Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 270825

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

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.


GOES-16 water vapor channels show the upper low has shifted
farther the southeast and was positioned just off the northeast
Florida coast early this morning. Upper forcing along the
northeastern edge of the cyclone coupled with weak surface
moisture convergence east of a weak surface reflection near
Brunswick, GA, was producing numerous to widespread showers/tstms
over coastal Georgia east into the Atlantic waters. Marine-
based activity will gradually diminish later morning as low-
level convergence shifts inland along the developing sea breeze
circulation. Convection this afternoon should a bit less
compared to the past several days, especially across Southeast
South Carolina, with the upper low now displaced farther to the
southeast and water vapor data showing drier air advecting
south. Still expect enough coverage to justify chance to low-end
categorical pops today. The axis of higher pops will be placed
across coastal Southeast Georgia closer to the ribbon of deepest
moisture (PWATS 2.25-2.45 inches). Pops will range from 20-30%
in the Charleston Metro Area to 30-50% across interior
Southeast South Carolina with 50-80% for Southeast Georgia,
highest across the coastal counties, including the Savannah
Metro Area.

The risk for flooding will be lower compared to earlier in the
week as forecast soundings show a bit more flow in the steering
layer today. High PWATS still suggest heavy downpours will
occur, especially over Southeast Georgia, so a few pockets of
minor flooding could still occur. The risk for severe weather
will remain low, although drier advecting in from the north
could support a few strong wet microbursts across interior
Southeast South Carolina later this afternoon.

Similar to Wednesday, high temperatures will be closely tied to
the amount of convection that occurs during peak heating.
Expect the cooler temperatures to occur in the Savannah-Ludowici
corridor where the greater coverage of convection is expected
to occur. Will forecast highs from the lower 90s adjacent to the
CSRA and Southern Midlands to the mid 80s in the Savannah-
Darien-Ludowici corridor. There is a larger than normal bust
potential here as any breaks in the cloud cover could support a
quick spike in temperatures. Adjustments will likely be made
later today as short term convective influences become more


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.


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


Mainly VFR, although some brief low clouds could impact both
KCHS and KSAV prior to daybreak. Shower activity over the
Atlantic should remain east of the terminals through daybreak.
Guidance shows best chances of showers/tstms remaining west of
the terminals today along an early sea breeze circulation, but
activity could be close enough to KSAV for impacts. Will carry
VCTS at KSAV roughly 15-18z to account for this, but
probabilities look too low to justify a mention at KCHS with
the 06z 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.


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.




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