Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 212325
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
625 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/...
Isentropic lift across the central and southern zones has
increased and radar trends of late are showing isolated showers
developing. While much of this activity does not appear to be
capable of measuring, we have upper the PoP`s to 20-30% south
and west for tonight, but keeping the Charleston quad-county
district rainfree for now under the last effects of mid and
upper ridging those areas.

Otherwise skies will be mostly cloudy or overcast within a moist
E-SE synoptic flow around high pressure offshore and low
pressure to the W and SW in association with an abnormally deep
mid and upper low near Mobile Bay.

It`ll be yet another warm night more like spring rather than
winter, with lows to range from the mid 50s in the Allendale-
Saint Georgie-Moncks Corner corridor to the lower 60s along the
Georgia coast.

Not sure if there will be enough stratus late tonight to require
adding fog to the forecast, but at least a minor reduction in
visibilities are possible.

&&

.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
dewpts 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 /00Z WEDNESDAY THROUGH SUNDAY/...
A moist onshore flow from off the Atlantic between high pressure
to the north and low pressure to the W-SW will produce low-end
VFR to high-end MVFR ceilings tonight into Wednesday. While
there could certainly be a few showers, especially at KSAV
within this pattern, any direct impact are difficult to pinpoint
at this time. But brief periodic flight restrictions can occur
within any shower activity.

Extended Aviation Outlook: VFR.

&&

.MARINE...
Tonight: A 1027 mb high centered S-SE of Long island will weaken
as it drifts E, as the local area lies near the SW periphery of
the associated ridge and downstream from low pressure in the
northern Gulf of Mexico and another weaker low in the TN valley.
As a result winds will generally remain E-NE at or below 15 kt,
except veering more SE on the outer GA waters late this evening
and overnight.

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.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...


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