Area Forecast Discussion
Issued by NWS Charleston, SC

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818
FXUS62 KCHS 271749
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1249 PM EST Thu Feb 27 2020

.SYNOPSIS...
High pressure will prevail through Friday, with a weak and dry
cold front expected Friday night and early Saturday. High
pressure will again prevail into early next week. A cold front
could approach during the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
High pressure centered over the northwest Gulf of Mexico will
extend across the area today. Aloft, broad troughing will
prevail over the eastern half of the country. Deep dry air in
place as noted off the 12z KCHS RAOB will maintain clear skies
and quiet weather. Low level thickness values support high
temperatures in the mid 50s. Ongoing forecast was in good shape,
so only minor tweaks were needed with the early afternoon
update.

Tonight: Strong high pressure over central Texas will hold its
influence on the region tonight as it propagates southeast into
the northwestern Gulf of Mexico. With the center of the
anticyclone forecast to remain so far to the west and southwest,
there looks to be enough of a lingering pressure gradient
across the Southeast U.S. to prevent a full decoupling of the
boundary layer overnight, except in the most sheltered spots.
Lows will drop to near or just below freezing for many areas
away from the coast with mid-upper 30s at the beaches and
locations immediately adjacent to larger rivers, the Santee-
Cooper lakes, harbors, and sounds. A few upper 20s can not be
completely ruled out in sheltered spots (noted the 27/00z GFS
CO-OP guidance showing 28 at Allendale).

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
Friday through Sunday: A closed upper level low across Quebec,
Canada on Friday shifts into northern maine and New Brunswick by
late Saturday, then finally through Nova Scotia Sunday. Our
area will lie under the broad trough and associated cyclonic
circulation around this feature late this week, before the the
trough moves into the Atlantic late Saturday and Sunday,
allowing for ridging to move in from the west. At the surface,
high pressure along the coast of southeast Texas during the
beginning of the period travels across the northern Gulf of
Mexico at the start of the weekend, reaching overhead on Sunday.
A dry atmosphere, and large scale subsidence will promote
rainfree conditions all three days, despite a weak cold front
that pushes through Friday night and early Saturday.

Temps will be far below climo Friday into early Sunday, with the
850 mb 0C isotherm near or into northern Florida. Warm advection
however begins on Sunday, and that along with rising heights
and the air mass moderating, will allow for temps back near
normal for daytime highs.

The most important aspect of the upcoming short term forecast
is the start of our Frost/Freeze program that begins again for
the spring as of March 1st. Excellent radiational cooling will
transpire for Saturday night into early Sunday morning, with
the potential for a freeze along our northwest tier of counties,
plus scattered to widespread frost in locations near and west
of US-17. A Freeze Warning and/or a Frost Advisory will be
required at a later time for this upcoming cold weather event.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Quasi-zonal flow or flat ridging will prevail aloft early next week,
as an upper low/trough moves from the Southwest and northern Mexico
toward the Rio Grande Valley Tuesday. That feature dampens out
Wednesday, before a phased trough develops over the central states
and heads towards the Appalachians next Thursday. Meanwhile, surface
high pressure will cover much of the southwest and west Atlantic,
which holds back an upstream cold front until late in the forecast
period. There is considerable warm advection in advance of the
front, which leads to temps climbing far above normal. The main
forecast challenge will be the timing of the next front, but
considerable moisture will eventually return in association with the
front. With significant forcing for ascent and strong shear, there
is a concern for severe weather or at least t-storms late in the
current forecast period.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
VFR. Gusty winds will diminish by late afternoon, although will
will pick up again late in the TAF cycle.

Extended Aviation Outlook: VFR. Breezy conditions Friday and
Saturday.

&&

.MARINE...
Today: Winds will steadily diminish across the waters today as
high pressure builds in from the west and cold air advection
wanes. Northwest winds will average 10-15 kt nearshore with
15-20 kt offshore this afternoon. Seas will average 2 to 3 ft
nearshore, and 4 to 6 ft beyond.

Tonight: There are signals that a weak surge could develop over
the waters tonight as pressure and thermal gradients tighten
slightly with high pressure still displaced well to the west and
southwest. Northwest winds could peak as high as 15-20 kt, but
speeds and gusts look to remain below Small Craft Advisory
thresholds at this time. Seas will average 1-2 ft nearshore
waters and 2-3 ft offshore waters.

Friday through Saturday: High pressure centered in the Gulf of
Mexico will be the dominant synoptic feature into the start of the
weekend, interrupted only a weak cold front Friday night and early
Saturday. It is during the time frame behind the front  that we have
the best chances of experiencing Small Craft Advisories.

Sunday through Tuesday: the core of high pressure will move over the
local vicinity late Sunday, before moving toward Bermuda thereafter.
Good marine conditions on Sunday will slowly deteriorate Monday and
Tuesday as the gradient starts tightening on the back side of the
high. Although there is the advection of warmer and more humid air,
wind fields don`t appear to be favorable for sea fog at this time.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 5 PM EST this afternoon for AMZ374.

&&

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



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