Area Forecast Discussion
Issued by NWS Charleston, SC

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471
FXUS62 KCHS 252052
AFDCHS

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

.SYNOPSIS...
High pressure will weaken across the area tonight, and gives
way to a cold front that slowly moves through the region Monday
night. High pressure will then build from the north through
Tuesday followed by a warm front lifting north on Wednesday.
Another cold front is expected to move through on Thursday,
followed by high pressure.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Deep layered ridging will weaken slowly tonight as deep cyclone
between the upper Great Lakes and Hudson Bay, Canada lifts NE,
and its associated trough swings through the Plains toward the
mid-MS valley overnight. At the surface a cold front aligned
generally parallel to the flow aloft will remain far off to the
W-NW, while a long distance trough hangs in the area from E/NE
to the W/SW.

Scattered convection associated with the trough (or currently a
gust front) snakes across the SE states from NC into parts of
our forecast counties, then into the NE Gulf of Mexico. This
feature will push further east through this evening, and collide
with the sea breeze and other meso-scale boundaries. With
Normalized CAPE of 0.25 and decent low level lapse rates of
7-8C, plus PWAT of 1.2-1.4 inches, this will allow for at least
scattered showers prior to midnight. DCAPE of 500-1000 J/kg
could allow for some wind gusts of 30-40 mph, but due to the
overall lack of instability and poor mid level lapse rates the
risk for thunder is low enough where we have removed its mention
from the forecast.

During the late evening and overnight hours the risk for showers
will diminish as the atmosphere stabilizes further and there is
a slot of some lower 1000-850 mb moisture that pokes in from the
S/SW. We drop chances down to 20/30 PoPs most communities, even
less closer to the Altamaha River basin.

The advection of low stratus and fog will approach the Altamaha
closer to daybreak Monday, so for now no mention in the tonight
portion of our forecast.

We set record highs at both KCHS and KSAV for February 25th, and
broke the record for the warmest at KSAV for the entire month of
February.

Attempts have been made in the hourly temp grids to account for
rain-cooled conditions where we have scattered PoPs through
this evening, but overall it`ll be another abnormally warm
February night within a moist SW flow. Most lows will be 60-62F,
except for some upper 50s in Allendale County.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: A dampening shortwave will move into the central
Appalachians while a slow-moving cold front moves into the
forecast area during the late afternoon. Incredibly high
Precipitable Water values are forecast, potentially exceeding
the daily climatological maxima for Feb 26th with readings of
1.6 to 1.7 inches. Thus, despite the weakening nature of the
front, we anticipate fairly good coverage of showers ahead of
the boundary as it moves through. We pared back PoPs a bit
during the morning since the best isentropic ascent is expected
to be just inland from the area, however we then show likely
PoPs pushing through from west to east. There could be a small
window of opportunity for a few thunderstorms during the
afternoon as a small region of 500-1000 J/kg SBCAPE and -2 to
-3C LI develops over southeast GA.

Our forcing for ascent diminishes rapidly Monday evening as the
shortwave departs to the north and the front shifts offshore.

Cool, dry high pressure will build in from the north Monday
night and Tuesday. High temps on Tuesday will be around 70 which
is still a few degrees above normal.

A flurry of shortwaves will approach the area late Tuesday night
while a warm front develops over southern SC, then lifts slowly
north on Wednesday. Scattered showers are expected on Wednesday,
especially northwest zones where better isentropic ascent and
upper level forcing will exist. With most zones in the warm
sector much of the day we should see temps rise into the 70s,
potentially as high as 80 far south.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
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 /21Z SUNDAY THROUGH FRIDAY/...
Both sites will remain VFR with gusty SW winds this afternoon;
with any passing -SHRA/SHRA having 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...
Tonight: A 1027 mb high centered SW of Bermuda and stretching
west to near the local waters will weaken somewhat as a gust
front approaches from the nearby land areas of SC and GA. As
this transition occurs and sea breeze influences diminish this
evening, S-SW winds will drop off to less than 15 kt, while seas
hold fairly steady at 2-4 ft with a mix of swells and wind
driven waves.

Conditions are certainly not ideal for sea fog due to the warmer
waters, winds too high early on, and then veering too much to
the SW or W-SW late. Although nothing has formed as of this
time, we will maintain mention of patchy fog beginning late
this evening since all Hi-Res models and the NAM show it
happening to some extent.

A decent northeast wind surge is expected beginning Monday night
behind a cold front. Small Craft Advisories are likely for most
of the waters from Monday night through Tuesday due to a
combination of winds/wind gusts/seas.

Another period of advisories is expected Thursday through much
of next weekend, initially due to strengthening pre-frontal
southerly flow, then a prolonged period of gusty northwest flow
behind a strong cold front.

&&

.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: 87/2018...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: 87/February 25, 2018...previous 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...JRL
LONG TERM...MS
AVIATION...
MARINE...
CLIMATE...



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