Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 212348
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
748 PM EDT Sat Oct 21 2017

.SYNOPSIS...
High pressure will shift off the Mid-Atlantic coast this
weekend. A cold front will move through the area Monday night
with stronger cold front pushing offshore Tuesday afternoon.
High pressure will then prevail into late next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Early this evening: The center of the large anticyclone
associated with the ridge aloft sits just to the east of the
forecast area. Satellite imagery shows plentiful high clouds
streaming into and around the ridge axis as well. The center of
the surface sits near the Delmarva peninsula and will continue
to retreat to the northeast overnight. Radar imagery shows a few
light returns over the coastal waters, but this is likely just
marine based stratocumulus that is being picked up by the radar.
However, we will see shower activity begin to take shape across
the coastal waters through the remainder of the night as
convergence develops within a subtle coastal trough. PoP`s
slowly increase and even begin to spread into the Georgia coast
by late tonight. No change to temperatures for the night which
should be a few degrees warmer than last night.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Sunday: High pressure will shift farther offshore of the Mid-
Atlantic coast as a strong cold front approaches from the west.
The moistening onshore flow could support isolated to perhaps
low-end scattered showers moving inland from off the Atlantic
Sunday afternoon, but model soundings show a pronounced
subsidence inversion induced by subtropical ridging aloft hold
for much of the day with the deepest moisture generally confined
to the boundary layer. This should limit shower coverage quite
a bit, especially inland from the coast. 20-30% pops look
reasonable during this time with no mention of thunderstorms
given the largely stable thermodynamic profiles. Highs will warm
into the lower 80s.

Sunday Night: The onshore flow will deepen Sunday Night into
early Monday as low-level jetting increases with the approach of
a powerful upper trough an its attendant cold front. Increasing
nocturnal warm air advection/isentropic assent should support a
general uptick in shower activity from midnight on, mainly
across Southeast Georgia where the lower condensation pressure
deficits will be found along a developing warm front. Pops will
range from 50% roughly along/south of the I-16 corridor to
20-30% along the I-26 corridor. Lows will range from the upper
60s across interior Southeast South Carolina to the mid 70s
along the immediate Georgia coast.

Monday and Monday Night: Rain chance will be on a rapid
increase Monday into Monday night as a cold front pushes
offshore. A warm front will quickly move north of the Santee
River by late morning accompanied by scattered to numerous
showers and possibly a tstm or two. This area of rain should
move north of the region with the warm front. Upper forcing will
quickly intensify Monday afternoon as the cold front`s
supporting upper trough becomes negatively tilted and the
200-300 hPa upper flow becomes increasing difluent. Increasing
multi-layer vertical velocities induced by strong DPVA coupled
with a deepening pre-frontal moisture channel (PWATS 1.75-2.00
inches) and some surface based instability will support a large
swath of measurable rainfall, initially beginning across the far
west Monday afternoon, pushing east Monday evening and ending
west- east Tuesday morning. There still remains some uncertainty
with the timing of the highest rain chances with the 17/12z GFS
running about 3-6 hours quicker than the ECMWF, so pops will be
capped at 80% for now, although some guidance, including the
NCEP NBM, is running closer to 90-100%. Am hesitant to go that
high this far out given these persistent timing differences.
Highs will warm to around 80 Monday before the onset of steadier
rains with lows Tuesday morning ranging from around 60 far
interior to the upper 60s along the beaches.

Tuesday: Isolated showers could be ongoing around sunrise
Tuesday morning at the coast, but dry weather will prevail
behind the initial cold front through the remainder of the day.
A reinforcing, secondary cold front will push offshore Tuesday
afternoon with 850 hPa temperatures progged to drop quickly
during the mid-late afternoon hours. Should still see highs
recover to the mid-upper 70s before the the onset of the
stronger cold air advection.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
The secondary cold front will by off the coast by sunset. A
broad upper trough envelopes the eastern U.S. Dry, but cooler
conditions will then prevail through Thursday before some
moderation occurs late in the week as high pressure quickly
shifts offshore allowing a milder return flow to set up. Some
uncertainty in the longer range model guidance implies lower
forecast confidence late next week but could see some rain
return to the area Saturday. Temperatures will be mostly below
normal through Thursday night before possibly rising closer to
or even above normal through the end of the week. Some inland
locales should get into the 40s each morning from Wednesday
through Friday.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
VFR conditions are expected at both KCHS and KSAV through 00Z
Monday. It`s not out of the question to see a period of MVFR
ceilings in the morning after sunrise but confidence is low so
it was not included in the forecast. Then Sunday afternoon will
likely bring showers into the vicinity at both sites, first at
KSAV and then later at KCHS. Added a VCSH for now as model
solutions are mixed on the coverage and intensity of shower
development.


Extended Aviation Outlook: MVFR to IFR conditions are possible
Sunday night and especially Monday showers impact the region
with a cold front.

&&

.MARINE...
This Evening and Tonight: The center of sfc high pressure north
of the area will shift east off the Mid-Atlantic Coast through
tonight. As this occurs, winds/seas will remain below Small
Craft Advisory levels for all waters early, but will remain
somewhat elevated due to a slight pinching of the gradient
associated with a weak coastal trough developing off the
Southeast Georgia coast. In general, east winds will peak
between 15-20 kts, highest in southern waters where the gradient
is tightest. Gusts up to 25 kt should creep into offshore
Georgia waters after midnight. Seas will also build, peaking
between 3-4 ft in nearshore waters and 5-6 ft in offshore
Georgia waters late. For this reason, a Small Craft Advisory has
been issued for the offshore Georgia waters starting at 2 AM
tonight.

Sunday through Thursday: Conditions will deteriorate a bit this
weekend as a cold front approaches from the west and likely
moves through Monday night. A stronger, secondary cold front
will move through Tuesday afternoon. A Small Craft Advisory will
likely be needed for the nearshore waters Monday into Monday
night as seas reach 6 ft in the southerly flow ahead of the
front. For the Georgia offshore waters, A Small Craft Advisory
is in effect through Tuesday morning for seas reaching 6 ft.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 2 AM Sunday to 11 AM EDT Tuesday for
     AMZ374.

&&

$$

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


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