Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 250922
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
422 AM EST Sat Feb 25 2017

.SYNOPSIS...
A cold front will move through the area late today, with colder
high pressure to build over the southeast later tonight into
Sunday. The high will shift offshore early next week as a warm
and moist air mass builds from the Atlantic and Gulf of Mexico.
Another cold front will move through Wednesday night into
Thursday, followed by high pressure for the end of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Early this morning: A large and expanding area of stratus and
fog covering much of SE GA will progress into parts of SC,
leading to areas of fog, some locally dense through 9 am or so.
There is likely too much wind within the boundary layer to
prevent anything worse than patchy dense fog, but visibilities
where fog does occur will generally be as low as 1/2 to 1 mile.

Today: A weak ridge hugging the SE coast at the start of the day
will push offshore in advance of a cold front near the
Appalachians early on, that reaches into our CWFA around 18Z,
before it passes offshore around 21z or shortly thereafter.
There isn`t much chance of showers in association with the
front, with shallow moisture, a strong inversion between 800 and
700 mb and little to no upper dynamics. However, there are
subtle mid level height falls and low level lapse rates are good
by February standards. But updrafts will have a difficult time
breaking the inversion, so we don`t foresee anything more than
isolated showers across interior areas from noon to 3 pm.

Compressional heating in advance of the cold front and a
westerly flow through about the first 2 miles or so of the
troposphere, plus 925 mb temps near the maximum for this time
of year, will allow for temps to climb to the upper 70s and
lower 80s most places. the warmest will be parts of the
Charleston tri-county district, where 84F will occur. This will
easily exceed the daily record at KCHS, and will also approach
the February highest on record. (see CLIMATE section below). The
cool SST`s of the ocean will limit max temps along coastal
sections to the upper 60s/lower 70s north, lower-mid 70s south.

In addition to the record warmth, gusty SW-W breezes will occur
during the afternoon with deep mixing allowing for speeds to
reach 15-25 mph, with gusts as high as near 30 mph in parts of
the CWFA.

Tonight: Cold advection quickly fills in behind the departing
cold front in the Atlantic, as skies become clear. A gigantic
diurnal swing in temps will transpire, with lows by Sunday
morning down some 40F colder than they will be today. Tonight
will be the coldest night for many of us in about a week, as
high pressure builds in from the W.

Lake Winds: Deep mixing this afternoon, followed by CAA late
today into tonight, will lead to  marginal Lake Wind Advisory
conditions on Lake Moultrie this afternoon into early tonight.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
Sunday: High pressure will become centered over the Southeast,
leading to all day sunshine and light wind. Overall, temps will be
closer to normal for this time of year. In general, high temps will
range in the low/mid 60s, warmest away from the coast in Southeast
Georgia.

Monday: Dry high pressure centered over the Southeast will slide
offshore, allowing a south/southeast wind to return moisture and
warmer temps to the region. We think the morning should be dry
but conditions will be changing rather rapidly Mon afternoon as deep
layered moisture builds in from the Atlc and Gulf of Mexico ahead of
an advancing baroclinic zone along the Gulf Coast and northern GA.
Warmer temps will be accompanied by increasing chances for showers
over southeast GA and perhaps isolated thunderstorms over south GA.
Scattered showers could even reach southeast SC later in the day
around sunset.

Monday Night: Unsettled weather becoming more likely each forecast
package. A series of mid level impulses will ride over a zone of
deeper moisture and low level moisture convergence producing
scattered to locally numerous showers spreading northward along a
warm frontal feature. We bumped pops up to 50 percent many areas
with some likely POPs along coastal SC to trend. Lows will be quite
mild in the 57-60 degree range most areas.

Tuesday: The rains on Monday night should have moved off to the
northeast, setting the stage for a warm day. Warmest temps will be
over southeast GA with lower to mid 80s common under partly sunny
skies in the afternoon. There should be a bit more cloudiness over
northern SC zones but highs will still reach the upper 70s to around
80. Most of the convective precip will be west of the region but an
isolated shower or late day tstm is possible to the north of I-16.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
A long wave upper trough will advance from the Great Plains to the
Tennessee Valley Tuesday night through Thursday. There is decent
model agreement that an advancing cold front will reach the forecast
area either late Wednesday night or early Thursday with the GFS
running just a tad faster than the ECMWF. March will come in with
potential record warmth given 13C-15C 850 MB temps in southwest
synoptic flow. High temps could soar to the mid 80s with periodic
breaks in the clouds. Higher rain chances should come ahead of the
cold front either Wednesday night or early Thursday. Despite
increasing deep layered shear on Wednesday and Wednesday night,
severe potential still looks rather low-end due to model differences
in frontal timing and marginal instability. In the wake of the
front, cooler and closer to climo temps are expected Friday and
Saturday with dry weather both days.

&&

.AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
Areas of fog/stratus will impact both airfields through mid-
morning, with flight restrictions to occur. Conditions will be
down to IFR and even LIFR, with airfield minimums possibly
being obtained. VFR will return by 14Z and prevail the rest of
the valid 06Z TAF cycle.

A cold front could produce a passing shower at either site
between 17Z and 20Z in advance of a cold front, but no
significant impacts would occur. Of more concern will be gusty
W-SW winds that occur both in advance of a cold front, but more
especially behind the front through sunset. Speeds will reach
15-20 kt with gusts around 25 kt during the afternoon hours.

Extended Aviation Outlook: VFR conditions are expected into Monday
morning. Another risk of flight restrictions is possible late Monday
and Monday night with low clouds/showers. Low clouds are also
possible Tuesday and Wednesday Mornings with both afternoon likely
VFR. A cold front will move through Thursday, brief restrictions
possible in clouds/showers.

&&

.MARINE...
Areas of fog will impact much of the coastal waters within 20
nm, including Charleston Harbor and the intra-coastal through
mid to late morning. The combination of sea fog within a light,
warm and very humid S-SW flow, plus low stratus and fog from off
the land areas, will lower visibilities down under 1 nm. We
continue to maintain a close watch for a Marine Dense Fog
Advisory, but for now a Marine Weather Statement is in effect
for the reduced visibilities.

In the larger scale, weak high pressure overhead to begin the
day gives way to a cold front that moves through around 4-9 pm.
warm advection out ahead of the front limits mixing, but the
land/sea interface of Charleston Harbor will give a boost to Sw
and W winds this afternoon. We`ll be close to requiring a Small
Craft Advisory, but for now have limited winds to 15-20 kt. Seas
on the AMZ350 and AMZ374 waters are still as high as 6 ft early
this morning, as reliable buoy reports and an earlier observation
from the Charleston Pilot Boat indicating the bulk of the wave
energy is from E`erly swells.

Cold advection behind the front tonight and decent isallobaric
pressure rises on will lead to possible SCA`s on all waters,
with the exception of Charleston Harbor. To avoid confusion with
the ongoing SCA`s we have deferred to later shifts to hoist any
other advisories.

Sunday through Wednesday: Marine conditions will improve early
Sunday as high pressure builds over the area. The SCA over outer
GA waters will likely come down by mid morning. Return flow on the
back-side of Atlc high pressure will result in a southeast to
southwest flow regime during the early to middle part of next week,
advisory conditions are unlikely. Patchy sea fog is possible Tuesday
into mid week but it`s too early for any mentions in the forecast
just yet.

A cold front moving into the waters on Thursday should result in
an uptick in wind and seas with unfavorable mixing profiles until
cold air advection ensues post front. Advisory conditions may be
possible.

&&

.CLIMATE...
High temperatures this afternoon could approach record levels
for the month of February: 86F at KCHS (Feb 28 1962) and KSAV
(last set Feb 24 2012) and 83F in downtown Charleston (Feb 27
1962).

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

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Lake Wind Advisory from 1 PM this afternoon to 11 PM EST this
     evening for SCZ045.
MARINE...Small Craft Advisory until 9 AM EST Sunday for AMZ374.
     Small Craft Advisory until 7 AM EST this morning for AMZ350.

&&

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



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