Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 202041
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
341 PM EST Fri Jan 20 2017

.SYNOPSIS...
A series of upper level disturbances will cross the area Saturday
and Saturday night, then a strong low pressure system and cold front
will move through Sunday into Monday. The low pressure will lift
north of the area Tuesday and will be replaced by drier high
pressure through mid week. Another cold front will cross the
area late next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Southern SC and southeast GA will be situated in the surface
region between high pressure east of the Bahamas and strong low
pressure in the central Plains. Deep ridging will prevail aloft
initially, but gives way to a short wave that approaches from
the southwest late and is accompanied by a 130-140 kt upper jet
over the interior of the southeast states. This will contribute
to isolated or scattered showers, especially inland.

With dew points around 60 degrees, stratus should lower through
the night and fog could develop. However, increasing showers
over land could help to limit coverage/thickness. Hence, we`re
only going with patchy fog. Temperatures will be held up by
warm advection and considerable cloud cover, not making it any
lower than the upper 50s to lower 60s, or as much as 20-25F
above normal.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
Saturday: As a lead upper trough ejects NE across the area, showers
will overspread the region, and the chance for thunderstorms will
increase with time. Thus, POPs ramp up from slight chance/chance
early to likely/categorical by early/mid afternoon. During the
afternoon/evening, there is a chance that a few strong/severe
thunderstorms originating upstream/south and west of the region
could advance into southern/western counties. Kinematic/thermodynamic
parameters suggest that any thunderstorms could produce
damaging wind gusts, large hail and even isolated tornadoes.
However, our area will reside on the eastern periphery of the
prime convective initiation region, so the potential for a
widespread severe weather event appears low Saturday PM. Also of
note, PWAT values increasing to around 1.5 inches could
translate to locally heavy rainfall. Despite clouds/showers,
temperatures are expected to recover into the lower/mid 70s.

Saturday night: The lead upper trough and associated pool of deepest
moisture will advance E/NE of the region early, and another upper
trough will approach from the W/SW. between these systems, a
relative lull in precipitation coverage could develop. We maintain
chance/low-end likely POPs to account for this potentially quieter
period. The, late Saturday night/early Sunday, the next shortwave
trough aloft will spread showers/thunderstorms back into the region.
Thus, late night POPs again increase to likely/categorical.
Otherwise, temperatures should not fall below 60F at most locations.

Sunday: The second shortwave trough aloft, pool of deepest moisture
and associated showers/thunderstorms to start will quickly eject
E/NE of the region. Then, stronger/more sustained forcing for ascent
ahead of a deep longwave upper trough will overspread the region.
This will promote strong surface cyclogenesis inland, and this
surface low will drive a surface cold front toward the coast,
possibly supporting organized thunderstorms. Kinematic parameters,
including powerful lower/mid tropospheric winds, strong/deep layered
shear and enhanced storm relative helicity suggest the potential for
a QLCS with damaging wind gusts and a few tornadoes/episodes of
large hail. Indeed, most guidance depicts Significant Tornado
Parameter values quite high for any time of year in our area.
Indeed, if clouds can break up and the environment can destabilize
between the period of morning showers and the arrival of the cold
front Sunday evening, a significant severe weather event could
develop, especially if temps realize the mid/upper 70s as currently
forecast. However, there remains significant uncertainty regarding
the amount of destabilization and the depth/position of the dry slot
progged to overspread much of the region from the S/SW Sunday
afternoon.

Sunday night: Any organized convection ahead of the cold front will
exit the region early, then the cold front will push offshore, and a
widespread dry slot will prevail east of the upper trough axis
approaching from the west. Thus, POPs decrease to slight chance. The
post cold-fropa air mass will remain warmer than normal,
temperatures will only bottom out in the 50-55F range most areas
as W/SW winds persist through the night.

Monday: The upper low will begin to lift N/NE, but the associated
upper trough axis will swing the region with somewhat enhanced
moisture within the comma cloud. Thus, expected isolated/scattered
showers. Otherwise, breezy/windy conditions can be expected in the
wake of the deepening/departing surface low, and downslope flow will
help push temperatures back into the mid 60s most areas.

Lake winds: W/SW winds could gust to at least 25 knots on Lake
Moultrie Monday. Will continue to assess.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Monday night through Wednesday will be quiet and pleasant, as dry
high pressure prevails. A cold front will approach and impact the
area late week, however timing is uncertain due to differences in
the global models. Rain chances will accompany the front.
Temperatures through the period will be above normal.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
VFR or maybe MVFR ceilings through 09Z. Stratus build down late
tonight will result in MVFR or maybe IFR beginning about 09Z.
These conditions will persist as rain showers overspread the
area around daybreak.

Extended Aviation Outlook: Periods of flight restrictions are
expected at least through this weekend. VFR returns early next week.

&&

.MARINE...
Tonight: A SW flow between deep ridging in the Atlantic and
strong low pressure in the central states will persist. But the
warm advection and slacked gradient doesn`t allow for anything
more than 10 or 15 kt, with 2-3 ft seas.

Sea Fog: Warm and very humid air riding across the cooler shelf
waters may lead to some sea fog. The limiting factor will be the
slightly offshore wind direction. For those reasons we don`t
have anything more than patchy fog.

Saturday through Wednesday: Increasing S/SW winds could attain
SCA levels late Saturday night or Sunday. An organized complex
of thunderstorms could produce dangerous conditions Sunday
afternoon into Sunday evening. Then, a cold front will cross the
waters Sunday night, and stronger W/SW winds will produce
widespread SCA conditions, and Gales could develop in some areas
especially at 20 nm and beyond. Elevated offshore
winds/elevated seas could persist through Tuesday, then high
pressure will expand east to produce more tranquil conditions by
mid week.

Areas of sea fog could develop as warm/moist are flows over the
cool Atlantic shelf waters until a cold front crosses the
waters Sunday night.

&&

.CLIMATE...
Record High Maximum Temperatures for Friday January 20:
KCHS: 75 last set in 1954
KCXM: 74 last set in 1933
KSAV: 77 set in 1951

Record High Minimum Temperatures for Saturday January 21:
KCHS: 59 set in 1954
KCXM: 62 set in 1935
KSAV: 66 set in 1937

Record High Minimum Temperatures for Sunday January 22:
KCHS: 59 set in 1972
KCXM: 66 set in 1937
KSAV: 64 set in 1937

Record High Maximum Temperatures for Sunday January 22:
KCHS: 80 set in 1999
KCXM: 76 last set in 1937
KSAV: 80 last set in 1937

Record Rainfall for Sunday January 22:
KCHS: 1.77 inches set in 1999
KCXM: 1.67 inches set in 1973
KSAV: 1.33 inches set in 1966

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...ECT
AVIATION...
MARINE...
CLIMATE...



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