Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 161203

Area Forecast Discussion
National Weather Service Charleston SC
703 AM EST Mon Jan 16 2017

High pressure will extend across the area through tonight.
Temperatures will gradually warm into mid week as a cold front
approaches the region. The front is expected to stall over the
Coastal Plain of South Carolina Wednesday night, then will lift
north as a warm front on Thursday. Mild and moist conditions
will linger across the area this weekend, with greatest chances
of showers and thunderstorms Sunday through Sunday night.


Pre-dawn: The sweet spot for fog overnight has been along the central
South Carolina coast where dense sea fog advected inland prior to
midnight along the Charleston County coast and has remained in place
prompting a Dense Fog Advisory. The fog has slowly moved inland,
impacting Dorchester and Berkeley Counties and we have expanded
the advisory to these 2 zones and recently added Colleton County
to the advisory.

Today: Surface high pressure will build into the mid Atlantic
and coastal Carolinas while a weak coastal trough develops offshore.
A warm air mass will remain in place but a difference today
will be more cloudiness for most areas and this should keep
temps several degrees lower than Sunday. Ridging will generally
prevail aloft but models indicate increasing 1000-800 MB
moisture and weak isentropic ascent during the day. This could
result in isolated light showers developing this afternoon from
upper coastal Georgia brushing southeast South Carolina. We
have mentioned some slight chance pops but no appreciable QPF in
the forecast. Warmer temps should be south of I-16 in Georgia
as models indicate better chances for breaks in the clouds later
today where a few upper 70s may occur if clouds break sooner.

Tonight: Once again the forecast will favor areas of stratus
and fog development for the entire area. A strong nocturnal
inversion and flux of Atlantic moisture in the boundary layer
will provide a solid foundation for both coastal sea fog
advection potential and a widespread build-down stratus event
inland. The main uncertainty will be clouds and there could be
plenty of high clouds. We think there was enough of a
possibility of dense fog to put it in our Hazardous Weather


On Tuesday, sfc high pressure will remain centered near Bermuda
as a wavy cold front struggles to move over the NC/TN line.
This pattern will support light and steady SW llvl flow across
the forecast area. H85 temperatures will rise to around 11.5 C
by 0Z Wed. I will forecast PoPs below SCHC and sky to feature
partly sunny conditions with Cu. After the morning fog dissipates,
the primary forecast challenge will remain with the high
temperatures. Using a blend of the best performing guidance, I
will forecast highs to range from the mid 70s across the inland
counties to low to mid 70s along the coastline, falling short of
records by 3 to 4 degrees.

The front is expected to stall over the southern Appalachians, with
the northern portion sliding south as a backdoor front. Guidance and
overall pattern indicate that the backdoor front will not exit the
forecast area. The boundary may stall across the SC Coastal Plain on
Wed. Clouds will increase across the forecast area Tues night, then
remaining through Wed. Overall, temperatures will not change by more
than one or two degrees from Tuesday, ranging just below records.
Weak sct showers may develop along and ahead of the front, greatest
chances to the north. Rainfall may generally range in just a few
hundredths of an inch.


Medium range guidance agrees that the axis of long wave H5
ridge will ripple over the region on Thursday. However, the sfc
front is expected to slowly lift north as a weak warm front.
PoPs across the forecast area will remain limited to SCHC, with
values slowly lowering from south to north. High temperatures on
Thursday are forecast to favor values around 70 degrees over

Thursday night into Friday, a negatively tilted H5 trough is will
pass over the southern Appalachians, brushing the CWA with weak
omega by sunrise Fri. The combination of deep moisture, weak
instability, and weak forcing should support sct showers. In the
wake of the front, dry air will spread across the region Sat am.

Saturday night into Sunday, a large low pressure system will
organize over the Great Plains, tracking over the Mid West by Sunday
evening. Moist SW flow will gradually increase across the forecast
through the late weekend. In fact, moist and warm llvl flow may
develop a band of sfc based CAPE as high as 1000 J/kg on Sunday. I
will increase PoPs to likely, for showers and thunderstorms. The
center of the H5 low is expected to pass over the mid Atlantic
region, with wrap around moisture crossing the CWA. SCHC showers and
near normal highs expected on Monday.


KCHS: Dense fog continues early this morning despite layered
clouds moving atop the stratus. Latest soundings suggest
breaking the fog and low clouds may be slower than earlier
expected but we have maintained similar previous trends which
improve conditions after 15Z. We cannot rule out an isolated
shower this afternoon but chances favor VFR conditions during
this time. Tonight, another round of fog and perhaps dense fog
is possible once again. We introduced some IFR after 06Z given
the strong model consensus and synoptic pattern.

KSAV: Fog did not develop overnight as layered clouds dampened
effects from stronger radiational cooling. Brief fog and low
clouds are possible through mid morning VFR conditions appear
likely for most of the day. Tonight. fog development is possible
and we cannot rule out dense fog late.

Both terminals will see vast improvements by late morning and
this afternoon with VFR conditions returning.

Extended Aviation Outlook: Patchy to areas of fog may linger
into mid morning Tuesday. A weak cold front is forecast to
slowly sink move southward across the South Carolina Coastal
Plain Wednesday night, fog and showers may develop along and
ahead of the front. Restrictive visibility and ceilings may
develop with passing band of showers Thursday into Friday. A
large low pressure system is expected to bring showers and
thunderstorms across the region Sunday through Sunday night.


Through tonight: The main threat to navigation will be the sea
fog potential which continues to be tricky to forecast as subtle
mesoscale features are in constant motion across the waters.
This morning, fog is prevalent along Charleston County but no
reports from Hilton Head or coastal Chatham County to this
point. Patchy dense fog is possible today and may expand
somewhat tonight as stratus lowers along the immediate coast.
There is a marine Dense Fog Advisory for Charleston waters
through mid morning.

Otherwise, stable conditions in the marine layer and mainly
light winds will prevail, mainly ne directions with the coastal
trough out near the Gulf Stream. Seas through tonight in the 2
to 3 ft range.

Through Wednesday: SW winds will remain <10 kt for much of the
period with seas 2-4 ft on average. Sea fog will remain an issue
Tuesday morning, but should diminish Tuesday afternoon and
especially Wednesday as a cold front approaches and low-level
trajectories become less ideal. Marine Dense Fog Advisories could be
needed Tuesday morning.

Thursday and Friday: A weak backdoor cold front will push across the
region by Thursday, then expected to stall. Winds will veer from
east to south Thursday night. Another cold front is expected to
slide east offshore on Friday into Friday night, resulting in winds
to shift from the west. Sfc high pressure should build off the SC
and GA coast on Saturday. Seas will average between 2-4 feet through
the period. Small Craft Advisory conditions are unlikely.


Record highs for Tuesday, 17 January:
KCHS: 78/2013 and previous years
KCXM: 78/1928
KSAV: 81/1943

Record highs for Wednesday, 18 January:
KCHS: 77/1952
KCXM: 79/1928
KSAV: 81/1937


SC...Dense Fog Advisory until 10 AM EST this morning for SCZ043>045-
MARINE...Dense Fog Advisory until 10 AM EST this morning for AMZ330-350.


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