Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230808
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
408 AM 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/...
As of 4 AM: Latest water vapor images indicated a closed mid level
low was tracking east across Arkansas, south of the low, dry
air was pushing east across the MS/AL state line. Latest sfc
analysis showed a well defined cold front extending from middle
TN south across western AL. Across the forecast area, SE LLVL
winds will strengthen through the mid morning hours, providing
broad isentropic lift around 305K. The shallow lift combined
with weak instability should continue to support rounds of
showers across the CWA through this morning, especially across
the SC Lowcountry. The GFS1deg shows a field of H5 Q-vector
convergence expanding across the region between 18Z to 0Z. At
the SFC, a cold front is forecast to run from the western
Carolinas south across GA to the FL Panhandle by 18z, reaching
the inland GA counties by 0Z. The environment along and ahead of
the front will feature 0-1km helicity greater than 300 m2/s2,
SFC based CAPE should reach to at least 500 J/kg, with 0-1 km
EHI ranging from 1.5 to 2.5 units. Latest HRRR suggests that a
broken band of showers and embedded thunderstorms will reach the
western forecast area around 20Z, across the SC Lowcountry 21Z
to 02Z, then pushing offshore during the mid to late evening.
SPC has highlighted most of SE GA and SC with a SLGT risk for
severe weather. Given the high shear and low/moderate
instability, it appears that a few damaging wind and tornado
events are possible this afternoon into the evening hours. High
temperatures are forecast to reach the upper 70s across most
areas, with near 80 common across SE GA.

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 /08Z MONDAY THROUGH FRIDAY/...
Scattered to numerous showers are expected to pass near and over
KCHS through the rest of the night, with less coverage expected
around KSAV. 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.

Extended Aviation Outlook: No significant concerns Tue-Sat.

&&

.MARINE...
Southeast winds this morning will gradually veer from the south by
mid day. The pressure gradient should steepen slightly as a cold
front approach late today, resulting in gusts in the 20s near shore
to the mid 20 across the outer waters. Wave guidance indicates that
SC near shore and the GA outer waters will see wave heights range
from 4 to 6 feet, with nearshore GA between 4 to 5 feet. Based
mainly on wave heights, Small Craft Advisories will remain for the
SC near shore waters and for AMZ374. A band of prefrontal showers
and thunderstorms is forecast to push over the marine zones this
evening, slowly track east through the rest of the night. The
environment could support isolated waterspouts and wind gusts
greater than 34 kts. The passage of the sfc cold front tonight
will result in winds to shift from the WNW by dawn on Tuesday.

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 AMZ374.
     Small Craft Advisory until 11 AM EDT Tuesday for AMZ350.

&&

$$
NEAR TERM...NED
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...NED/RJB
MARINE...NED/RJB



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