Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 242039

Area Forecast Discussion
National Weather Service Charleston SC
339 PM EST Sat Feb 24 2018

Unseasonably warm high pressure will prevail through Sunday. A
cold front will move into the region Sunday night and Monday,
pushing south of the area Monday night as high pressure builds
from the north. High pressure is then expected until a warm
front moves through Wednesday or Wednesday night, followed by
another cold front late next week.


High pressure will prevail with another dry and mild night on tap.
Low-level moisture will likely be sufficient for some low clouds/fog to
develop but winds are not ideal for fog, especially for dense fog. The
best chance of such appears to be southwest of the forecast area farther
into Georgia. Of course a potential fly in the ointment here would be any
sea fog moving inland from the Atlantic. For now we don`t see any hint of
sea fog and just maintained mention of patchy fog for mainly after midnight.
Low temperatures will be above normal near 60, close to record high


Sunday and Sunday night: Strong deep layered ridging will hold firm,
weakening slightly at night as mid level perturbations knock down
heights a little as they advance within the W-SW flow aloft, and a
cold front aligned mostly parallel to the flow aloft approaches from
the W-NW. It`ll be another abnormally warm February day, and with H8
temps of 1 to almost 2 standard deviations above normal (12-14C), we
should easily exceed record highs for the date. In fact we might
even approach the warmest on record for February, especially at KSAV
if we obtain enough insolation. Sufficient mixing will allow for S-
SW breezes during the heating of the day, with gusts of 20-25 mph at
times. In advance of the cold front at night temps will struggle to
drop to even the lower 60s, or about 15-20F above the normal low.

Regarding our chances for rainfall, the better forcing and dynamics
remain off to the NW during the day Sunday, and with main moisture
axis also outside the CWFA, we have lowered PoP a bit from the
earlier forecast. For Sunday we have PoPs capped at 30-40% in the
afternoon along the NW tier, 20% or less most elsewhere, including
the Charleston, Beaufort and Savannah areas. A few coastal showers
could skirt parts of SC in the morning, but nothing more than 14%
probabilities. We do increase chances a bit at night as the front
oozes closer, there are some impulses aloft and moisture is a little
better. Instability is paltry and lapse rates are poor, so
confidence on any thunder is low through the period. But since there
was already thunder mentioned in the forecast we have maintained it,
just shrunk the slight chances to late day and evening far inland.

Monday and Monday night: The cold front makes slow progress through
the forecast district, as a a stronger short wave passing by to the
north late in the day and off the mid-Atlantic coast at night. The
orientation of the front is still generally parallel to the flow
aloft, so this transition will not be abrupt, but does become more
noticeable during Monday night as strengthening high pressure builds
from the OH valley, PA and the Virginia`s. Deeper moisture and PWat
in excess of 1.5 inches or 2 to almost 3 standard deviations above
normal, plus larger forcing from a coupling upper jet structure in
the southern Appalachians suggests an even greater potential for
showers. For now we have 50-70% chances Monday, but locally higher
PoPs are possible as trends become better defined. There is actually
a little better instability than on Sunday, so we do have a slight
chance of thunder to the south and east. Rain chances drop off from
N/NW to S/SE at night as the cold front finally works its way south
of the area and drier mid level air moves in. Monday will again be
far above normal, but due to the higher rain chances and lower
heights aloft it`ll be on average about 6-12F lower than on Sunday.
Cool advection at night will be noticeable as the synoptic flow
clocks around to the NE and speeds increase along the coast. Lows
will be at their lowest since mid month.

Tuesday: Flat ridging or a zonal flow develops aloft as surface high
pressure advances toward the mid-Atlantic coast. A weak coastal
trough will form offshore, maybe generating a few light showers
onshore of coastal SE GA, but otherwise large scale subsidence will
supply us with a rainfree day. Onshore low level trajectories will
maintain a feed of maritime moisture, and this poses our problem of
the day in regards to cloud cover and the resulting high temps. We
show more sunshine north and a greater amount of clouds south, with
temps mainly within a couple of degrees either side of 70F with both
H8 and low level thickness above normal.


High pressure to our north will move offshore Tuesday night. A weak
warm front may develop over the area Wednesday, quickly moving north
Wednesday night. A cold front will then approach from the west,
moving through Thursday or Friday. Highest chances of reductions will
likely be closer to 12Z but given overall low confidence we maintained
VFR conditions for now.


VFR will prevail through this evening as the sea breeze clears the
terms. Winds could gust near 20 kt at times. The risk for
low clouds/fog return tonight, mainly after midnight. With increasing
low-level flow it looks like low clouds are generally favored over
fog, although if sea fog develops and moves inland all bets are off. For
now we have maintained VFR conditions given low confidence.

Extended Aviation Outlook: Flight restrictions are expected with a
slow moving cold front late Sunday night into early Tuesday.


Tonight: Atlantic high pressure will continue offshore
and maintain a mostly SW wind across the local waters, mainly 15 kt
or less. Seas will continue elevated given the SE swell but stay just
below Advisory levels across far eastern portions of the offshore waters.
Could see sea fog develop just about any time given the more favorable
southerly winds, mainly across the cooler nearshore waters.

Sunday and Sunday night: The coastal waters will lie on the western
portion of strong sub-tropical ridging, blocking an upstream cold
front from reaching the area during this time. Warm advection will
temper the amount of mixing, so S-SW winds will generally hold at or
below 15 kt, with seas at best 3-5 ft.

Sea fog will exist in some form through the period with favorable
trajectories, except speeds might be a little too high. Thus we have
limited the coverage to "patchy" fog as of this time. If winds are
less than coverage could be greater and dense fog would be a
concern. We will maintain mention in the Hazardous Weather Outlook.

Monday through Tuesday: The cold front will gradually advance
through the waters, passing to the south of the area late Monday
night, as continental high pressure from the north into Tuesday.
Cool advection, steady isallobaric pressure rises and modest
pinching will likely support Small Craft Advisories developing
Monday night and continuing into Tuesday. Some fog will persist in
advance of the front, more as stratus clouds behind the front.

Wednesday through Thursday: Surface high pressure slides into the
Atlantic and further offshore, allowing for a cold front to approach
the SE late in the period. Winds and seas will be beneath advisory
levels Wednesday, but a much stronger S-SW gradient will exist on
Thursday, and SCA`s could again be common.


Record highs for February 24:
KCHS: 81/2017 *broken today*
KSAV: 86/2012
KCXM: 81/1930

Record high minimums for February 25:
KCHS: 62/1992
KSAV: 63/1992
KCXM: 61/2017

Record highs for February 25:
KCHS: 81/2017
KSAV: 82/1985
KCXM: 80/1930

Record high minimums for February 26:
KCHS: 62/1939
KSAV: 63/1939
KCXM: 62/1957 and previous

Record highs for the month of February:
KCHS: 87/February 16, 1989
KSAV: 86/last set February 24, 2012
KCXM: 83/February 27, 1962




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