Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 251455

Area Forecast Discussion
National Weather Service Charleston SC
1055 AM EDT Tue Jul 25 2017

Broad low pressure will remain across the Carolinas and Georgia
through tonight. A weak cold front will then stall north of the
area Wednesday before weak low pressure lingers over the area
through late week. Another cold front will approach the region
this weekend before stalling over or near the area early next


Late Tuesday morning: Ongoing forecasts depicting highest
POPs/greatest coverage of thunderstorms across southern/coastal
counties this afternoon, within axis of PWATs exceeding 2
inches, appears on track and required on minor adjustments to
hourly parameters. Of note, the modified 12Z KCHS raob depicted
SBCape exceeding 3000-3500 j/kg, suggesting some potential for
a brief/isolated pulse severe/damaging wind event or two
especially where boundary/cell interactions occur. However,
significant/widespread severe weather appears unlikely. Also of
note, some high resolution guidance suggests that thunderstorm
coverage could remain below forecast levels across
southern/coastal counties. However, given deep-layered moisture
this solution does not seem reasonable and has not been
factored into the forecast.

Today: Deep moisture, a weak mid level disturbance drifting east
through Georgia, and increasing upper difluence will spell an active
convective day across the forecast area. This will be especially
true along the Savannah River and across southeast GA, where the
east-west axis of higher PWATs is forecast to persist. Coverage is
expected to increase to scattered to numerous along and south of
I-16 during mid to late afternoon as showers and storms proliferate
freely in the plume of deep moisture on mesoscale boundary
collisions. Our highest POPs in the 70 percent range for the
afternoon, extend from much of southeast GA into Jasper and
Beaufort Counties will a ribbon of likely POPs extended
northeast along the SC sea breeze coverage.

The main risk for hazardous weather today appears to be a lower-
end threat for localized flooding and very heavy rainfall potential
in the strongest convection. Storm motions look weak later today
and multi-cell convection should be driven on boundary interactions.
The NSSL wrf favors the I-95 corridor either side of Savannah today
and this seems to correlate with the global model QPF output as
well. High temps will be in the upper 80s to near 90, convective
temps are mainly in the upper 80s. Convective processes will result
in mostly cloudy to cloudy skies many areas by later this afternoon.

Tonight: The forecast remains on the tricky side. A weak mid
level low will moving into the western half of the forecast
area overnight and moisture will remain very deep. Surface winds
could shift onshore along the upper SC coast late, resulting in
better moisture convergence and coastal showers and storms late.
Inland areas may also see isolated to scattered convective rains
lingering through the night and we maintained chance POPs to
cover. Temps will probably remain in the mid/upper 70s through
the night.


Wednesday: A weak cold front will likely stall north of the region
early while a mid/upper lvl trough of low pressure shifts further
off the Northeast coast. However, broad low pressure will likely
persist along and south of the front within moderately unstable and
moist conditions. Weak forcing associated with the broad area of low
pressure and deep moisture characterized by PWATs near 2.25 inches
support numerous showers and thunderstorms during late morning into
evening hours over much of the area. Precip activity and mostly
cloudy skies will likely limit high temps to the mid/upper 80s. A
few showers and/or thunderstorms could linger into the overnight
period. Overnight lows will range in the low/mid 70s.

Thursday: What`s left of a sfc trough/broad area of low pressure
will slowly drift offshore during the day. However, lingering
moisture and the position of the mid-lvl low should support chances
of showers and thunderstorms over much of the area. The bulk of
precip activity should trend toward the coast of Southeast Georgia
later in the day. Near the Georgia coast, numerous showers and
thunderstorms will be possible through the afternoon while the mid-
lvl low slowly meanders off the Southeast coast. Cloud cover will
limit max temps to around 90 degrees. Shower and thunderstorms are
expected to taper off during the overnight period. Low temps should
range in the mid 70s.

Friday: Conditions should be slightly drier to start off the weekend
while the area sees a transition from low pressure shifting further
offshore and stronger low pressure developing inland. However, a
southwest flow should begin to advect greater levels of moisture to
the area, warranting chances of afternoon showers and thunderstorms
well ahead of a cold front approaching the region late. Temps will
likely be warmer, peaking in the lower 90s.


Deep moisture will advect to the Southeast United States ahead of a
cold front that is expected to move into the area Saturday. PWATs
between 2.25 to 2.5 inches and forcing associated with the front
support numerous showers and thunderstorms Saturday into Sunday
while the front slowly progresses south and eventually stalls over
or near southern areas of Southeast Georgia. A few thunderstorms
could become more organized than previous days given greater wind
fields aloft, but the overall threat of stronger thunderstorms could
be limited due to the timing of fropa during earlier hours on
Saturday. At least chances of showers and thunderstorms could linger
into early next week while moisture continues to lift over or near
the stationary front.

Temps will generally be a few degrees below normal this weekend and
early next week given extensive cloud cover and precip activity. In
general, temps should peak in the mid/upper 80s. Overnight lows will
range in the low/mid 70s.


KCHS: VFR conditions likely until VCTS this afternoon. Also,
introduced some shower chances later tonight with an upper level
disturbance in the region and deep level moisture at high

KSAV: Deep moisture should result in a good potential for thunder
and some heavy downpours in the general area this afternoon. Prior
to then, a potential for some low clouds and slight chances for
brief IFR/MVFR through mid morning. Mainly VFR tonight but cannot
rule out some low clouds or even a shower late.

Extended Aviation Outlook: Brief flight restrictions are possible
with showers and thunderstorms, especially Wednesday into early
Thursday as weak low pressure approaches the region. VFR conditions
should prevail late Thursday and Friday, before flight restrictions
return with showers and thunderstorms along/near another cold front


Late Tuesday morning: Ongoing forecasts depicting offshore flow
10-15 kt to start giving way to the sea breeze circulation
remain on track. Thunderstorms could produce locally hazardous
conditions this afternoon/evening especially over near shore

Southwest synoptic flow will persist today but the low-level gradient
will become weaker with time. Speeds will be more in the 10-15 kt
range today with seas 2-4 ft, highest well offshore. Tonight, the
flow becomes even lighter and more variable late. Showers and tstms
may increase after midnight, as moisture convergence and instability
peak over the warm Atlantic waters.

Wednesday through Sunday: Broad low pressure will gradually shift
over the coastal waters mid week while a cold front stalls north of
the region. 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 the coastal waters Saturday and Sunday
before stalling over or near southern Georgia waters. 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 Sunday. Conditions are
expected to remain below Small Craft Advisory levels through the
period. However, a southwest/south flow could gust around 20 kts
Friday night into early Saturday until cold fropa occurs. Seas will
gradually build from 2-3 ft to 3-4 ft this weekend ahead of the
front, then subside to 2-3 ft early next week.




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