Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 131737
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1237 PM EST Wed Dec 13 2017

.SYNOPSIS...
Dry high pressure will persist across the region through
Thursday. A cold front will push through the area Friday before
high pressure prevails Saturday and Sunday. Another cold front
will shift over the region Monday into Tuesday before high
pressure returns during the middle of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 1235 PM: I will update the forecast to decrease RH values
through the rest of the afternoon.

As of 10 AM: I will update the forecast to lower RH slightly
through this afternoon. In addition, I will increase high
temperatures by a degree.

No major changes were made for the sunrise update.

There will be a brief cool down today a fresh cP airmass settles
across the Southeast U.S. in the wake of a cold front which
pushed offshore yesterday. The strongest push of post-frontal
cold air advection has propagated well offshore and the low-
level advection pattern will become neutral to warm later in
the day as a secondary cold front approaches from the northwest.
H8 temperatures as cold as -2 to -4C will recover 4 to 6C by
late afternoon with corresponding 1000-850 hPa thickness rising
to 1320-1330 meters under full insolation. This should support
highs from the upper 40s/near 50 across the Charleston Tri-
County area to the lower 50s across much of Southeast Georgia in
the absence of a downslope component.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
Tonight: A secondary cold front will push offshore overnight and
be followed by another brief yet weaker surge of cold air
advection as the supporting shortwave passes by well to the
north. Mainly clear skies will prevail overnight, although some
cirrus associated with the subtropical jet will approach
Southeast Georgia prior to daybreak Thursday. Lows will range
from the mid 30s inland to the lower-mid 40s at the coast.
Breezy conditions will occur at the beaches with gusts at or
above 30 mph at times.

Thursday: Dry sfc high pressure will prevail over the Southeast
along the base of a large and broad mid/upper trough of low pressure
extending from the Central United States to the Northeast United
States. The pattern favors quiet weather, but temps will be
noticeably warmer than the previous day as low pressure passes to
the north and helps produce a west/southwest flow over the
Southeast. In general, afternoon highs will approach the lower 60s
over most areas. A few locations could peak in the mid 60s south of
I-16 in Southeast Georgia. A light west/southwest flow will continue
through Thursday night, keeping temps a few degrees warmer than the
previous night. In general, temps should dip into the favoring a
warmer trend compared to the previous night. In general, Thursday
night lows should range in the upper 30s inland to mid 40s near the
coast.

Friday: A cold front will push through the region early this weekend
with little in regards to precip. However, a few showers can not be
ruled out near the coast as the front shifts offshore late. Upper
lvl-moisture along with strong divergence associated with the right-
rear quadrant of an upper lvl-jet should also produce some high
clouds for most areas away from the coast. Given the setup, temps
should remain a bit cooler inland where high clouds and fropa occur
first. In general, temps will peak in the upper 50s inland to
low/mid 60s near the coast. Dry and cold high pressure will push
into the area Friday night. Lows should dip into the lower 30s
inland to mid/upper 30s closer to the coast.

Saturday: Conditions will remain quiet as dry high pressure becomes
centered over the Southeast. Despite a full day of sun and
downsloping wind component, temps will remain below normal. In
general, afternoon highs will peak in the mid/upper 50s over most
areas.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
High pressure centered over the area Saturday night will lead to
strong radiational cooling and temps that dip into the mid/upper 30s
inland to low/mid 40s near the coast. Conditions will then become
noticeably warmer on Sunday as a light southerly wind develops on
the west side of high pressure shifting offshore. Temps should
approach the mid 60s Sunday afternoon before clouds develop as
moisture and isentropic ascent increase late. The next chance of
showers should begin Sunday night and increase in coverage on Monday
as a plume of moisture characterized by PWATs near 1.75-2.0 inches
advects over the Southeast along/ahead of a cold front approaching
from the west. Temps could peak into the upper 60s to near 70
degrees on Monday before fropa occurs. Models insist that mid-lvl
energy will traverse over the Southeast before the main mid-lvl
trough axis passes over the area on Tuesday. For this reason, at
least slight chances of showers will be possible Monday night into
Tuesday before dry high pressure returns mid week.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
VFR. Mixing will increase through the morning daylight hours on
Thursday. WSW winds are expected to develop gusts into the upper
teens by late morning.

Extended Aviation Outlook: VFR conditions will prevail at both CHS
and SAV terminals through this weekend.

&&

.MARINE...
Today: Northwest winds will steadily back to the west and
southwest this afternoon as the surface high shifts south of the
waters.

Tonight: Southwest winds will pick up this evening as low-level
jetting intensifies with the approach of a dampening shortwave
and associated secondary cold front. Expect winds to reach 20-25
kt over the waters with 15 kt in the Charleston Harbor. Seas
will correspondingly build, reaching 3-5 ft nearshore waters
with 5-6 ft offshore waters. Small Craft Advisories will be
issued for all legs with the exception of Charleston Harbor.
Could see gusts to 25 kt occur in the Harbor entrance, but will
hold off on raising flags for the Harbor with this forecast
package.

Thursday through Sunday: Small Craft Advisory level conditions will
likely continue for most coastal waters into Thursday morning while
cold air advection helps promote low-lvl mixing into a 40 kt low-lvl
jet. Winds/seas should then improve late Thursday morning with
nearshore waters likely falling below Small Craft Advisory levels by
noon. Offshore Georgia waters could see Small Craft Advisory level
conditions linger into early afternoon. Dry high pressure will then
prevail Thursday evening and night ahead of a cold front approaching
the waters on Friday. Conditions could approach marginal Small Craft
Advisory levels over northern South Carolina waters and offshore
Georgia waters behind the front Friday night into early Saturday.
Winds/seas will then remain well below Small Craft Advisory levels
as high pressure becomes centered over the area on Saturday and then
slowly shifts offshore on Sunday.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 10 PM this evening to 10 AM EST
     Thursday for AMZ350-352-354.
     Small Craft Advisory from 10 PM this evening to 1 PM EST
     Thursday for AMZ374.

&&

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


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