Area Forecast Discussion
Issued by NWS Charleston, SC

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631
FXUS62 KCHS 220643
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
143 AM EST Wed Nov 22 2017

.SYNOPSIS...
Low pressure will pass off the Southeast coast today and
tonight. High pressure will build in from the west Wednesday
before another low pressure system moves up the coast Wednesday
night and Thursday. A cold front will shift through the area
Saturday night, followed by dry high pressure through the
middle of next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
As of 135 AM: Rain has generally pushed north and east of the
land based forecast area. Clearing in the wake of the rainfall
has created favorable conditions for low stratus and fog,
especially across SE GA. HRRR sfc condensation pressure deficits
indicate area of less than 5 mbs across SE GA and portions of SC
through the early daylight hours Wed. The environment of an
approaching sfc warm front from the south will favor low stratus
and fog this morning. Given the latest observations of dense
fog across SE GA and near term guidance I will post a Dense Fog
Advisory for SE GA. It is possible that the advisory may expand
east into SE SC later this morning.

Previous discussion:
A quick update was issued to expand dense fog wording a few
counties west in interior Southeast Georgia. Webcams and surface
observations out of Claxton and Statesboro indicate stratus has
quickly transitioned to locally dense fog. A Special Weather
Statement has also been issued to address this.


Shower activity will steadily wind down over the next few hours
as low pressure near the Savannah River entrance moves northeast
and away from the region. Drier conditions will ensue as
DNVA/subsidence overspreads the region in the wake of shortwave
energy currently moving across south-central Georgia. Adjusted
near term pops to match going radar trends, but expect most
shower activity to be offshore by 06z, if not sooner.

Attention shifts to the potential for low stratus and fog.
Conditions appear to favor a stratus build-down event with low
clouds gradually transitioning to areas of fog after midnight.
The highest dense fog probabilities are currently clustered
along the Georgia coast and up into the Beaufort and Hilton Head
areas, including the Savannah Metro Area and have been this way
since late afternoon. Winds look to remain the lightest in this
area as low pressure exits the region. Maintained areas of fog
in the forecast for now, except patchy well inland, but opted to
introduced areas of dense fog across parts of Bryan, Liberty,
McIntosh and Long counties were confidence on the development of
more widespread dense fog becoming is the highest. A Dense Fog
Advisory may very well be needed for some areas later tonight.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH FRIDAY/...
The main axis of a mid/upper lvl trough will shift over the Mid-
Atlantic and Northeast states to start off Wednesday, while an h5
shortwave meanders over the Gulf of Mexico. Given the setup, dry
high pressure will prevail through much of Wednesday, until an area
of sfc low pressure develops over the Gulf of Mexico and tracks
northeast Wednesday night into Thursday. There remains quite a bit
of uncertainty in regards to the exact track of the low pressure
system as it passes along/near the Southeast coast. However,
guidance suggests at least chances of showers developing over
southern areas starting Wednesday night before spreading northeast
into Thursday. At this time, greatest precip coverage should occur
over coastal counties Thursday morning into early Thursday afternoon
when PWATs approach 1.25 to 1.5 inches and isentropic lift peaks.
Precip should gradually shift offshore Thursday night into early
Friday as a h5 shortwave shifts offshore and the low pressure system
continues advancing to the northeast.

High temps will peak around 70 degrees on Wednesday in advance of
the low pressure system, but conditions will be considerably cooler
on Thursday due to showers, cloud cover and a northerly flow. Highs
should only peak in the mid/upper 50s Thursday afternoon. By Friday,
temps should rebound into the lower 60s under partly cloudy skies.
Overnight lows will generally range in the upper 30s/lower 40s
inland to upper 40s/lower 50s near the coast each night.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
A deep upper trough will swing through from the northwest over
the weekend, pushing a dry cold front through the area Saturday
night. Cold advection in its wake will result in below-normal
temperatures, especially Monday. However, as an upper ridge
builds and the surface high shifts east, temps will rebound by
Tuesday.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
Rain has generally pushed north and east of the terminals.
Clearing in the wake of the rainfall has created favorable
conditions for LIFR to VLIFR stratus and fog, especially across
SE GA. KSAV should see very restrictive ceilings and vis through
the early daylight hours, improving to VFR by 16Z. KCHS may see
a greater issue with lowering stratus than with dense fog
through 14Z. Winds should remain light early this morning, with
north winds between 5-10 kts during the daylight hours.

Extended Aviation Outlook: MVFR cigs are possible at both terminals
Wednesday afternoon, but are more likely to occur late Wednesday
night into Thursday night due to low clouds and showers associated
with a passing low pressure system along/near the Southeast coast.
VFR conditions should prevail at both terminals this weekend.

&&

.MARINE...
Areas of fog are expected to drift into the coastal waters
overnight as fog banks developing inland. Best chances for fog
will be from Edisto Beach south. Vsbys could drop below 1 nm at
times, especially along the Georgia coast. Marine Dense Fog
Advisories may be needed.

Tonight: A surface wave passing over the waters and moving to
the northeast is driving the general wind flow across the
coastal waters. Light and variable winds this evening will
become northerly through sunrise Wednesday. Seas will be 2-4
feet out to 20 nm and up to 5 feet beyond.

Wednesday through Sunday: A northeast flow will persist
Wednesday through Friday as high pressure builds from the west
and a low pressure system shifts northeast along/near the
Southeast coast. The tightest gradient is expected on Thursday
as the surface low passes the area, which should result in Small
Craft Advisories for at least offshore Georgia and northern
South Carolina waters and potentially all waters outside the CHS
Harbor Thursday and Thursday night. Elevated seas could linger
in offshore Georgia waters through much of Friday.

The next surge will occur late Saturday night into Sunday behind a
dry cold front as cold air advection expands over the waters. At
this point the best chance for Small Craft Advisory conditions will
be over the offshore GA waters where water temperatures are warmer.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Dense Fog Advisory until 10 AM EST this morning for GAZ087-088-
     099>101-114>119-137>141.
SC...None.
MARINE...None.

&&

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



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