Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 302347
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
747 PM EDT Sun Apr 30 2017

.SYNOPSIS...
Atlantic high pressure will give way to a cold front which will
move through Monday night into early Tuesday. High pressure
will then bring dry weather through Wednesday night before rain
chances return late in the week as a stronger storm system
affects the area. Drier high pressure should then return by the
end of the weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Early this evening: No changes needed to the going forecast.
Tonight will be quiet with warm and elevated southerly flow and
lows only falling into the low 70s. A few models show some light
shower activity developing closer to sunrise within a subtle
pre-frontal trough type feature, but maintained a dry forecast
for now. Low stratus is expected to develop late and mainly
along and west of I-95.


&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: Deep layered ridging will slowly retreat east over the
Atlantic as a large area of low pressure lifts northeast over
the Midwest. A southward extending cold front associated with
this low will make a run toward the Southeast United States,
likely losing some speed/strength as it becomes further
separated from the parent low. Ahead of the front, deep moisture
advection within a southerly flow along with increasing
isentropic ascent will help spawn some showers inland late
morning into early afternoon hours. Sfc temps should peak a
degree or two cooler than the previous day (low to mid 80s) due
to expanding cloud cover, but sfc heating should still be
sufficient to produce SBCAPE near 1000 J/kg well inland. These
values in combination with lifted index values around -6 C and
mid- lvl lapse rates near 6.5 C/km support afternoon
thunderstorms away from the coast. However, the bulk of precip
activity should come near or just ahead of the cold front as it
progresses through the area early Monday night. Given the timing
of the front, instability will be weaker, but 0-6 bulk shear
between 35-40 kt could help a few thunderstorms become organized
along/near fropa, resulting in strong and/or damaging straight-
line winds. At this time, the threat for greater severe weather
coverage is low. Most precip will end early Tuesday as the front
shifts offshore. Overnight lows will range in the mid/upper 50s
inland to lower 60s near the immediate coast.

Winds/Lake Winds: The pressure gradient will tighten over the
area ahead of an approaching cold front Monday, suggesting gusty
wind potential (near 30 mph) when afternoon heating promotes
mixing into 35-40 mph low-lvl winds fields. A Lake Wind Advisory
could eventually be needed late Monday morning into Monday
evening for gusts of 25 kt near the beaches of Lake Moultrie.

Tuesday and Wednesday: High pressure will build over the
Southeast behind the departing cold front offshore. Expect all
areas to be dry and temps to be slightly cooler on Tuesday,
followed by gradual warming on Wednesday ahead of the next
system progressing over the South Central United States. In
general, afternoon temps will peak in the lower 80s on Tuesday,
then middle 80s on Wednesday. Tuesday night lows will range in
the mid/upper 50s inland to lower 60s along the immediate coast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Mainly dry through Wednesday night before rain chances return
with the approach of a stronger storm system toward the end of
the week. Although there is a bit better agreement between the
GFS and ECMWF than 24 hours ago, there are still some timing
differences so forecast confidence is lower than normal.
Conditions look at least marginally favorable though for some
stronger storms Thursday. Rain chances should diminish by Friday
night but we kept small rain chances into Saturday given the
uncertainty regarding the . timing of the cold front and
strength/timing of the upper trough. Temperatures should be near
or above normal through at least Thursday night before likely
dropping below normal Friday through the weekend.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
The main near term concern is the potential for low stratus to
develop late tonight. The best chances are at KSAV which should
be within a sizable area of stratus that develops ahead of the
approaching cold front. Maintained the MVFR ceiling timing of
08z at KSAV. Chances are lower at KCHS which will likely be on
the fringe of the larger area of stratus. Chances are high
enough that MVFR ceilings will be at least in and out of the
terminal beginning around 11z that it has been introduced. Both
sites will then scatter out by mid morning with gusty southerly
winds prevailing through the rest of the day. Showers and
thunderstorms associated with the front could reach KSAV by the
last hour or two of the period, but have opted not to include
anything as of yet.

Extended Aviation Outlook: Ceiling and/or visibility
restrictions are likely Monday evening into Monday night due to
low clouds and showers/thunderstorms along/ahead of a passing
cold front. Breezy/gusty conditions are also expected Monday
afternoon ahead of the front. Another round of flight
restrictions is possible with showers/thunderstorms
Thursday/Thursday night, possibly lasting into Friday.
Breezy/gusty conditions are also expected Monday, Thursday and
Friday afternoons.

&&

.MARINE...
Tonight: Southerly winds will steadily increase tonight as pre-
frontal low-level jetting takes hold. Speeds are expected to
reach 15-20 kt by midnight and continue through the early
morning hours Monday. In Charleston Harbor, winds will remain 15
kt, but will be closer to 15-20 kt near the Harbor entrance.
Seas will also build, reaching 4-6 ft South Carolina nearshore
waters, 4-5 ft Georgia nearshore waters and 5-7 ft Georgia
offshore waters by sunrise Monday. Small Craft Advisories have
been posted for the South Carolina waters and Georgia offshore
waters for tonight and will remain in force.

Small Craft Advisory conditions will likely be ongoing for all
waters Monday into Monday night when winds gust near 25-30 kt
and seas build as high as 6-8 ft. Small Craft Advisories will
likely be needed for all waters until fropa nears/occurs Monday
night into early Tuesday. However, 6 ft seas could linger in
offshore Georgia waters through late Tuesday morning. Otherwise,
no significant issues are expected over the coastal waters
until Thursday when another cold front approaches. Much
uncertainty exists regarding the timing of the frontal passage,
but conditions look poor either way through Friday.

&&

.CLIMATE...
Observed minimums this morning indicate all three climate sites
should break their respective record high minimums for today.

Record high minimums for 30 April:
KCHS: 69/2014 and previous years.
KCXM: 73/1912.
KSAV: 72/1953 and previous years.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 8 AM Monday to midnight EDT Monday
     night for AMZ330-354.
     Small Craft Advisory from midnight tonight to 6 AM EDT Tuesday
     for AMZ352.
     Small Craft Advisory from midnight tonight to noon EDT Tuesday
     for AMZ350.
     Small Craft Advisory until noon EDT Tuesday for AMZ374.

&&

$$

NEAR TERM...BSH
SHORT TERM...DPB
LONG TERM...RJB
AVIATION...BSH/DPB
MARINE...ST/DPB
CLIMATE...



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