Area Forecast Discussion
Issued by NWS Charleston, SC

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807
FXUS62 KCHS 260820
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
420 AM EDT Wed Jul 26 2017

...HEAVY RAIN AND SOME FLOODING LIKELY TODAY...
...WATERSPOUTS POSSIBLE THIS MORNING...

.SYNOPSIS...
Broad low pressure will remain across far southern Southeast Georgia
through tonight. A weak cold front will stall over or near
Southeast South Carolina through Thursday. Another cold front will
move into the area this weekend, then likely stall just offshore
into early next week.

&&

.NEAR TERM /THROUGH TODAY/...
A wet and unsettled day is in tap as the area will remain
influenced by upper level low pressure meandering across far
Southeast Georgia. Convection will initially develop along a
broad low-level convergence zone noted just offshore of the
lower South Carolina coast then steadily spread/develop inland
and south through the morning as instability and upper difluence
build. High resolution guidance is similar in depicting the
greatest coverage of showers/tstms south of the I-16 corridor
coincident with the axis of deepest moisture featuring PWATs
2.25-2.40 inches and 250 hPa difluence occurring along the
northern periphery of the upper-level cyclone. Pops will run
from 70% for areas along/north of a Millen-Yemassee-Edisto Beach
line to 80-90% elsewhere, highest along/south of the I-16
corridor.

Today`s modified soundings are rather unstable despite high
atmospheric moisture, but the better instability will be
confined to areas adjacent to the Southern Midland and CSRA
where greater insolation will be found. There will be a risk for
an isolated severe tstm to occur in this area where some
elevated DCAPE will be found, but the overall risk is quite low.
The main concern centers around the potential for heavy rain
and flooding. The combination of slow storm motion, potential
enhancement along mesoscale boundary collisions and PWATs
2.25-2.40 inches will promote efficient convective rains. An
automated rain gage near Midville, GA recorded just over 3
inches of rain in an hour yesterday evening and the potential is
there to something similar today. Expect a general rainfall of
1-2 inches south of I-16 with localized amounts potentially
exceeding 4 inches. This should yield areas of minor to moderate
flooding, mainly in low-lying and poor drainage areas, but
flash flooding can not be completely ruled out. A Flash Flood
Watch will not be issued at this time as confidence for
widespread flash flooding is deemed too low to justify a watch,
but will highlight the potential in the Hazardous Weather
Outlook and utilize a "rain heavy at times" qualifier in the
various gridded and text forecasts.

Pinning down highs today is proving tricky as much will depend
on the timing and position of convection during peak heating.
Right now, the coolest conditions look to occur in the
Reidsville-Hinesville-Richmond Hill corridor where tstms are
expected to the most widespread. Will limit highs to the mid
80s in this region with upper 80s elsewhere away from the
beaches, except near 90 along the Southern Midlands and CSRA
where the potential for greater insolation will be found. As is
typical in summer, any breaks in the thicker cloud canopy could
cause temperatures to rise quickly, so there is certainly a big
bust potential here. Utilized a blend of the various MOS
products with hourly temperature data from the RAP and H3R to
derive today`s high temperature forecast.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
Tonight: Convection will quickly diminish this evening with the
loss of insolation. However, the risk for isolated convection
will linger through the night given the warm, moist airmass that
will remain in place. Increasing coastal convergence along the
coastal waters and beaches could support more scattered activity
early Sunday morning, but he bulk of this convection should
remain over the coastal waters. Lows will range from the lower
70s inland to the upper 70s at the beaches.

Thursday: What`s left of a stationary front 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 late in the day,
where numerous showers/thunderstorms are possible while the mid-lvl
low slowly meanders and weakens near the Southeast coast. Cloud
cover will limit max temps to around 90 degrees, warmest inland.
Showers 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 of low pressure weakening/shifting
offshore and stronger low pressure developing inland. However, 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 returning to the
lower 90s support chances of showers/thunderstorms during the
afternoon/evening hours. Shower/thunderstorm coverage could very
well increase from the northwest Friday night as the cold front
quickly shifts toward the Southeast United States. Wind fields also
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. Overnight lows should remain mild under clouds, ranging
in the mid/upper 70s.

Saturday: A cold front will shift over the region this weekend,
supporting numerous showers/thunderstorms within deep moisture
characterized by PWATs near 2.5 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 would be 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.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
Deep moisture characterized by PWATS up to 2.0 inches will persist
over the Southeast United States Saturday night into Sunday while a
cold front slowly progresses south and eventually stalls just
offshore and/or south of the region. The pattern becomes more
difficult to predict early next week with some guidance suggesting
that the stationary front drifts back near the Southeast coast or
low pressure develops along it and lifts northeast along/near the
Southeast coast. Regardless of the outcome, it appears chances of
showers/thunderstorms will be possible early next week with greatest
precip coverage anticipated near coastal areas.

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.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
Main concern is for tstm impacts at both terminals.

KCHS: Early morning activity looks to organize along the lower
South Carolina around daybreak, which could impact KCHS mid/late
morning before pushing inland. Will carry TEMPO group roughly
16-19z for marginal MVFR conditions in TSRA to cover for now.
Activity should push west of the terminal by early afternoon
with dry conditions prevailing for the rest of the TAF period.

KSAV: Guidance is in pretty good agreement in showing a cluster
of showers/tstms impacting the terminal later this morning and
lingering into the early afternoon hours. High PWAT values
support low vsbys at times in heavy rain. Window for the
greatest impacts looks to center roughly 15-18z. Will carry
prevailing MVFR with a TEMPO group with vsbys within IFR and
approaching alternate minimums. However, briefly lower
conditions, possibly to airfield minimums, will be possible at
times. Activity will push south of the terminal by early/mid-
afternoon with dry conditions prevailing thereafter.

Extended Aviation Outlook: Brief flight restrictions are possible
into Thursday as low pressure shifts offshore. VFR conditions should
prevail late Thursday into Friday. Higher chances of flight
restrictions should return late Friday through Sunday as showers and
thunderstorms occur with a cold front that slowly progresses over
the region.

&&

.MARINE...
Waterspouts: Conditions will be favorable for waterspouts this
morning as convection develops along a broad low-level
convergence boundary just offshore the lower South Carolina
coast within a light lower tropospheric wind regime. Although
the risk will be highest in the vicinity of this boundary,
atmospheric conditions will favor waterspouts just about
anywhere this morning. Will have to watch for waterspouts
possibly approaching the beaches given the steering flow is
directed to the coast. The waterspout risk will gradually lower
late morning into the afternoon as convection gradually works
inland. A Marine Weather Statement will be issued to address the
risk for waterspouts this morning.

Today: A plethora of boundaries is producing various wind
direction across the waters this morning. Winds should become
more onshore later today as these boundaries dissipate and weak
low pressure lingers across far Southeast Georgia. Speeds will
generally remain less than 10 kt with seas 1-2 ft. Heavy rains
will produce vsbys less than 1 nm at times today.

Tonight: East and southwest winds will prevail today as weak low
pressure meanders well to the south. Speeds look to remain less
than 10 kt with seas 1-2 ft, except 2-3 ft along the Charleston
County coast.

Thursday through Monday: A mid-lvl low will gradually shift over the
coastal waters and weaken Thursday. The waters should then remain
between high pressure well offshore and a strengthening trough of
low pressure inland through Friday. A cold front will approach from
the north/northwest Friday night and shift over the coastal waters
Saturday and Sunday before stalling over or just south of 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 and Sunday. The
front could drift back over coastal waters early next week,
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 until cold fropa occurs. Seas
will gradually build from 2-3 ft to 3-4 ft this weekend and early
next week.

&&

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

&&

$$



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