Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201430

Area Forecast Discussion
National Weather Service Charleston SC
930 AM EST Mon Feb 20 2017

High pressure will linger across the region through the rest of this
week. A cold front will sweep across the region on Saturday. Behind
the front, a Canadian air mass will build east of the Appalachians
on Sunday. Moisture is expected to rapidly stream northward across
the deep south ahead of another large low pressure system on


Deep layered ridging will build into the area today. It will be a
rather warm day with highs reaching into the mid-upper 70s away
from the beaches. An initial band of cirrus bisecting Georgia
from north-south this morning will gradually mix out as it
moves east, but do expect a steady increase in cirrus from
southwest-northeast later this afternoon. Updated sky grids to
reflect this thinking.


Tonight: A bit more low-level moisture on tap tonight and with
decent radiational cooling conditions much of the night some fog
will likely develop, especially near the GA coast where the
lowest condensation pressure deficits are forecast. Some of the
fog could become dense. Lows should range from the upper 40s
inland to mid/upper 50s at the coast.

Tuesday: Longwave ridge axis will shift off the Atlantic coast
as a closed low tracks across the heal of LA. At the sfc, large
high pressure centered off the New England coast will continue
to ridge to south across the region. Low pressure remain over
the southern Mississippi River Valley. Forecast soundings
indicate that cirrus level moisture will thicken across SE GA
and the Low Country of SC through the daylight hours. In
addition, an inversion is forecast to persist around H75, likely
resulting in moisture to gradually increase below with llvl on
shore flow. The forecast will feature at least mostly cloudy
conditions with low PoPs. High temperatures should range from
the low 70s across the Santee Basin to the upper 70s south of
the Savannah River.

Tuesday Night and Wednesday: Mid level pattern will become
quite complicated across the southeast CONUS. Generally, short
range guidance indicates that the closed low will slip SE across
the Gulf of Mexico, centered west of Key West by 0Z Thursday.
Across the CWA, mid level heights will feature a broad weakness
early Wednesday morning, likely aiding the formation of a weak
S/W, rippling over the Savannah River Valley Wed afternoon. PoPs
will rise to 20-30 percent across the forecast area. Model
soundings show shallow CAPE developing below a H7 inversion. The
limited instability and forcing should yield only showers, with
thunder chances greater closer to the Gulf Stream. Mild llvl
thicknesses should keep min temperatures around 15 degrees above
normal, with highs generally around 10 degrees above normal.


Progressive pattern will continue across the CONUS through the
medium range period. Large low pressure will organize over the
central Great Plains late Thursday. A warm front associated with
the low will meld with a old cold front across the mid
Appalachians. The large warm sector should provide the region
weak instability and moderate mid and low level moisture. PoPs
will remain within SCHC range across the CWA on Thursday,
mentioning showers with iso thunderstorms over the offshore
waters. On Friday, south winds will strengthen across the CWA as
a cold front pushes east across MS/AL/TN. GFS indicates that
H85 temps will warm above 13C by late Fri afternoon. Guidance
indicates that high temperatures will reach the upper 70s near
the coast to the low 80s inland. The high temperatures may
challenge record values. Building instability should yield iso
showers/thunderstorms. The cold front is expected to remain east
of the region by daybreak Saturday. Min temps are forecast to
remain very mild, with low 60s common Sat AM. GFS shows the
passage of the sfc cold front during the daylight hours on
Saturday. CAPE values may range from 1000-1500 J/kg, especially
across the SC zones. Given the timing and instability, it
appears that the environment may yield a band of prefrontal deep

Dry Canadian high pressure is expected to build over the region
Saturday night and Sunday. Cooling thicknesses will yield lower
temperatures, but still remaining around 5 degrees above normal.
Moisture will rapidly stream northward ahead of a dynamic low
pressure centered over the Mid West. Strengthening return flow will
likely advect deep moisture and weak instability across the forecast
area on Monday. I will add CHC PoPs for deep convection. Temps
should raise several degrees over Sunday.


VFR conditions will generally prevail at both KCHS and KSAV through
12Z Tuesday, although restrictions from low clouds/fog are
possible toward the end of the period at KSAV.

Extended Aviation Outlook: Patchy fog is possible each work day
morning. Otherwise, no restrictive conditions are expected
through Friday. A cold front will sweep across the region on


Today/Tonight: Winds/seas will remain well below Small Craft
Advisory levels with high pressure to the northwest shifting
north of the area tonight. Winds will increase a bit tonight as
the pressure gradient tightens a bit across the area.

Tuesday through Saturday: East to northeast winds will develop
over the marine zones on Tuesday. Sfc wind will veer southeast
Tuesday night, remain through late in the week. A cold front
will sweep across the region late Saturday. Cold air advection
should occur Saturday night into early Sunday, likely resulting
in gusty conditions. Wave heights are forecast to build over 5
feet beyond 10 NM to 6-7 feet beyond 20 NM on Sunday.


Record Highs for Feb 24:
KCHS: 81 set in 2012/1996.
KCXM: 81 set in 1930.
KSAV: 86 set in 2012.

Record Highs for Feb 25:
KCHS: 80 set in 1949.
KCXM: 80 set in 1930.
KSAV: 82 set in 1985/1930.

Record High Minimums for Feb 24:
KCHS: 60 set in 1992.
KCXM: 61 set in 1901.
KSAV: 64 set in 1980.

Record High Minimums for Feb 25:
KCHS: 62 set in 1992.
KCXM: 59 set in 2011.
KSAV: 63 set in 1992.




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