Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 010250

Area Forecast Discussion
National Weather Service Charleston SC
950 PM EST Sun Feb 28 2021

A cold front will push offshore Monday night. Low pressure will
impact the region Tuesday into Tuesday night before moving east
of the region Wednesday. High pressure will prevail for the end
of the week.


Late this evening: No change to the forecast thinking through
sunrise. Previous discussion continues below.

Early this evening: Another very warm afternoon as passed and we
are left with very mild temperatures within an elevated
southwest flow this evening. Overnight, not a whole lot of
change to the current pattern. A cold front will begin to
approach from the northwest as sunrise approaches but the
forecast area will solidly remain in the southwesterly flow
within the warm sector through the overnight. In fact, the
pressure gradient will steadily tighten through the night and
support elevated flow. This will help keep temperatures quite
mild for lows, and will also have implications for any fog
potential. Overall, the low-level wind field should prevent
much, if any, fog from developing. This includes locations along
the coast as the marine fog environment is also not very
conducive to development. The forecast is dry and lows should
fall in the low to mid 60s.


Monday: The brief stint of early summer weather will come to an
end Monday as a cold front pushes offshore as Atlantic high
pressure slowly gives way. Strong subtropical ridging that has
been in place for the past few days will finally begin to
retreat as a powerful phased longwave across the central Rockies
decouples into two impulses; one piece heading into the Great
Lakes with the second heading into north Texas. The warm, moist
flow around the anticyclone offshore will continue to advect mid
60 dewpoints and PWATS in excess of 1" into the region, but
model cross sections still depict little in the way of deep-
layered forcing for ascent ahead of the front with only some
weak UVVs noted along the frontal surface itself. This suggests
the band of rain associated with the front will gradually weaken
and shrink with time as it drops to Southeast South Carolina
and Southeast Georgia. Pops will be lowered slightly to 50-70%
as a result which matches statistical guidance trends. The
system continues to exhibit a split front structure with the
surface front outrunning the 850 hPa front by 6-8 hours. Even as
the surface front pushes offshore by late afternoon, a risk for
showers will persist overnight as the 850 hPa front settles in
around the Savannah River by daybreak Tuesday. Chance pops were
maintained during this time. Highs will be closely tied to FROPA
with clouds and rain likely helping to keep temperatures down a
bit even with a very warm start. Highs will range from the
upper 70s/near 80, except lower 80s in the KSAV-KLHW-KJES-KBQK
corridor with mid 60s at the beaches. Lows will range from the
mid 40s across northern Dorchester and Berkeley Counties to the
lower 80s across coastal Georgia.

Tuesday: A potent southern stream shortwave will move across
the Deep South on Tuesday inducing cyclogenesis across the Gulf
of Mexico, which will in-turn redevelop of the Georgia/South
Carolina coasts early Wednesday as cold air damming strengthens
across the interior. The combination of strong quasi-geostrophic
(QG) forcing coupled with a modest 40-45 kt LLJ advecting PWATS
1.25-1.50" in from the southwest should support widespread
rains developing from southwest-northeast through the day with
rainfall rates peaking Tuesday night as the secondary low
redevelops just offshore. Pops 90- 100% look justified and
rainfall amounts have been nudged up, especially Tuesday night
into early Wednesday. Widespread flooding is not anticipated
even with the wet ground conditions, but some minor
flooding/ponding of water is likely, especially in low-lying and
poor drainage areas. Tides will also have to be watched for the
late evening high tide cycle for possible drainage impacts for
Downtown Charleston.

The coldest temperature guidance members (NBM and CONSRaw) were
favored to construct high temperatures within a strengthening
area of cold air damming. Highs will range from the lower 50s
inland to around 60 in Darien, GA, but these may prove too warm,
especially across the interior where lower dewpoints could
intensify diabatic cooling as rain begins to fall. In fact,
highs may occur at different times during the day with readings
falling in the afternoon as rain spreads in and diabatic cooling
takes hold. Lows Tuesday Night will range from the lower 40s
well inland to the lower 50s along the coast, but temperatures
could rise overnight along the coast if the track of the
secondary surface low tracks closer to the coast than what is
expected. Finally, breezy conditions will occur across the
coastal counties and especially at the beaches where a pinched
gradient will reside. Could see gusts 30-35 mph in these areas,
highest in open areas and along the beaches. These values should
remain below Wind Advisory thresholds, but given the wet ground
conditions, a tree or two could be downed by gusty winds.

Wednesday: Low pressure will be found near the middle South
Carolina coast at daybreak Wednesday as a powerful southern
stream shortwave traverses the area and eventually offshore.
Widespread rains will slowly end from west-east during the day,
potentially ending by by afternoon for all but the Charleston
Tri-County area where some wrap around showers caught in the
cyclonic flow around the low could hold a tad longer. Will once
again have to watch high tide late Wednesday morning for
possible tidal influences on drainage in Downtown Charleston and
along other portions of the lower South Carolina coast. Skies
will be slow to clear, but some breaks are likely by afternoon.
Dry and mostly clear conditions will prevail Wednesday night.
Highs will range from the upper 50s inland to the mid 60s in the
Darien area with lows from the lower 40s inland to the lower
50s at the coast.


Mainly quiet conditions will prevail through much of the
extended as high pressure prevails. Weak low pressure could
develop off the Southeast U.S. coast Saturday into Sunday, but
the details of any potential impacts with a system just brushing
the area are difficult to pin down this far out. Generally
favored the drier NBM solutions with only some very minor
adjustments in a number of parameters. Temperatures will average
near to slightly below normal. There will be a increased risk
for some degree of frost over the upcoming weekend.


VFR conditions are expected to prevail at KCHS and KSAV through
00z Tuesday. Winds overnight should be enough to prevent much
(if any) fog development and stratus does not appear to be a
significant concern. winds will be gusty Monday morning ahead of
an approaching front, and showers could move through the
terminals in the late morning and early afternoon hours.
However, no ceiling or visibility restrictions are anticipated.

Extended Aviation Outlook: Flight restrictions are more likely
later Tuesday into Wednesday as a more potent storm system
impacts the area. Low-level wind shear could also be a concern
Tuesday night into early Wednesday. VFR should then return by
Thursday and generally persist through Friday.


Tonight: Southwest flow will prevail and should steadily
strengthen through the night as the gradient tightens ahead of
an upstream cold front. Winds should top out in the 15-20 knot
range across most of the waters, though wind gusts to 25 knots
will be possible in the Charleston County waters and the outer
Georgia waters. Seas will average 2-4 feet across the nearshore
waters, though up to 5 feet in the Charleston County waters. In
the outer Georgia waters, seas up to 6 feet will be possible,
especially out towards the 60 nm line. Small Craft Advisories
are in effect for the Charleston County waters and the outer
Georgia waters as a result. Concerning fog, area web cams and
satellite imagery show that there is little if any fog across
the waters. The Charleston Harbor pilot boat dispatch did report
some patchy fog in the Charleston County waters, but that
appears to have advected northward. As such, the Dense Fog
Advisory has been allowed to expire. We still carry patchy fog
for a few more hours, but winds are not expected to be conducive
to fog development overnight.

Monday: Southwest winds 15-20 kt will prevail head of a cold
front as low-level jetting moves across the coastal waters. Seas
could briefly reach 6 ft with 25 kt gusts in the outer portions
of the Georgia offshore and Charleston nearshore legs and the
combination of winds and seas will necessitate a Small Craft
Advisory through the afternoon. Winds will diminish from
southwest-northeast during the afternoon the low-level jet moves
east of the area and the cold front approaches the waters.
FROPA should occur during the evening hours with winds turning
northwest to north in its wake. Seas will average 2-4 ft, except
4-6 ft in the Charleston and Georgia offshore waters. Some
patchy sea fog could linger across portions of the waters in the
morning before dissipating in the afternoon as winds peak ahead
of the front.

Tuesday: Pinched gradient conditions will develop as cold air
damming strengthening across the interior and low pressure
eventually forms offshore overnight. Expect northeast winds
20-25 kt with gusts to 30 kt to prevail with seas building 4-6
ft nearshore waters and 5-7 ft offshore waters. Widespread rain
will fill in across the waters during the day and persist

Wednesday through Friday: Expect improving conditions Wednesday
as low pressure pulls away from the region. There are no
concerns after Wednesday with winds and seas holding below
advisory thresholds.


Minor coastal flooding is possible during the Tuesday late
morning high tide near Charleston and along the entire southern
SC/northern GA coast during the evening high tide. There is even
an outside chance of moderate coastal flooding near Charleston
during the evening high tide. In addition, heavy rainfall will
be possible during the evening high tide which could exacerbate
any flooding issues.


MARINE...Small Craft Advisory until 5 PM EST Monday for AMZ350-374.



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