Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 231714
AFDCHS

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

.SYNOPSIS...
A cold front will push offshore tonight and 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 /THROUGH TONIGHT/...
This Afternoon: Latest water vapor imagery indicated a mid-lvl low
tracking over the Tennessee Valley early this afternoon with dry mid-
lvl air punching eastward along the southern periphery over a cold
front shifting through Central Georgia. Ahead of this front, strong
isentropic lift and deep moisture characterized by PWATs near 2.0
inches within a south-southeast flow will maintain widespread clouds
over the Southeast and the potential for scattered showers along
with a few thunderstorms within a marginally unstable environment
into early afternoon.

As we head into mid to late afternoon hours, the main issue will
focus on the possibility of severe weather across parts of the
region. Latest mesoanalysis and guidance suggests a mid/upper lvl
trough to our west/northwest will take on a slightly negative tilt
as a h25 jet max rounds it`s southern periphery. This feature along
with dry mid-lvl air intruding over an approaching cold front will
play a significant role in regards to severe weather potential late
afternoon/early evening. Ahead of the front, convective activity is
expected to be less vigorous under widespread cloud cover and
weaker mid-lvl lapse rates. However, temps approaching the upper 70s
within a gusty south/southeast sfc wind will drive moisture and
higher instability onshore with SBCAPE near 1500 J/kg over most areas
this afternoon. This could support a few stronger thunderstorms early
this afternoon, but the main threat should occur as shear increases
later this afternoon. Shear will become more than favorable for
organized convection as the cold front shifts over the area late
afternoon/evening, with 0-6 km bulk shear of 40-50 kt and 850-500mb
cross over winds suggesting increasing levels of shear/helicity
over the region as a southwest flow spreads over a south/southeast
sfc wind. Based on the latest radar trends, the potential for severe
weather will be largely dependent on the amount of instability remaining
during the arrival of the cold front, which in this case should remain
somewhat favorable while dry air punches over the front as it encounters
greater levels of sfc instability closer to the Southeast Coast. However,
the main caveat to this scenario is a fairly wide swath of rain/showers
ongoing ahead of the front, which could stabilize conditions enough
to limit the overall severe weather potential. For these reasons,
isolated severe weather will remain possible later this afternoon
and evening with damaging wind gusts and/or isolated tornadoes being
the primary concern. The greatest potential for severe weather should
occur over northern areas in Southeast South Carolina where the
combination of shear/instability are greatest during fropa.

Tonight: Near term guidance indicates that the cold front will
gradually slow as it pushes east of the SC/GA coast tonight. However,
H85 CAA will begin tonight, but may only cool H85 temps by a few
degrees before sunrise. Drier air should easily spread across
SE GA and SC tonight, resulting in the sky cover to gradually
decrease through the night. Using a blend of temperature
guidance, I will forecast low temperatures to range from the mid
50s across inland GA to the mid 60s over the beaches.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
This period starts off with improving conditions behind an initial
cold front which will already be offshore. A stronger cold front
will then move through later Tuesday and Tuesday night bringing much
cooler temperatures with it through Thursday. Temperatures Tuesday
should get above normal given the downslope westerly winds which
will offset low-level cold advection to some extent. On Wednesday
temperatures will likely struggle to reach 70 degrees given the
strong cold advection, with the lowest temperatures likely to be
Thursday morning when many inland locales could reach the lower 40s,
possibly closer to 40 in some of the normally colder rural areas.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
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 /17Z MONDAY THROUGH FRIDAY/...
South winds will strengthen through this afternoon as a cold
front approaches from the west, gusts around 20 kts should be
common. Rounds of showers should gradually increase today,
supported by synoptic scale forcing and building instability.
High resolution guidance suggests that a pre-frontal band of
stronger showers with embedded thunderstorms will reach the
terminals late this afternoon into the early evening, I will
highlight with a TEMPO. FROPA should occur shortly after sunset,
resulting in winds to shift from the SW. Forecast soundings
indicate that cloud based will lower as CAA arrives over the
terminals, I will indicate MVFR ceilings through this evening
into late tonight. Sky should decrease to SCT by 10Z.

Extended Aviation Outlook: No significant concerns Tue-Sat.

&&

.MARINE...
This Afternoon and Tonight: Southeast winds will gradually
become south by mid day ahead of a cold front approaching
from the west. The pressure gradient will tighten ahead of
the front, suggesting a slight uptick in winds/seas through
this evening. 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 4-6 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
afternoon through much of tonight.

A band of prefrontal showers could produce gusty winds over
parts of the waters into at least mid afternoon hours. Then, a
band of showers/thunderstorms will approach the marine zones
this evening, slowly tracking east through the rest of the
night. The environment could support strong wind gusts and/or
isolated waterspouts, especially along/near the cold front. The
passage of the sfc cold front will result in winds becoming
west-northwest late.

Tuesday through Saturday: Conditions will be poor as a cold front
moves through late Tuesday/Tuesday night, mainly beyond 20 nm where
an Advisory will be possible. Winds/seas will remain elevated
through Wednesday night given the cold air advection but should
remain below Advisory levels. 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...RJB
LONG TERM...RJB
AVIATION...DPB/RJB
MARINE...DPB/RJB



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