back into the
southeast states. This sets up a more typical late June pattern with
Gentle to Moderate easterly breezes, and the long onshore fetch in
turn allows for seas to build another foot or two higher.

&&

.TIDES/COASTAL FLOODING...
Tide levels could approach 7.0 ft MLLW in the Charleston Harbor
with the evening high tide, while Fort Pulaski looks to remain
well below 9.2 ft MLLW. A low-end Coastal Flood Advisory may be
needed for the lower South Carolina coast, possibly only
including Charleston County.

The evening high tides will continue to be elevated through at
least Monday due to lingering astronomical influences. The
total departures will be driven by the position of the cold
front and the resulting winds that are either parallel to the
coast or onshore. Coastal Flood Advisories are still not out of
the question for the coastal areas from Charleston County to
Beaufort County.

&&

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

&&

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



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