Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 241821
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
121 PM EST Fri Feb 24 2017

.SYNOPSIS...
Low pressure will meander north of the Bahamas today, then moves
northeast tonight. A cold front will cross the region on Saturday,
followed by high pressure on Sunday. The high will shift offshore
early next week. Another cold front could move through Wednesday
night into Thursday with high pressure returning for the end of
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
1 PM: The forecast remains on track, so no changes were made.

Today: A deep layered cyclone to the E of Florida and it`s
associated surface low north of the Bahamas will begin to shift
E-NE throughout the day. In it`s place a low and mid level
ridge will move overhead from the W, as a narrow ridge of
surface high pressure pokes in from the mid-Atlantic. Given
plenty of insolation and 850 mb temps of 11-12C or near the 90th
percentile, we`re well on track for springlike warmth and near
record temps (See CLIMATE section below). Today will be the 50th
day of meteorological winter so far (beginning December 1st)
with temps of 70F or greater at KSAV and the 40th such day at
KCHS. keep in mind that if you`re on the beaches the onshore
fetch crossing the nearshore waters where SST`s are around
60-62F, that`ll be some 12-15F cooler than inland.

Tonight: Weakening Atlantic low pressure lifts NE to a position
E of the outer banks of NC late, as a strong cold front trailing
from robust low pressure in MI nears the spine of the
Appalachians before daybreak. Our area lies under subtle ridging
surface and aloft, with the best moisture confined closer to
the cold front. So no risk for any rains, but enough low level
moisture, mostly clear skies and condensation pressure deficits
less than 20-30 mb, which will support the potential for
fog/stratus. Probably too much wind within the boundary layer
to produce dense fog, but certainly enough for patchy to areas
of fog to occur after midnight. Lows this time of year should be
in the lower and middle 40s, but we`ll be in the mid and upper
50s with low level warm advection within a S-SW synoptic flow.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
Saturday: Patchy/areas of fog should easily disperse by mid-
morning as the low level gradient continues to tighten ahead of an
advancing cold front which may spend the first part of the day
hung-up a bit in the mountains before moving through the forecast
area in the afternoon hours. Moisture levels will not be all that
impressive as the front pushes through the area, suggesting that
only a few showers will be possible in the very mixed-out prefrontal
environment. Breezy and very warm temps are expected with high temps
in the 79-81 degree range. Frontal compression could easily push
temps into the mid 80s along the coastal corridor of SC, we bumped
highs up to a record-breaking 83 degrees at KCHS on this morning`s
update.

Saturday Night and Sunday: The cold front will sweep offshore by
early Saturday evening with cold air advection spreading across the
entire forecast area Saturday night under clear skies. Temps will
steadily fall producing some huge diurnal swings, low reaching near
40 well inland to the mid 40s along the coast. On Sunday, high
pressure will become centered over the Southeast, leading to
quiet/dry conditions under a zonal flow aloft. Overall temps will be
closer to normal for this time of year. In general, high temps will
range in the low/mid 60s.

Monday: The surface high will rapidly shift offshore and another
moderating trend is on tap. Clouds will tend to increase somewhat
as isentropic ascent develops on the warm side of a developing
baroclinic zone from the lower Gulf Coast to North GA. Dry weather
is expected until later in the day when a slight chance of showers
develops well inland and southern zones. Highs should warm back to
the lower/mid 70s.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Pops raised a bit across northwest and northern zones Monday
Night as a series of mid level impulses ride through GA and the
Carolinas producing decent upper forcing across the northern
half of SC. On Tuesday and Wednesday, a strong subtropical ridge
will extend from the southwest Atlantic into the Gulf of Mexico.
Models in general agreement that the bulk of the convective pcpn
will extend west through north of the forecast area and only
slight chance pops appeared warranted. Temps still expected to
be extremely warm with highs near 80 both days away from the
beaches and barrier islands. A cold front will eventually move
through the region during mid week with chances for convective
rains. The timing of the front was still too uncertain for
entertaining any potential for strong/severe storms, neither
the GFS/ECMWF was impressive as yet. High pressure will return
for the latter portion of the upcoming week with slightly cooler
weather.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
VFR through the first part of tonight. Fog and stratus may
develop late tonight, leading to flight restrictions. Confidence
in this occurring is moderate at best, mainly due to increasing
winds just off the surface. As a result, we kept MVFR from 06Z-
15Z. This will need to be adjusted with future TAFs.

Extended Aviation Outlook: Flight restrictions are possible Saturday
morning due to low clouds/fog. Another low risk for periodic
restrictions will occur during the first half of the upcoming week
with lower clouds possible most likely late at night and in
the morning hours beginning Monday Night.

&&

.MARINE...
Rest of today: Dropped the advisory for AMZ352 as seas now
appear to have fallen below 6 ft across all or most of the zone.
Otherwise, the rest of the forecast remains on track. Low
pressure will spin E of FL as a ridge of high pressure to its N
prevails across the local waters. There is enough of a gradient
between these two systems to keep NE winds as high as 15 kt
through much of the day. But the bigger concern is in regards to
the seas in association with the Atlantic low. There is an
excellent and long duration onshore fetch which has caused and
will continue to allow for elevated seas. Small Craft Advisories
remain up for AMZ350 and 374.

Tonight: The Atlantic low passes E of the area this evening,
lifting NE to off the NC coast by 12Z Saturday. Winds will
steadily diminish, dropping below 10 kt, and even seas will
start to subside, albeit slowly. We`re still expecting 4 of 5 ft
within 20 nm and up near 6 or 7 ft further out. We`ll need to
keep a sharp watch on fog/stratus from over land that could
impact the Charleston Harbor late.

Saturday: Gusty sw to w winds are likely along the land/sea
interface and over warmer offshore waters. Elevated seas will
linger closer to the Gulf Stream as a SCA persists over our
20-60 NM GA waters, mainly for seas. A cold front will sweep
offshore Saturday evening with a solid punch of cold air
advection to follow overnight. Additional SCAs may be required as
low level mixing increases and colder drier air punches in.
Despite the air mass change, it looks like more a marginal event
into early Sunday morning.

Sunday: Light flow will develop as high pres settles over the
waters.

Monday through Wednesday: Winds will have a southerly component,
gradually veering to the southwest with time. Advisory conditions
are unlikely as a poor low level mixing environment sets-up over
cooler shelf waters. We will need to watch for potential sea fog
however as dew points surge in the lower and middle 60s by Tuesday.
A cold front will approach the waters on Wednesday and winds and
seas will likely see an uptick as the pressure gradient tightens.

&&

.CLIMATE...
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.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 11 AM EST Sunday for AMZ374.
     Small Craft Advisory until 11 PM EST this evening for AMZ350.

&&

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



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