Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 271434
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1034 AM EDT Thu Jul 27 2017

.SYNOPSIS...
Broad low pressure will remain across far Northeast Florida
today as a cold front dissipates north of the region. Another
cold front will move into the area this weekend, then stall
offshore into early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
No significant changes were made to the previous forecast. Deep
low pressure will remain over N FL/SE GA where the deepest
moisture will also be found. Thus, the best rain chances are
expected across southeast GA and the main concern will be heavy
rainfall with localized flooding as PWATs are around 2.3 inches
and steering flow is rather weak closer to the center of low
pressure. The risk for severe weather will remain low overall
but the modified 12Z CHS sounding showed about 2700 J/kg of CAPE
and 1400 J/kg of DCAPE so certainly cannot rule out an isolated
severe storm, especially across inland SC. High temperatures
will be kept below climo in the mid/upper 80s for most areas
given the abundant clouds/rain, but could still reach the lower
90s toward the Midlands/CSRA.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/...
Tonight: Land-based convection will quickly diminish this
evening as instability wanes and low-level convergence shifts
back offshore closer to the western wall of the Gulf Stream.
Although convection will likely redevelop over the coastal
waters after midnight, most of this activity should remain away
from land as low-level trajectories veer to the southwest with
time. Activity will be close enough to justify pops early
Friday across the coastal counties, mainly 20-30% except as high
as 40% along the Charleston County coast given its orientation
relative to steering trajectories. Lows will range from the
lower 70s inland to the upper 70s at the beaches. Could see some
low stratus develop well inland late and this will have to be
watched carefully as it could impact lows early Friday,
especially interior Southeast South Carolina where model 1000
hPa condensation pressure deficits are progged to be the lowest.

Friday: A strong mid/upper lvl low will shift from the Great Lakes
region to the Northeast United States, developing/strengthening a
sfc low over the Mid-Atlantic states. A cold front associated with
the strengthening low pressure system will then shift south through
the Mid-Atlantic region as the mid/upper lvl low helps dig the large-
scale trough over the East CONUS. The setup will favor drier
conditions during morning hours, but a southwest flow will likely
begin to advect deep moisture into the region well ahead of a cold
front approaching from the north and northwest late. Deepening
moisture along with temps peaking in the lower 90s support chances
of afternoon/evening showers and thunderstorms, a few of which could
potentially be strong. However, the bulk of precip activity should
begin Friday night as the cold front continues to progress toward
the Southeast United States. Wind fields during the overnight period
suggest upstream convection could be more organized than previous
days, making a run toward the area late. Given the timing of the
event during overnight hours, the overall severe weather risk
remains low. However, stronger thunderstorms can not be ruled out
ahead of the arriving cold front. Overnight lows should remain mild
under clouds, ranging in the mid/upper 70s.

Saturday: A cold front will shift over the region early, supporting
numerous showers/thunderstorms within deep moisture characterized by
PWATs around 2.25 inches and moderate instability. Isentropic lift
ahead of the front could produce a fair amount of clouds and showers
before peak heating, limiting stronger afternoon convection.
However, wind fields will be slightly enhanced ahead of cold fropa,
suggesting the possibility of a few stronger thunderstorms should
sufficient sfc heating occur. Given the setup, the greater threat
for stronger thunderstorms capable of producing damaging winds
should occur over parts of Southeast Georgia where sfc heating is
greatest before the arrival of the cold front. High PWATs also
suggest the potential for heavy rainfall with afternoon/evening
thunderstorms. The threat of stronger convection and bulk of precip
should shift offshore with the cold front by early Saturday night.
Areas could be dry away from the coast within a few hours of sunset.
Overnight lows will be a bit cooler behind the cold front, ranging
in the lower 70s.

Sunday: Conditions will be considerably drier to start off the week
behind a cold front that stalls offshore and/or south of the area.
However, a mid/upper lvl low will continue to meander over the Mid-
Atlantic states and part of the Southeast, suggesting a few showers
and/or thunderstorms could develop over the area during the
afternoon. At this time, a slight chance of showers is forecast
inland while a chance of thunderstorms is forecast near coastal
areas where instability is slightly higher. Afternoon highs should
remain below normal, peaking in the mid 80s over most locations.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
Conditions should be mostly dry to start of the week while a stalled
front remains offshore and/or south of the region. However, guidance
suggests the boundary will drift back north and toward the coast
through the middle of next week. Given the setup, a slight chance of
showers/thunderstorms is forecast along coastal areas only on
Monday, but a gradual trend to chances of showers/thunderstorms is
anticipated Tuesday and Wednesday. Greatest precip coverage should
occur along coastal areas, especially in Southeast Georgia. Temps
will generally be a few degrees below normal next week, peaking in
the mid/upper 80s. Overnight lows will range in the upper 60s/lower
70s.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
KCHS: Mainly VFR. Convection will be rather isolated near the
term with abundant mid-level dry air in place so will not
include a mention of SHRA/TSRA at this time. Risk for noctural
showers will increase after midnight, but most of this will
remain over the coastal waters. Could see a period of low cigs
prior to daybreak Friday. Confidence too low to justify a
mention for the 12z TAF cycle.

KSAV: Deep moisture and proximity to low pressure will keep high
shower/tstm chances through at least early afternoon. Risk for
nocturnal showers will increase after midnight, but most of this
will remain over the coastal waters. Could see a period of low
cigs prior to daybreak Friday. Confidence too low to justify a
mention for the 12z TAF cycle.

Extended Aviation Outlook: VFR conditions should prevail for much of
Friday ahead of a cold front. Periods of flight restrictions are
then likely late Friday night into Saturday as a cold front slowly
progresses through the region. VFR conditions should then return
early next week, but brief flight restrictions will remain possible
each day as a front lingers offshore.

&&

.MARINE...
Today: South to southeast winds will prevail today as weak low
pressure lingers well to the south. Speeds look to remain 10 kt
or less with seas 1 to 2 ft.

Tonight: Southerly winds will begin to increase tonight as a
cold front well to the north draws closer. Winds look to tick up
to 10-15 kt, but could be locally higher near convection. Seas
will respond, building to 2-3 ft nearshore waters and 3-4 ft
offshore waters. Scattered to numerous showers/tstms are
expected to develop after midnight as low-level convergence
shifts to the western wall of the Gulf Stream. Locally gusty
winds and vsbys less than 1 nm will occur in the heavier
convective elements.

Friday through Tuesday: The waters should then remain between high
pressure well offshore and a strengthening trough of low pressure
inland through Friday. A cold front will then approach from the
north/northwest Friday night and shift over coastal waters Saturday
before stalling south of the waters early next week. At least
chances of showers and thunderstorms are forecast over the waters
through late week, while numerous showers and thunderstorms are
possible with cold fropa Saturday into early Sunday. The front could
drift back over coastal waters by midweek, producing additional
showers and thunderstorms. Wind/sea conditions are expected to
remain below Small Craft Advisory levels through the period.
However, a southwest/south flow could gust around 20 kts Friday
afternoon into early Saturday as the pressure gradient tightens
along/ahead of cold fropa. Seas will range between 2-4 ft, highest
this weekend in advance of the cold front.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

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


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