Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 251419

Area Forecast Discussion
National Weather Service Charleston SC
1019 AM EDT Mon Sep 25 2017

Hurricane Maria will track northward off the Southeast U.S.
coast through midweek then will turn northeast and will move
away from the coast. A cold front will move through the area
from the northwest late this week, followed by high pressure
that will build into the region through next weekend.


Late this morning: No significant changes were needed to the
going forecast. Satellite imagery shows plentiful mid and high
level clouds streaming in from distant Hurricane Maria. Aloft,
southeasterly flow between the ridge north of Maria and the
trough across the deep south is helping to funnel this outflow
cloudiness from Maria into the Carolinas and portions of
Georgia. The 12z CHS RAOB shows a very dry atmosphere with
precipitable water just over an inch and plentiful dry air in
the mid levels. This will continue through the day and keep the
forecast dry. The main forecast issue revolves on the coverage
of clouds and potential impacts on temperatures. The forecast
continues to feature mid to upper 80s, with the coolest values
around the Tri-County region which will see the bulk of the
cloud cover.

Tonight: Maria will pass well of the coast, reaching near the
latitude to the distant east of the Santee River by Tuesday at
sunrise. There is still some outflow of cirrus overhead, but
soundings and cross- sections indicate that the coverage of
these clouds will be less than last night and during today. Some
of the MOS guidance supports late night stratus, but the SREF
probabilities are minimal enough where we don`t have any mention
of fog at this time. Most places will experience rapid
decoupling of low level winds in the evening, allowing for
adequate radiational cooling to support lows 66-71F inland from
US-17, 72-73F in downtown Charleston, near Lake Moultrie and
along many coastal sections.


Models are in good agreement that precipitation chances will
generally remain very low through mid-week under sufficient
subsidence as Maria passes the area offshore. Some isolated shower
or thunderstorm activity cannot be entirely ruled out Tuesday over
the coastal waters or even near coastal SC, but probabilities look
exceedingly low at this time, supported by GFS ensemble mean QPF
values of essentially 0 inches through at least 18Z Wednesday.
A warming trend with above normal temperatures will persist, with
highs in the mid to upper 80s Tuesday, but most locations reaching
lower 90s Wednesday and Thursday. Low temperatures will range from
the upper 60s to lower 70s through the period.


Chances for showers/thunderstorms will begin increasing late
Thursday into the weekend due to an approaching cold front. Guidance
continues to suggest that the front will complete passage through
the forecast area likely by late Friday or early Saturday. Cooler
high pressure will then begin filling in behind the front, with
early guidance suggesting a high pressure wedge-type pattern could
set up near or over the area Saturday into Sunday.


VFR with the chance for potential brief flight restrictions
through early afternoon at KCHS due to broken stratocumulus
and/or cumulus decks, and again in low stratus late in the valid
12Z TAF cycle.

Extended Aviation Outlook: Mainly VFR conditions are expected, with
low probabilities for morning restrictions due to fog, mainly
Wednesday and Thursday.


Today: The ongoing Small Craft Advisories for all waters will
remain in effect, as large seas (mainly in the form of E-SE
swells) continues to occur due to Hurricane Maria far to the E.
Both the NWPS and the WNA wave guidance is a tad high this
morning, so slight modifications have been made. But even so,
with some wind waves mixed in due to N-NE winds of 13-18 kt and
gusty, seas will reach as high as 7 to 9 ft within 20 nm of the
entire coast, and 10 or 11 ft on the outer GA waters. Winds in
Charleston harbor will be closer to 10 or 12 kt, but some of
that swell energy will impact the harbor entrance, with waves up
to 2 ft to occur.

Tonight: As the hurricane continues her northward movement,
winds and seas will slowly start to come down, enough so where
the Small Craft Advisory on the AMZ354 waters will drop off late
tonight. But seas will still be elevated on the other Atlantic
waters outside the Charleston Harbor, where Small Craft
Advisories will remain in effect.

Tuesday through Friday: A prolonged period of Small Craft Advisories
will continue due to high seas pushing into the waters from offshore
Hurricane Maria. Generally northerly winds will gradually back NW
Tuesday while slowly diminishing to 10-15 knots with some gusts to
near 20 knots. Latest guidance suggests seas will remain above Small
Craft Advisory criteria through Wednesday afternoon or evening, but
relatively benign sea conditions are anticipated Thursday into
Friday, with winds generally 5-10 knots and seas 2-4 ft. A cold
front followed by strong high pressure to the north will lead to
an enhanced onshore gradient Saturday, and winds and seas could
again be high enough where SCA`s are once again necessary.

Rip Currents: With large and powerful long period swells from
Hurricane Maria to occur, a High Risk for life-threatening rip
currents will persist. NO ONE should enter the surf due to
these extremely dangerous conditions. An elevated risk will
persist through at least mid week due to continued long period
swells from Maria.


Powerful E-SE swells originating from Hurricane Maria will
continue to impact the coast today, with the larger breakers
along the SC coast, where a High Surf Advisory remains in effect
for 5 or 6 foot breakers. While a few breakers of 5 feet can
reach the GA beaches, the majority of them will be of less
height, so no High Surf Advisory is required. The large surf
along with tides remaining well above normal will continue to
produce significant wave run-up, causing additional erosion
around the times of high tide today and continuing through early
this week.

Conditions will be very close to the requirement of a Coastal
Flood Advisory with the early afternoon high tide along much of
the SC coast, with minor salt water flooding likely. Similar
conditions will prevail again for the Tuesday early afternoon
high tide.


GA...High Risk for Rip Currents until 8 PM EDT this evening for GAZ117-
SC...High Risk for Rip Currents until 8 PM EDT this evening for
     High Surf Advisory until 8 PM EDT this evening for SCZ048>051.
MARINE...Small Craft Advisory until 8 PM EDT Tuesday for AMZ352.
     Small Craft Advisory until 4 PM EDT Wednesday for AMZ350-374.
     Small Craft Advisory until 5 AM EDT Tuesday for AMZ354.



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