Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 212226

Area Forecast Discussion
National Weather Service Charleston SC
626 PM EDT Mon Oct 21 2019

A cold front will approach from the west late tonight and move
off the coast late Tuesday. High pressure will rebuild Tuesday
night through Thursday before another storm system potentially
moves in Friday or Saturday.


Overall, the forecast is in good shape. 21/21z surface analysis
placed a sharpening warm front across central Florida with
weakening high pressure noted along the Carolina coast. The
front will gradually lift north across Southeast South Carolina
and Southeast Georgia tonight as the high moves offshore and a
cold front approaches from the west. Isentropic assent/warm air
advection will slowly increase with time, first producing a
large area of low/mid-level cloudiness advecting in from the
south followed by isolated to scattered showers later. Most of
the lift looks to remain rooted along the 305K surface for much
of the night with more dynamic, deep-layered forcing associated
with the upper trough to the west not arriving until after
daybreak. Much of the high resolution guidance is similar in
showing an area of convection developing over the coastal waters
in the vicinity of the warm front and along the nose of the
strongest warm air advection. With most of the lift being
primary driven by warm air advection and not by strong,
kinematic forcing, it is unclear how widespread this activity
will become. The H3R and RAP are both not overly aggressive in
the coverage of convection during the overnight period, so pops
have been lowered by about 10% across the board to show this
trend. The best chance for measurable rainfall looks to occur
from the Savannah Metro Area north along the lower South
Carolina, including the Charleston Metro Area.

Short term soundings show weak instability developing in the
vicinity of the warm front with as much as 50 kt of bulk shear.
Although instability and forcing are weak, there looks to be
enough to support at least a few tstms. There will be a non-
zero chance for an isolated tornado after midnight mainly along
the lower South Carolina coast where weak instability and shear
will maximize along the northward propagating warm front. Noted
the latest RAP SigTor values in this area are forecast to be
around 1 unit. Nocturnal tornadoes can be especially deadly. An
isolated damaging wind event can also not be completely ruled

Temperatures tonight will exhibit an atypical thermal curve
with values rising early Tuesday with the passage of the warm
front. Lows will range from the mid 60s in the area bounded by
I-26 and US-17 to the upper 60s-lower 70s elsewhere.
Temperatures are forecast be in the lower-mid 70s by daybreak


Tuesday: The base of a deep eastern U.S. upper trough is forecast
to shift through the Tennessee and Ohio Valley regions. A cold front
at the surface is expected to move into our inland zones to near the
I-95 corridor during the early afternoon hours. The window for
potential destabilization ahead of the front appears to be short and
rather limited due to the progressive nature of the front. Surface
dew points are expected to fall after mid afternoon as the front
shifts to the coast. Prior to fropa, forecast soundings along zones
along and east of I-95 show surface based CAPEs 1000-1500 J/KG along
with Pwats near 2 inches along with bulk shear of 30 kt and rather
unidirectional deep layer flow by early afternoon. There appears to
be a brief potential for isolated strong to severe tstms just ahead
of the cold front over coastal SC, especially along the Charleston
Tri-County in the 17Z-20Z time frame. The overall risk for severe
weather otherwise looks rather low given the timing of the front
into the I-95 corridor by 18Z. We maintained high chance POPs all
areas as convective rains will likely remained scattered. Temps will
be warm in the lower to mid 80s with frontal compression and periods
of insolation.

Wednesday and Thursday: High pressure and dry weather will return to
the region. It will be cooler on Wednesday with highs in the mid 70s.
We also lowered temps Wednesday night as strong radiational cooling
likely to send temps into the mid to upper 40s northwest tier to near
50 at both Charleston and Savannah. Northeast flow on Thursday should
keep afternoon temps in the mid 70s most areas once again with a few
high clouds around and some increasing coastal stratocumulus south of
the Savannah River.


An upper level trough will develop to the west late in the
week. A frontal system is expected to impact the area Friday
into Sunday, likely bringing periods of showers though the
heaviest rain may remain just inland.


MVFR cigs will gradually fill in from the south this evening as
a warm front sharpens over Florida. Cigs could briefly rise to
VFR at KSAV with its passage around 08z, but at least periods of
MVFR should persist. Isolated to scattered showers will
accompany the warm front where isentropic assent will be
present. At this time, activity looks too isolated/scattered to
support any mention of reduced vsbys. Will carry VCSH at both
sites. A more consolidated line of showers/tstms associated with
a cold front will approach both KCHS and KSAV from the west
late morning into the early afternoon Tuesday. Latest model
trends suggest this activity will be weakening as it approaches.
Will carry VFR conditions with -SHRA for now. The cold front
should clear the terminals by mid-late afternoon.

Extended Aviation Outlook: Brief restrictions possible ahead of a
cold front Tuesday morning. VFR Tuesday night through Thursday.
Increasing chances for low clouds late Thursday night through the
weekend along with scattered showers.


Tonight: A warm front will lift north through the waters
overnight, allowing winds to veer from the southeast to the
south. Speeds are expected to remain well below Small Craft
Advisory criteria, topping out in the 10 to 15 kt range. Seas
will average 2-3 ft nearshore and 3-4 ft offshore.

Tuesday through Friday: A cold front will move offshore late
Tuesday with winds veering from southwest to northwest in the
wake of the front. We expect a nice punch of cooler and drier
air over the waters Tuesday night and this will result in
surging and jetting offshore flow. We will likely need a Small
Craft Advisory for GA waters 20-60 NM during this time before
flow begins to subside on Wednesday. The near shore waters will
see decent surging but it appears we may not reach SCA criteria
for a substantial length of time.

The surface high will build north of the waters mid to late week
with moderate onshore flow continuing. Seas may increase to advisory
levels next weekend due to persistent northeast fetch and an
increasing pressure gradient.




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