Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 251806
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
106 PM EST Sun Feb 25 2018

.SYNOPSIS...
High pressure will weaken through tonight. A cold front will
slowly approach from the west on Monday, moving through the
region by Monday night. High pressure is forecasted to pass to
the north Tuesday, followed by a warm front moving northward
through our area Wednesday. Another cold front is expected to
cross Thursday, followed by high pressure.

&&

.NEAR TERM /THROUGH TONIGHT/...
Rest of this afternoon: The upstream squall line has weakened
but is nonetheless continues to advance toward the forecast
counties from the W-NW, as Normalized CAPE has risen to 0.2 to
0.25, and low level lapse rates aided by dew points in the lower
60s and temps in many areas 80-85F are as high as 7.0-7.5C. We
are also seeing a few showers forming over the Charleston tri-
county district as the resultant sea breeze forms and where we
also find some favorable low level lapse rates. We have upped
PoPs to 40-60% from northern Dorchester to Jenkins County, and
maintained 20-30 PoP most elsewhere due to boundary interactions
and some activity sliding in from southern GA. There remains
only a low potential for some t-storms along our far NW tier as
there just isn`t much instability and the mid levels are
unfavorable. Severe weather is highly unlikely, but due to DCAPE
of 500-900 J/kg there can be some gusts of 30-40 mph as the
remnant squall line arrives during the mid afternoon.

We have increased cloud cover over far western zones and as a
result lowered temps a couple of degrees. But further east we
have had to raise temps about 1-2F per latest trends. We have
already broken the record high for the date at both KCHS and
KSAV, and both KCHS and KSAV will flirt with the warmest day on
record for all of February. Shoreline communities will be quite
a bit cooler, especially from Tybee Island north where the sea
breeze will hold readings down in the upper 60s or lower 70s.

Deep mixing tapping into 20-30 kt of winds will result in breezy
S-SW surface winds, with gusts of 20-25 mph to frequently occur.

Tonight: Various interactions between mesoscale boundaries and
the remnant resultant sea breeze during the evening hours may
result in isolated to scattered showers on the fringe of the 1.5
inch PWATS across our western and northern zones. To the south
of I-16, any spotty showers will be harder to come by as upper
forcing and deeper moisture remain off the north. We trended
POPs in the middle of the road toward the ECMWF which was a
general compromise between the wetter GFS and drier GEM/NAM.
While the stalled front to our west awaits a developing surface
wave along the Gulf Coast region, we will see another moist and
mild night with light southwest flow. We considered adding some
patchy late night fog to the forecast but with a complex
mesoscale and uncertain cloud cover, we held off on the mention
but it may need to be included at some point. Lows will mainly
be in the 60-62 degree range.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Monday: The mid/upper levels will consist of strong high pressure
over Southern FL and the Bahamas in the morning, shifting into the
southern Gulf of Mexico overnight. The result will be southwest
winds over our area in the morning, becoming westerly in zonal flow
overnight. At the surface, a cold front will be just to our west
during the day, crossing through in the evening or overnight. PWATs
will be ~1.5" ahead of the front, which is almost 3 standard
deviations above normal per NAEFS. POPs reflect the expected showers
during the day, diminishing overnight with frontal passage.
Measurable QPF is expected for the entire area. There is some
instability, so a slight chance of thunderstorms are forecasted in
the afternoon. Ahead of the front, temperatures will remain well
above normal.

Tuesday: The mid/upper levels will consist of zonal flow over the
Southeast. At the surface, a cold front will be over the coastal
waters in the morning, quickly departing. At the same time, high
pressure will be centered to the north over the Mid-Atlantic states.
The high will bring dry conditions. Thickness values still support
high temperatures above normal. Showers are possible overnight.

Wednesday: The coastal trough will merge with a developing warm
front, which will move northward through the area during the day.
Showers are expected as PWATs rise well above normal.
Temperatures will also be above normal.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
A warm front will move north Wednesday night. A cold front will
then approach from the west, moving through Thursday night.
High pressure will then follow.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Both sites will remain VFR with gusty SW winds this afternoon;
with any passing -SHRA/SHRA not have little to no impacts. A
better chance for -SHRA/SHRA will exist early tonight as
boundary interactions occur, and there is a little more forcing
aloft. Brief flight restrictions may occur. later tonight into
Monday morning the bulk of the convective rains will shift out
of the area, and as drier air in the lower levels moves in there
will be the expansion of MVFR/IFR conditions, especially at
KSAV.

Extended Aviation Outlook: Flight restrictions are expected
with a slow moving cold front late Sunday night into early
Tuesday.

&&

.MARINE...
The only important changes to the forecast were to increase
winds slightly in Charleston Harbor due to better mixing at the
land/water interface, showing winds similar to what they have
been this morning in the 15-20 kt range. Occasional gusts will
approach 25 kt, but not frequent enough to hoist an advisory.
The rest of the forecast is in excellent shape, as we maintain
watch for the possible formation of sea fog later today and
tonight as wind speeds become more favorable.

Previous discussion...
Through tonight: A cold front to the west will slow it`s
progression and become less defined with time, remaining
well inland from the waters. SW to S flow will continue
over the marine forecast area with a poor low-level mixing
environment over cooler shelf waters. We expect wind speeds
mainly 15 kt or less. It could become a bit gusty for a period
this afternoon along the intra-coastal waterways of Charleston
County in the vicinity of the resultant sea breeze boundary.
Patchy sea fog is possible tonight over the cooler shelf waters
with a decent formation regime in the evening, looking less
favorable nearing daybreak as low level wind fields increase
and veer more southwest.

Monday through Tuesday: The cold front will gradually advance
through the waters, passing to the south of the area late Monday
night, as continental high pressure from the north into Tuesday.
Cool advection, steady isallobaric pressure rises and modest
pinching will likely support Small Craft Advisories developing
Monday night and continuing into Tuesday. Some fog will persist in
advance of the front, more as stratus clouds behind the front.

Wednesday through Thursday: Surface high pressure slides into the
Atlantic and further offshore, allowing for a cold front to approach
the SE late in the period. Winds and seas will be beneath advisory
levels Wednesday, but a much stronger S-SW gradient will exist on
Thursday, and SCA`s could again be common.

&&

.CLIMATE...
Record high minimums for February 25:
KCHS: 62/1992
KSAV: 63/1992
KCXM: 61/2017

Record highs for February 25:
KCHS: 86/2018...previous 81/2017
KSAV: 84/2018...previous 82/1985

Record high minimums for February 26:
KCHS: 62/1939
KSAV: 63/1939
KCXM: 62/1957 and previous

Record highs for the month of February:
KCHS: 87/February 16, 1989
KSAV: 86/last set February 24, 2012
KCXM: 83/February 27, 1962

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

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


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