Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 220134

Area Forecast Discussion
National Weather Service Charleston SC
934 PM EDT Mon Aug 21 2017

High pressure will extend across the area Tuesday and Wednesday
with a Piedmont trough developing inland. A stronger cold front
is expected to pass over the region on Thursday. A wave of low
pressure could form on the front well offshore Friday. Slightly
cooler and drier conditions are then expected into early next


Convection has ended across the entire area. Have kept trend of
gradually bringing back pops, first over the Atlantic, then
toward the coast late tonight and early Tuesday morning. Given
the relatively widespread convection/rainfall earlier today,
think the chance for at least patchy fog has increased. Many
areas are already at their dewpoint temperatures. Have slightly
increased the area of patchy fog to include much of the land
area by 08z tonight. As mentioned before, temperatures are
expected to remain steady of fall slightly into the mid 70s across
most locations, except around 80 close to the coast.


Tuesday: Broad ridging and associated surface high pressure
will remain over the forecast area Tuesday but will begin to
weaken and shift south and east as a longwave trough moves east
towards the Appalachians. Locally, models are in reasonable
agreement that showers and thunderstorms should initiate
primarily due to the seabreeze before expanding inland. High
temperatures are expected to range from the upper 80s across the
beaches to the low to mid 90s across the inland counties.

Wednesday: The upper level trough will attempt to phase lock
with surface low pressure over eastern Canada, with resultant
strengthening causing the trough to extend south and bring a
surface cold front towards the forecast area. Lowering heights
suggest another day with fair coverage of showers and
thunderstorms, probably better coverage than Tuesday.
Temperatures are expected to be nearly the same if not a degree
or so warmer.

Thursday: The surface cold front is expected to cross through
the forecast area during the day Thursday. Forcing provided by
the front, combined with ample moisture and widespread weak to
moderate instability suggests the precipitation coverage will be
greatest during this period. High temperatures should range
around 90 degrees for most areas Thursday afternoon before a
slightly cooler overnight period relative to previous days under
weak cold air advection.


Friday through Monday: High pressure sourced from Canada will
remain centered well north of the region through the rest of the
week. Confidence in the day 5 through 7 forecast remains low.
Medium range guidance indicates that the sfc high center will
shift over New England, ridging SW across the Carolinas into GA.
In addition, both the GFS and ECMWF show a coastal low
developing off the GA/SC coast by late this week, with run to
run trends shifting east. Guidance supports prolonged NE winds
across the CWA, yielding afternoon dewpoints in the upper 60s
inland by the weekend. I will favor the marine and near shore
zones for Chc PoPs through much of the day 4 through 6 period. I
will keep the forecast dry for day 7, indicating the best
chance for drier air to arrive on the west side of the departing
coastal low.


Tonight: Expect VFR to prevail both sites. Once again, meso-
scale models indicating the threat for isolated to scattered
showers or even a thunderstorm to possibly approach both sites
later/after 08z. Confidence not high enough to put in TAF, but
will continue to monitor. Guidance also indicating the potential
for some light/patchy fog. Have kept mention in for KSAV, and
may need to add for KCHS.

Extended Aviation Outlook: Brief restrictions are possible due
to direct impacts from showers/thunderstorms, most probable
during the afternoon and early evening.


Tonight: No highlights or significant updates this evening.
High pressure to the east will keep light winds of 10 knots or
less and seas 2 to 3 feet or less. Expect scattered showers and
thunderstorms to redevelop later tonight and early Tuesday

Tuesday through Thursday: Atlantic high pressure with a weak
surface pressure gradient will persist over the coastal waters
into mid- week. A cold front is expected to sweep across the
forecast area on Thursday. During this period, winds will
generally range from SE to SW with speeds 5 to 15 knots, with
some gusts to near 20 knots. Seas will range 2 to 4 feet.

Friday through Saturday: Large high pressure centered over the
Great Lakes will build over the region Friday into Saturday. In
addition, a coastal low may develop off the GA and SC coast by
Friday and may slowly depart on Saturday. Winds should shift
from the NE, increasing to 10 to 15 kts by Friday afternoon.
Four foot wave are forecast across portions of AMZ350 and
possibly beyond 20 nm offshore. On Saturday, NE winds are
forecast to strengthen to 15 to 20 kts, gusts between 20 to 25
kts. Wave heights are expected to range from 4 to 6 feet with 20
NM, reaching 7 feet across AMZ374.

Rip Currents...The combination of swell, onshore flow, and
astronomical influences will lead to an enhanced environment for
rip currents through at least Tuesday, so the Moderate Risk has
been extended through tomorrow.


Rest of tonight: Tide levels made it to 6.95 feet MLLW, just
shy of the threshold for minor coastal flooding of 7 feet MLLW.
Therefore, have discontinued the Coastal Flood Advisory for the
SC coast this evening.

Astronomical influences and onshore winds (albeit light) will
continue to produce elevated tides through Tuesday. Only small
tidal departures are necessary to reach shallow coastal flooding
levels with the evening high tides along parts of the SC coast.
A Coastal Flood Advisory has been issued for the SC coast
Tuesday evening. For now it looks like conditions will stay just
below any advisory thresholds on the GA coast and in Jasper


The KCLX radar continues to be down due to lighting strikes.
Parts are on order but the radar is not expected to return to
service until Tuesday afternoon at the earliest.

The temperature and dew point sensors at the Downtown
Charleston observation site (KCXM) could periodically fail.
Technicians plan on fixing the problem.




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