Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 212004
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
304 PM EST Tue Feb 21 2017

.SYNOPSIS...
Weak low pressure will pass to the south Wednesday into Thursday. A
cold front will sweep across the region Saturday, followed by a
strong high persisting into next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Sharp upper ridging along the Southeast U.S. coast will quickly
erode tonight as the area becomes increasingly influenced by an
approaching upper level cyclone. It appears the primary moisture
channel and associated shower activity along the low`s eastern
flank will remain displaced to the west for much of the night,
only approaching far interior Southeast Georgia around sunrise.
Expect mainly dry conditions to prevail with rain chances
increasing from the west and south after 3-4 am. Limited pops to
20% for areas roughly along/south of a Millhaven-Statesboro-
Richmond Hill line with non-mentionable pops elsewhere across
Southeast South Carolina and Southeast Georgia. Lows will range
from the mid 50s in the Allendale-Saint Georgie-Monks Corner
corridor to the lower 60s along the Georgia coast.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Wednesday: A closed low will track SE across the Gulf of Mexico,
centered over the FL Keys late at night. Across the forecast area,
mid level heights will feature a broad weakness, likely aiding the
formation of a weak vort max, which is forecasted to move over the
Savannah River Valley in the afternoon. POPs were kept mainly in the
20-30% range. A lack of instability and forcing should yield only
showers, no thunderstorms. Thicknesses support high temperatures
from the mid 70s across the SC Lowcountry to the upper 70s across SE
GA.

Thursday: A large low pressure system will organize over the central
Great Plains late in the day. A warm front associated with the low
will link with an old cold front across the mid Appalachians. The
large warm sector should provide the region with weak instability
and moderate low and mid level moisture. POPs remain in the slight
chance range across the land areas, higher over the offshore waters.
Temperatures should peak in the mid/upper 70s.

Friday: South-southeast winds will strengthen across the area as a
cold front pushes east across MS/AL/TN, especially during the
evening hours. Thickness values indicate that high temperatures will
reach the upper 70s near the coast to the low 80s inland.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The cold front is expected to remain east of the region by
daybreak Saturday. Min temps are forecast to remain very mild, with
near 60 common Sat AM. GFS shows the passage of the sfc cold front
during the daylight hours on Saturday. Afternoon temperatures should
warm well into the upper 70s. The combination warm temps and dwpts
in the 50s should yield CAPE values from 1000-1500 J/kg, especially
across the SC zones. Given the timing and instability, it appears
that the environment may yield a band of prefrontal convection. I
will mention both showers and thunderstorms with 20 PoPs.

Dry Canadian high pressure is expected to build quickly over the
region Saturday night and Sunday. Cooling thicknesses will yield
lower temperatures, but still remaining around 5 degrees above
normal. The coolest daytime temperatures will occur on Sunday, with
mid 60s across the Santee Basin to low 70s close to the Altamaha
River. Monday through Tuesday, both the GFS and ECMWF indicate that
the sfc high will shift over the western Atlantic, ridging west
below the Mason-Dixon line. To the west, another low pressure will
begin deepen across the southern Great Plains on Tuesday. This
pattern should provide a strengthening return flow during the early
week. High temperature are forecast to return to the mid to upper
70s by Tuesday.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
VFR for much of the period, although cigs may approach high-end
MVFR at times.

Extended Aviation Outlook: VFR.

&&

.MARINE...
Tonight: East to northeast flow-level flow will persist. Speeds
will remain 10-15 kt or lower with seas building 2-3 ft
nearshore and 3-4 ft offshore.

Wednesday through Sunday: Northeast winds will be common across the
marine zones from through Friday. A cold front will sweep across the
region late Saturday, shifting winds to the northwest. Cold air
advection will occur Saturday night into early Sunday, likely
resulting in gusty conditions. Seas are forecasted to build Thursday
and Friday, so Small Craft Advisories will probably be needed,
especially for the offshore GA waters.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$



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