Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 120307
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1007 PM EST Mon Dec 11 2017

.SYNOPSIS...
High pressure will prevail through early Tuesday. A dry cold
front will move through the region Tuesday afternoon, followed
by cold and dry weather through Thursday. A cold front will
push into the area Friday while low pressure passes offshore.
High pressure should return Saturday before another cold front
approaches early next week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
A clipper-type low will race east-northeast along the southern
Great Lakes, and it`s associated cold front passes over the
central and southern Appalachians late.

Other than a few thin cirrus clouds with the upper jet and/or
the short wave off to the distant northwest that is accompanying
the low and cold front, skies will stay clear tonight. Weak
warm air advection (WAA) and mixing in the boundary layer will
prevent temps from getting anywhere near as cold as the past 2
or 3 night. Expect lows generally in the upper 30s to lower 40s
inland from US-17, middle or upper 40s along the coastal
sections.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday: The mid/upper levels will consist of a strong trough over
the eastern half of the country, with it`s axis stretching from the
Great Lakes region down into the Lower MS Valley in the morning. The
axis will move eastward, becoming located over the Northeast states
down and off the Southeast coast by overnight. At the surface, high
pressure will quickly dissipate in the morning as a dry cold front
approaches, the front is expected to rapidly pass through our area
during the afternoon hours. There is no noticeable moisture with the
front (PWATs ahead of it barely reach 0.5"), so no precipitation is
expected. The only impacts with the front will be some clouds and a
wind shift/increase. As for temperatures, ahead of the front,
southwest winds will allow temperatures to rise within a few degrees
of normal. Behind the front, cold air advection and clear skies will
cause temperatures to drop. Though, elevated winds should keep them
from dropping too rapidly. Lows will be about 10 degrees below
normal, ranging from the upper 20s far inland to around freezing at
the coast. Additionally, winds could approach Lake Wind Advisory
criteria over Lake Moultrie Tuesday night.

Wednesday and Thursday: Dry high pressure will prevail under a
somewhat zonal flow aloft. This will lead to fairly quiet weather
conditions mid week while temps slowly modify each day. Strong cold
air advection will be slow to weaken on Wednesday, limiting highs
only to the mid/upper 40s over most areas. Clear skies will once
again lead to chilly temps Wednesday night. In general, temps should
dip into the lower 30s away from the coast. Conditions will then be
noticeably warmer on Thursday as a west/southwest flow develops over
the area while low pressure passes to the north. In general, highs
should approach the upper 50s to lower 60s Thursday afternoon under
mostly sunny skies.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Latest trends indicate increasing moisture levels Thursday night
into Friday as low pressure develops along a passing cold front just
offshore. Strong divergence associated with the right-rear quadrant
of an upper-lvl jet should also help promote rain/shower activity
behind the front, especially as low pressure helps draw moisture
north/northwest over it. For these reasons, chances of rain/showers
will be possible for most areas Friday, especially near the coast
closer to the vicinity of the front and low pressure passing
offshore. Dry and cooler high pressure will then build over the area
on Saturday with afternoon temps remaining in the mid/upper 50s.
Conditions will be noticeably warmer on Sunday as dry high pressure
becomes centered over the Southeast. In general, temps should
approach the mid 60s over most areas.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
VFR conditions will prevail at KSAV and KCHS through 00Z
Wednesday. Deep mixing and a tight gradient associated with the
passage of a cold front on Tuesday will lead to gusty westerly
winds in the afternoon, with highest speeds of at least 20-25
kt.

Extended Aviation Outlook: VFR through Thursday. MVFR possible
Friday as low pressure passes offshore.

&&

.MARINE...
Tonight: High pressure from the Gulf of Mexico and extending
across the southern waters will drop south through the night as
the large cyclonic circulation develops around low pressure
traveling from near Toledo, Ohio reaches near Buffalo, New York
by daybreak. W-SW winds will average 15-20 kt (except around 5-10
kt in Charleston Harbor), with seas of 2-3 ft early to build
another 1-2 ft during the post-midnight period.

A tightening pressure gradient ahead of an approaching cold
front will support strengthening WSW winds and an expansion of
Small Craft Advisory conditions Tuesday, especially outside
Charleston Harbor. Then, in the wake of cold fropa cold air
advection and increasing marine boundary layer instability/mixing
tapping into 40 knots winds aloft will support solid SCA
conditions featuring frequent NW gusts 25-30 knots accompanied
by seas 4-7 feet, highest beyond 20-40 nm. Small Craft
Advisories are already in effect for waters outside Charleston
Harbor to account for this scenario, and an SCA will likely be
required for the Harbor. Also of note, there exists a low
probability for a few gale force gusts across the outer GA
waters, especially beyond 40 nm.

As high pressure builds over the waters, winds should relax and
seas should subside Wednesday through Thursday. Then, another
cold front will push through the waters Friday, and an
associated low pressure will pass offshore. Thus, another round
of SCA winds/seas could develop late this week into this
weekend.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 11 AM Tuesday to 7 AM EST Wednesday
     for AMZ352-354.
     Small Craft Advisory from 7 AM Tuesday to 9 AM EST Wednesday
     for AMZ374.
     Small Craft Advisory from 4 AM Tuesday to 7 AM EST Wednesday
     for AMZ350.

&&

$$
NEAR TERM...
SHORT TERM...MS
LONG TERM...DPB
AVIATION...
MARINE...


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