Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 290542

Area Forecast Discussion
National Weather Service Charleston SC
142 AM EDT Sat Apr 29 2017

Early summer conditions will prevail over the weekend as strong
high pressure holds offshore. A cold front will cross the coast
early Tuesday bringing a chance of showers. A Gulf low pressure
system may bring a soaking rain late next into the early part
of next weekend.


As of 130 AM: Passing patches of high clouds will make forecasting the
placement and timing of low stratus challenging this morning.
Sfc observations and satellite data indicates that IFR stratus
and fog was common across the Lowcountry at 130 AM. I will
update the forecast to adjust sky timing and values.

Previous discussion:
A building ridge aloft and strengthening high pressure at the
surface and lower levels will encompass the area through the
night. A warm night for late April is expected with decent
mixing in the boundary layer, so no lower than the upper 60s
well inland and low to middle 70s along the coast to occur. In
fact we`ll likely set a record high minimum temp for April 28th
at both KCHS and KCXM.

Low stratus/stratocumulus over much of Charleston and far
eastern Berkeley counties will expand in coverage through the
night as isentropic ascent persists and a sharp inversion
develops around 800-1000 ft with considerable moisture
underneath. There is probably too much mixing within the
boundary layer for much more than shallow ground fog far inland
late, but due to some concern over sea fog, we have maintained
mention of patchy fog along the coastal zones where the lowest
condensation pressure deficits are situated.

Smoke from the West Mims wildfire in the Okefenokee Swamp has
been spreading north toward Tattnall and Long counties, but
there have been no signs of any reductions in surface
visibility, so far. Still, parts of SE GA could experience a
smell of smoke in the air tonight within the southerly flow.


As of 3 PM Friday: Highly amplified pattern creates hot
temperatures through the weekend ahead of a cold front which
will cross offshore Monday night.

590+ dm ridge will be centered just offshore Saturday before slowly
migrating eastward on Sunday. This ridging combined with surface
Bermuda-type high pressure will push 850mb temps up to 14-16C, which
combined with abundant sunshine thanks to exceedingly dry air aloft
will drive highs into the low 90s inland on Saturday, mid 80s at the
coast, but still only around 80 at the beaches due to a strong and
early-developed sea breeze. Temps on Sunday will be only a degree or
two cooler as the thermal ridge weakens slightly. While isolated sea
breeze showers are possible each aftn, the very dry air aloft will
cap potential, and will continue inherited silent POP Saturday, SCHC
on Sunday. As the cold front begins to encroach on the area late
Sunday, and especially on Monday, southerly winds will crank up as
well, keeping minimums well above seasonable norms, falling only to
a degree or two either side of 70 both Saturday and Sunday nights.

The cold front Monday will race east as a bowling ball upper low
rotates into the Great Lakes pushing the cold front off the coast by
Tuesday morning. Timing of FROPA is expected to be late Monday
night, but strong shear and moderate instability ahead of it will
create a good chance for tstms Monday aftn and Monday night.
Increased cloud cover and lowering heights will keep highs and lows
on Monday several degrees cooler than during the weekend.


A weakening cold front looks to push offshore Monday night followed
by generally drier/cooler weather through Wednesday before rain
chances return with the approach of another storm system. At this
point there is uncertainty regarding this system so forecast
confidence is lower than normal, especially later in the week.
Temperatures should be near or above normal, but could drop below
normal Friday if the storm departs by then.


Passing patches of high clouds will make forecasting the
placement and timing of low stratus challenging this morning.
Sfc observations and satellite data indicates that IFR stratus
and fog was common across the Lowcountry at 130 AM. KCHS should
see IFR ceilings redevelop by 7z, remaining through 14Z. KSAV
appears less exposed to stratus with thin high clouds drifting
over the terminal. However, I will indicate a period of MVFR
ceilings with a TEMPO from 7z-11z. Gusty south winds are
forecast to develop over the terminals this afternoon. Tonight,
stratus sourced from the shelf waters should drift over KCHS by
3z Sun. I will indicate BKN015 by 3z.

EXTENDED AVIATION OUTLOOK: Ceiling and/or vsby restrictions
will again be possible Monday into early Monday night due to
showers and/or thunderstorms. Also...breezy/gusty conditions
expected Monday.


Overnight: Strengthening high pressure will the Atlantic will
remain across the waters, resulting in S`erly winds around 12-17
kt, a little less in Charleston Harbor. Seas will hold fairly
steady throughout the night, averaging 3-4 ft.

While trajectories are not entirely favorable for sea fog, given
the elevated dew points in the lower and middle 70s plus pre-
existing and/or developing low stratus, we maintain mention of
patchy fog in the forecast. Some of the fog/stratus could move
into the Charleston Harbor at times, and we will maintain a
close weather watch for possible dense fog.

Saturday through Wednesday: High pressure offshore will be the
dominant feature through the weekend before a cold front
approaches the waters late Monday. Between these two features,
southerly winds will gradually increase during the weekend,
rising from 10-15 kts Saturday, to 15-20 kts by late Sunday. As
the cold front approaches during Monday, winds will veer more to
the SW and increase further to 20-25 kts, and an SCA will
likely be needed as seas build from 2- 4 ft Saturday, to as high
as 5-7 ft on Monday, with some 8 ft seas possible in the outer
portion of the GA waters. For Charleston Harbor, winds are
forecast to also go above SCA thresholds, primarily late Monday
immediately ahead of the FROPA.


ABased on recent departures, we have cancelled the Coastal Flood
Advisory from Jasper and Chatham counties south to McIntosh, as
tides will not reach the required 9.2 ft MLLW level with the
next high tide. We did hold onto the advisory though for
Charleston, Colleton and Beaufort counties, as tides in
Charleston Harbor will reach as high as 7.2 ft MLLW with the
latest high tide cycle.




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