Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 202324

Area Forecast Discussion
National Weather Service Charleston SC
724 PM EDT Fri Apr 20 2018

Cool and dry high pressure will prevail into Sunday. A low
pressure system will approach the area late Sunday and will
then slowly move up the coast through the middle of next week.
Weak high pressure will return late next week.


Tonight: Aloft, the axis of a mid-lvl trough of low pressure
will continue to shift further off the Northeast coast, giving
way to mid-lvl ridging associated with high pressure building
from the west. At the sfc, the area will remain positioned
between high pressure advancing from the Midwest toward the
Northeast United States and a coastal trough along the northern
Florida coast. Given the setup, expect dry and clear conditions
for most locations throughout the night. A fair amount of
radiational cooling will help temps dip to around 40 degrees
well inland and upper 40s/lower 50s near/along the coast. Temps
could be slightly warmer near the coast in Southeast Georgia,
mainly near the Altamaha River where the coastal trough supports
a northeast wind that drives moisture and some clouds onshore
through at least early tonight.


Saturday and Saturday night: Aloft, low amplitude riding will
translate across the forecast area ahead of a deep upper low
crossing the central CONUS. Surface high pressure will sit off to
the northeast and drive a cool northeast to easterly flow through
the day. Clouds during the day will be mainly confined to coastal
Georgia where stratocumulus will begin to spread onshore in the
afternoon. Highs will be cool and below normal, ranging from the mid
70s inland to the upper 60s/low 70s at the coast. Overnight, little
change to the pattern with lows ranging from the upper 40s to the
mid 50s along the Georgia coast.

Sunday through Monday: The forecast becomes increasingly active late
in the weekend and early next week as a strongly forced system moves
into the region. On Sunday, the aforementioned upper low will move
into the lower Mississippi Valley, and the first shot of shortwave
energy will move through ahead of the low. Overall Sunday will bring
increasing cloud cover and notable moistening of the column. There
may be enough forcing and subtle low level convergence to kick off a
few showers in the afternoon, and the forecast features 20-30
percent rain chances.

The most interesting part of the forecast comes late Sunday night
and Monday. Precipitable water values are progged to surge into the
1.4-1.6 inch range (in excess of the 90th percentile based on
climatology), coincident with excellent difluent flow aloft and
strong low level convergence/isentropic ascent. There is good model
consensus showing precipitation rapidly breaking out in the early
morning hours on Monday across southeast Georgia and spreading
northeastward through the afternoon. There is even good agreement on
a subtle surface wave feature along the coastal trough that could
further enhance the rainfall potential. Models have seemingly
increased the rainfall total potential and place the forecast area
in the bullseye. Following the most up to date WPC forecast results
in a very beneficial widespread 2-3 inches with potential for
locally higher amounts. There remains some thunder potential, though
mainly for southeast Georgia and the adjacent coastal waters. The
severe threat looks to be rather low though, and this system
certainly looks to be more of a rain maker. Precipitation coverage
and intensity will focus more into southeast South Carolina later in
the day, and should begin to diminish across southeast Georgia. Rain
chances across the entire day are in the 90-100 percent range.


A closed upper low over the Southeast on Tuesday will get absorbed
by a stronger shortwave dropping out of the Midwest during the
middle of next week. Concurrently at the surface, an area of low
pressure will lift northeast out of southern Georgia Monday night
and then move slowly up the coast. Weak high pressure will return
late week as the surface low continues to move away from the area
into the northeast U.S. Main chances for rain will come early in the
period as the low drifts directly over the area. Temperatures will
be near normal.


VFR conditions will prevail at both CHS and SAV terminals through
00Z Sunday.

Extended Aviation Outlook: Flight restrictions in reduced
ceilings and visibilities are expected to develop and spread
across the region Sunday night. These conditions should persist
through at least Monday night with breezy southeasterly winds.


Tonight: The area will remain between high pressure passing well to
the north and a coastal trough along the northern Florida coast.
Although recent trends show the pressure gradient slowly relaxing
over the waters, the gradient remains fairly pinched over southern
Georgia waters with seas ranging between 4-6 ft nearshore and
6-8 ft well offshore. A Small Craft Advisory has therefore been
extended for at least a few hours into early tonight for
nearshore Georgia waters where the effects of the coastal trough
continue to pinch the gradient. A Small Craft Advisory will be
ongoing in offshore Georgia waters through tonight for prevailing
northeast winds of 20-25 kt and seas that gradually subside to
5-7 ft after midnight.

Saturday through Wednesday: High pressure to the northeast will
drive elevated east or northeasterly flow across the local waters
into Sunday. Winds will generally top out in the 15-20 knot range.
Seas up to 6 feet will continue in the outer Georgia waters into
Saturday night. Then Sunday and Sunday night, an area of low
pressure will approach from the west and work to tighten the
pressure gradient. Winds will steadily increase Sunday night and
Monday, resulting in winds in the 25-30 knot range and solid Small
Craft Advisory conditions across all waters. There is even a low end
chance for Gales to develop depending on exactly how the southeast
flow sets up with the pinched gradient. Seas are expected to
increase significantly on Monday as a result, with 10 ft seas
possible inside of 20 nm. Conditions will improve Tuesday into
Wednesday as the surface low pulls away from the region.

Rip Currents: Elevated onshore winds in the 15-20 knot range,
astronomical influences, and some swell energy will result in a
moderate risk of rip currents at all beaches on Saturday.

A prolonged stretch of moderate to perhaps strong onshore winds and
larger surf, first due to high pressure, then as low pressure passes
nearby early next week, will lead to an enhanced risk of rip
currents at the beaches.


MARINE...Small Craft Advisory until 6 AM EDT Sunday for AMZ374.
     Small Craft Advisory until 10 PM EDT this evening for AMZ354.


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