Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 250207

Area Forecast Discussion
National Weather Service Charleston SC
1007 PM EDT Sat Mar 24 2018

A weak warm front will drift north across the forecast area
through this evening. A strong backdoor cold front will push
southward through the entire area on Sunday, followed by cool
high pressure into mid week. Warmer conditions arrive towards
the end of the week before another cold front pushes through
the area Friday. Dry high pressure will then return over the


As of late Saturday evening the warm front has moved little from
its position the past few hours, roughly running near a line
from Allendale to Beaufort. It will gradually make some
northward progress through the post-midnight period, perhaps as
far north as Berkeley County, before it swings back southward as
a cold front late. This occurs in response to a developing wave
of low pressure off SC Grand Strand region 09-12Z that occurs as
a short wave in eastern TN dives SE, allowing the front to reach
near or not far south of the Savannah River by daybreak. We
continue to slow the onset of the upstream convection by
another hour or so from the previous forecast, and also
significantly trimmed back PoP coverage south of I-16 through
the overnight period to generally no higher than 20-30%.
Elsewhere we did maintain PoP as high as 60-80%, with the bulk
of that occurring after 3 or 4 am with PVA aloft, upper
difluence from a 90-100 kt jet following the short wave and our
best isentropic ascent.

There is still only a very minimal chance of a little thunder
and lightning due to negative Showalter values and decent mid
level lapse rates. However, some of the MOS guidance does have
30-40% probabilities of thunder, so we might add to the forecast
at a later time pending trends.

Our diurnal temp curve which has shown a typical decline late
Saturday evening will slow or even rise overnight as cloud cover
increases and thickens, allowing for temps to steady out or even
rise a bit. Then toward morning another fall will occur in
places behind the front. Overall though our minimum temps will
be above late March averages.

Previous discussion...
Although the sea breeze is obscuring the exact position on the
warm front, it appears to be located north of a line from
Beaufort to Allendale as of early evening. There has been a few
sprinkles or virga near the front along the CSRA, but given such
large dew point depressions the risk of anything measurable
this evening is close to zero.

The latest Hi-Res guidance suggests that the onset of the
showers will be delayed by about 1-3 hours from the previous
forecast, and we have thus slowed down the beginning of any
rains until after 1 or 2 am, perhaps even a little later if the
slower trend continues. However, we did not alter the late
night/p[re-dawn coverage of showers, still showing 12 hour PoP
as high as 60-80% north of I-16 in GA, with 50% PoP so of there
through 12Z Sunday.

Probability of thunder/lightning is still low enough where we
have not added to the forecast, but given Showalter values less
than zero and decent mid level lapse rates around 6C/km, the
risk is non-zero.


Sunday: The mid/upper levels will consist of a trough off the
Northeast Coast and a ridge with it`s axis stretching from the
Great Lakes Region into the Lower MS Valley. Both features will
be amplifying during the day and night. Generally, flow over our
area will be from the northwest. At the surface in the morning,
there will be two lows, one roughly over TN and the second off
near the Outer Banks of NC. Attached to these lows will be a
cold front. As time progresses, the TN low will move southward
and dissipate into the evening. The other low will rapidly move
to the ENE. Additionally, the cold front will move southward,
passing through our area during the day, then becoming
positioned to our south by the evening. Very strong high
pressure north of New England. It`s southern periphery will help
to push the aforementioned front southward. There will be
plenty of moisture in place with the front, with PWATs ~1.4".
Lift from the front is decent, enhanced a bit by waves moving
near or overhead and some jet stream support. The result will be
showers for the entire area, moving southward with the front in
time. Though, models indicate they may hang around the southern
portion of our forecast area into the overnight period. Hence
we have higher POPs persisting here into the night. Measurable
rainfall is expected across the entire area. There is very
little, if any instability with the front, so we`re not
forecasting any thunderstorms. Temperatures will be highly
dependent on the position and southward movement of the front.
The bust potential is high. The current thinking is highs in the
far northwest tier in the mid/upper 50s, mainly on the back end
of the front. Further south, highs could be well into the 60s
with maybe even a few 70s near the Altamaha. These will probably
be morning highs, with temperatures falling in the afternoon.
Lows will generally be in the 40s, lowest in the far NW tier and
highest at the beaches and near the Altamaha.

Monday: The mid/upper levels in the morning will consist of a
trough off the Northeast Coast and a ridge with it`s axis
stretching from the Great Lakes Region into the Lower MS Valley.
By night, the trough will move be pushed offshore as the ridge
moves eastward. The ridge will be amplifying, with it`s axis
generally located from the northeast Gulf of Mexico stretching
northward, in lee of the Appalachians, all the way into New
England. The flow over our area will be from the northwest,
shifting to the north northwest. At the surface, very strong
high pressure will be moving SSE from Canada, making it into New
England by night. A weak high may try to form in the Mid-
Atlantic region, but the southern periphery of the initial high
will stretch into our forecast and impact the weather. Models
indicate >1" PWATs across our southernmost forecast area in the
morning, getting pushed southward by drier air building in from
the north. Models indicate some showers sticking around in the
southern portion of our forecast area, despite the front being
far away and no impressive forcing nearby. Hence we have
elevated POPs here tapering northward. POPs also decrease into
the afternoon. Highs will be more comparable to January due to
the cold air advection within northeast surface winds,
struggling to reach the upper 50s in most locations. Lows will
be a bit closer to normal, aided by the mostly cloudy skies.

Tuesday: The mid/upper levels will consist of an trough off the
East Coast and a ridge with it`s axis stretching from the
northeast Gulf of Mexico stretching northward, in lee of the
Appalachians, all the way into New England. The flow over our
area will be from the north northwest. At the surface, very
strong high pressure over New England will move out to sea.
However, high pressure in the Mid-Atlantic region will
strengthen, with it`s southern periphery stretching into our
forecast area. Subsidence and plenty of dry air will lead to a
dry forecast. Highs will be warmer as winds shift to the east,
but still several degrees below normal.


A wedge of high pressure will slowly weaken and shift offshore
Wednesday, supporting southerly winds well in advance of low
pressure developing over the South-central United States through
Thursday. The pattern will likely result in a noticeable warmup
Wednesday and Thursday, with highs approaching the mid/upper 70s
Wednesday afternoon and upper 70s/lower 80s Thursday afternoon.
Overnight lows will also favor a warming trend, initially dipping
into the upper 40s/lower 50s Tuesday night, then mid/upper 50s
Wednesday night, followed by upper 50s/lower 60s Thursday night. The
next chance of precip, including a few thunderstorms should come on
Friday as a southerly wind advects moisture over the Southeast in
advance of a cold front shifting through the region. Temps should
peak in the mid 70s for most areas. Dry and slightly cooler weather
should then return on Saturday as high pressure builds into the area
behind the departing front.


A warm front already north of KSAV will lift north through KCHS
early tonight, before a wave of low pressure along the front off
the NC coast overnight allows for the front to swing back south
as a cold front that passes through the terminals late tonight
and early Sunday. In advance of the front VFR weather will
essentially dominate, but as the front moves into and through
the area early Sunday, lower ceilings and light to moderate
rains and drizzle will develop. This eventually leads to MVFR or
IFR conditions after 11Z at KCHS and after 13Z at KSAV.

There is a low end risk for -TSRA with the frontal passage, but
not enough to include with the latest TAF set.

Also, a strong 40-45 kt low level jet will cross the area around
06-12Z, and this might support a marginal LLWS potential.

Extended Aviation Outlook: Flight restrictions are likely at both
CHS and SAV terminals through Monday with a high pressure wedge
firmly in place, resulting in low clouds and perhaps a little
light rain or drizzle at times. VFR conditions should return at
both terminals Tuesday and persist through Thursday.


Tonight: Latest sfc analysis indicated that a warm front was
attempting to lift into our Charleston County waters, and it
still could briefly clear much of those waters overnight, before
the front slides back southward as a cold front toward daybreak.
due to as low pressure forming off the SC Grand Strand. Winds of
SE and S early on at or below 15 kt will veer to the SW at 15-20
kt late overnight, with some gusts reaching the lower 20 kts.
Wave heights are expected to build from 2-3 ft up to 3-4 ft
across the near shore waters (a few 5 footers over AMZ350) and
generally 4-5 ft across the outer GA waters.

Sunday through Thursday: A backdoor cold front will quickly shift
over the waters on Sunday, supporting conditions that deteriorate
from north to south as strong cold air advection and a pinched
pressure gradient develop over the Southeast. Small Craft Advisories
are likely for all waters by Sunday night, which should then persist
into Monday night/early Tuesday. North/northeast winds should gust
as up to 25-30 kt while seas build to 5-7 ft in nearshore waters and
7-10 ft in offshore Georgia waters. A period of Gales is possible
for some of the waters Sunday night into early Monday. Conditions
are expected to improve Tuesday as high pressure expands over the
area and the pressure gradient relaxes, but high seas could linger
in offshore Georgia waters through midweek. Elsewhere, winds/seas
should then remain below Small Craft Advisory levels Wednesday and
Thursday well ahead of a cold front advancing toward the region
early next weekend.


MARINE...Small Craft Advisory from 2 PM Sunday to noon EDT Tuesday for


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