Area Forecast Discussion
Issued by NWS Charleston, SC

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843
FXUS62 KCHS 250020
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
720 PM EST Sat Feb 24 2018

.SYNOPSIS...
Unseasonably warm high pressure will prevail through Sunday. A
cold front will move into the region Sunday night and Monday,
pushing south of the area Monday night as high pressure builds
from the north. High pressure is then expected until a warm
front moves through Wednesday or Wednesday night, followed by
another cold front late next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Early this evening: While the overall set up is somewhat
similar to the last few nights, the low level wind field appears
to be much more significant. This will have impacts on fog
potential, which has been held at patchy for all areas. There
was consideration in pulling the mention of fog altogether, but
maintained it for now. The 00z KCHS RAOB measured 16 knots at 1
kft and 22 knots at 2 kft. Furthermore, model soundings show
wind in the lowest 500-1000 feet strengthening to 15-20 knots
late tonight. This should disrupt fog development and result in
less coverage than we have seen in the last few nights.
Otherwise, expect a mild night with temperatures falling into
the low 60s late.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Sunday and Sunday night: Strong deep layered ridging will hold
firm, weakening slightly at night as mid level perturbations
knock down heights a little as they advance within the W-SW flow
aloft, and a cold front aligned mostly parallel to the flow
aloft approaches from the W-NW. It`ll be another abnormally warm
February day, and with H8 temps of 1 to almost 2 standard
deviations above normal (12-14C), we should easily exceed record
highs for the date. In fact we might even approach the warmest
on record for February, especially at KSAV if we obtain enough
insolation. Sufficient mixing will allow for S- SW breezes
during the heating of the day, with gusts of 20-25 mph at times.
In advance of the cold front at night temps will struggle to
drop to even the lower 60s, or about 15-20F above the normal
low.

Regarding our chances for rainfall, the better forcing and
dynamics remain off to the NW during the day Sunday, and with
main moisture axis also outside the CWFA, we have lowered PoP a
bit from the earlier forecast. For Sunday we have PoPs capped at
30-40% in the afternoon along the NW tier, 20% or less most
elsewhere, including the Charleston, Beaufort and Savannah
areas. A few coastal showers could skirt parts of SC in the
morning, but nothing more than 14% probabilities. We do increase
chances a bit at night as the front oozes closer, there are
some impulses aloft and moisture is a little better. Instability
is paltry and lapse rates are poor, so confidence on any
thunder is low through the period. But since there was already
thunder mentioned in the forecast we have maintained it, just
shrunk the slight chances to late day and evening far inland.

Monday and Monday night: The cold front makes slow progress
through the forecast district, as a a stronger short wave
passing by to the north late in the day and off the mid-Atlantic
coast at night. The orientation of the front is still generally
parallel to the flow aloft, so this transition will not be
abrupt, but does become more noticeable during Monday night as
strengthening high pressure builds from the OH valley, PA and
the Virginia`s. Deeper moisture and PWat in excess of 1.5 inches
or 2 to almost 3 standard deviations above normal, plus larger
forcing from a coupling upper jet structure in the southern
Appalachians suggests an even greater potential for showers. For
now we have 50-70% chances Monday, but locally higher PoPs are
possible as trends become better defined. There is actually a
little better instability than on Sunday, so we do have a slight
chance of thunder to the south and east. Rain chances drop off
from N/NW to S/SE at night as the cold front finally works its
way south of the area and drier mid level air moves in. Monday
will again be far above normal, but due to the higher rain
chances and lower heights aloft it`ll be on average about 6-12F
lower than on Sunday. Cool advection at night will be noticeable
as the synoptic flow clocks around to the NE and speeds
increase along the coast. Lows will be at their lowest since mid
month.

Tuesday: Flat ridging or a zonal flow develops aloft as surface
high pressure advances toward the mid-Atlantic coast. A weak
coastal trough will form offshore, maybe generating a few light
showers onshore of coastal SE GA, but otherwise large scale
subsidence will supply us with a rainfree day. Onshore low level
trajectories will maintain a feed of maritime moisture, and
this poses our problem of the day in regards to cloud cover and
the resulting high temps. We show more sunshine north and a
greater amount of clouds south, with temps mainly within a
couple of degrees either side of 70F with both H8 and low level
thickness above normal.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
High pressure to our north will move offshore Tuesday night. A
weak warm front may develop over the area Wednesday, quickly
moving north Wednesday night. A cold front will then approach
from the west, moving through Thursday or Friday. Highest
chances of reductions will likely be closer to 12Z but given
overall low confidence we maintained VFR conditions for now.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
The main forecast issue is once again fog potential late
tonight. The difference from previous nights is that winds in
the 500-1000 ft layer will strengthen into the 15-20 knot range,
which should make it difficult for fog to develop. Model
guidance seems to favor stratus development, which makes sense
with the low level wind field, but there are differences on how
low ceilings could be. The forecast features TEMPO MVFR ceilings
around sunrise, then VFR thereafter. Winds will become more
southwesterly in the afternoon and gusts up to around 20 knots
will be possible.

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

&&

.MARINE...
Tonight: Atlantic high pressure will continue offshore and
maintain a mostly SW wind across the local waters, mainly 15 kt
or less. Seas will continue elevated given the SE swell but stay
just below Advisory levels across far eastern portions of the
offshore waters. Could see sea fog develop just about any time
given the more favorable southerly winds, mainly across the
cooler nearshore waters.

Sunday and Sunday night: The coastal waters will lie on the
western portion of strong sub-tropical ridging, blocking an
upstream cold front from reaching the area during this time.
Warm advection will temper the amount of mixing, so S-SW winds
will generally hold at or below 15 kt, with seas at best 3-5 ft.

Sea fog will exist in some form through the period with
favorable trajectories, except speeds might be a little too
high. Thus we have limited the coverage to "patchy" fog as of
this time. If winds are less than coverage could be greater and
dense fog would be a concern. We will maintain mention in the
Hazardous Weather Outlook.

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 highs for February 24:
KCHS: 81/2017 *broken today*
KSAV: 86/2012
KCXM: 81/1930

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

Record highs for February 25:
KCHS: 81/2017
KSAV: 82/1985
KCXM: 80/1930

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...BSH
SHORT TERM...33
LONG TERM...MS
AVIATION...BSH/33
MARINE...RJB/33
CLIMATE...



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