Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 232204
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
604 PM EDT Mon Oct 23 2017

.SYNOPSIS...
A cold front will push offshore tonight, then will be followed
by a stronger cold front late Tuesday and Tuesday night. High
pressure will then prevail into late week before another cold
front possibly affects the area this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Late Evening and Tonight: Latest water vapor imagery indicated a mid-
lvl low tracking northeast over eastern Kentucky and Tennessee this
evening with a large area of dry air punching eastward along the
southern periphery over a cold front now shifting through eastern
Georgia and central South Carolina. Ahead of this front, deep
moisture characterized by PWATs near 2.0 inches within a south-
southeast flow will maintain widespread clouds and scattered to
numerous showers along with some embedded thunderstorms. As we head
into evening hours, the main issue will continue to focus on the
possibility of severe weather across parts of the region, mainly
with a broken line of heavy thunderstorms near/along the advancing
cold front. Latest mesoanalysis and guidance suggests the mid/upper
lvl trough to our northwest will maintain a slightly negative tilt
while shifting northeast with dry mid-lvl air intruding over the
cold front to the south, which will likely steepen mid-lvl lapse
rates as the front encounters SBCAPE between 1500-2000 J/kg closer
to the Southeast Coast. Shear will be favorable for organized
convection, with 0-6 km bulk shear of 40-50 kt and 850-500mb crossover
winds supporting increasing levels of helicity over the region as
a west/southwest flow spreads over a south/southeast sfc wind.
Given the setup, a Tornado Watch remains in effect for parts of
the area until 9PM, with the main focus being over northern areas
in Southeast South Carolina where the combination of shear/instability
are expected to be greatest during cold fropa.

The main threat for severe weather will come to an end once the cold
front passes through the area, which is beginning to unfold from
west to east this evening. Widespread showers along with embedded
thunderstorms will shift east with the front and will be followed by
much drier air from the west. For this reason, expect precip chances
to significantly decline from west to east between sunset and
midnight. Most areas should then be dry late tonight. Strong cold
air advection behind the front should help temps dip into the mid
50s inland to lower 60s near the coast.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
The initial cold front will be offshore by daybreak Tuesday,
with significantly drier air filtering in from the west.
However, with the lack of appreciable cold air advection and
downslope flow off the surface, high temps will still climb into
the mid/upper 70s.

The deep upper trough will push toward the Southeast late
Tuesday, pushing a secondary dry cold front through the region
Tuesday night. This will yield low temps Wednesday and Thursday
morning in the 40s with daytime highs in the mid/upper 60s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
After another unseasonably cool start on Friday, temperatures will
moderate back closer to normal Friday afternoon into the mid 70s as
high pressure centered to the north moves offshore. A coastal trough
could develop Friday night and bring some light showers to the area.
Some uncertainty exists however beyond that time as the models
differ with respect to another cold front approaching from the west
and low pressure to the south and east. For now we stayed pretty
close to the WPC`s guidance which has a cold frontal passage
Saturday night into Sunday. Temperatures will likely be near normal
through Saturday before falling below normal early next week,
possibly well below normal by Monday when some 30s are possible.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
Gusty south winds peaking around 20 kt will be possible at both
terminals through this evening ahead of an approaching cold
front. Tempo MVFR flight restrictions from 18-22z at CHS and
19-23z SAV have been included due to showers/thunderstorms
impacting the terminals. There is a low chance of IFR vsbys
during heavier rainfall, but confidence is too low to include in
the tafs at this time. MVFR cigs will likely prevail for at
least a few hours early tonight at both terminals before
conditions improve to VFR between 04z-09z. VFR conditions will
then prevail late Tuesday morning into Tuesday afternoon at both
terminals.

Extended Aviation Outlook: No significant concerns Tue-Sat.

&&

.MARINE...
This Evening and Tonight: South/southeast winds will remain elevated
within a tight pressure gradient ahead of a cold front approaching
from the west early tonight. In general, southeast/south winds will
gust near 20-25 kt, highest in nearshore South Carolina waters and
offshore Georgia waters. Seas will also range between 4-7 ft,
highest in nearshore South Carolina waters and offshore Georgia
waters. For these reasons, Small Craft Advisories will continue for
nearshore South Carolina waters and offshore Georgia waters this
through much of tonight.

A band of showers/thunderstorms will approach the marine zones
with a cold front, slowly tracking east through the waters early
tonight. The environment could support strong wind gusts and/or
isolated waterspouts, especially along/near the cold front. A
Tornado Watch remains in effect for the Charleston Harbor
through 9 PM, but isolated waterspouts will remain a threat for
all coastal waters until cold fropa occurs. The passage of the
sfc cold front will result in winds becoming west- northwest
late. Despite cold air advection behind the front, winds will
decrease to around 10-15 kt late.

Marine conditions will temporarily improve on Tuesday, allowing
the Small Craft Advisory for the offshore GA waters to drop
midday. However, a period of marginal SCA conditions is possible
late Tuesday night through Wednesday morning after a secondary
cold front and more significant blast of cold air advection. We
show gusts approaching 25 kt mainly over the GA waters during
this time.

Much improved conditions will return by Thursday as high pressure
builds back in and persists through Friday. Another cold front
could approach the waters Saturday but much uncertainty remains
with the forecast this weekend.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 5 AM EDT Tuesday for AMZ352.
     Small Craft Advisory until 11 AM EDT Tuesday for AMZ350-374.

&&

$$
NEAR TERM...DPB
SHORT TERM...JRL
LONG TERM...RJB
AVIATION...DPB/JRL
MARINE...DPB/JRL



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