Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230136
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
936 PM EDT Sat Jul 22 2017

.SYNOPSIS...
Atlantic high pressure will continue to extend west into the
Southeast coast this weekend. A weak cold front will approach
the area during the middle of next week. Low pressure should
then develop over the area and linger through late week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Diurnal convection has faded and the bulk of the night (if not
all the night) will be rainfree. Skies will be mostly clear or
partly cloudy on average.

Late tonight will see the formation of isolated to scattered
convection over the Gulf Stream, maybe some making a run for
the immediate coast, especially over SC due to low level
convergence. But for now we believe that activity will stay
offshore, so no PoP`s higher than 10%.

Other than a little ground fog in locations where it rained on
Saturday, there is likely too much mixing and unfavorable
condensation pressure deficits for anything more to occur by
daybreak Sunday.

It`ll be a warm and very humid July night, with lows no better
than lower or middle 70s far inland, closer to 80 on the barrier
islands and in downtown Charleston.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Sunday, the mid level pattern over the region will feature a broad
weakness, with a large ridge centered over the Atlantic and another
ridge centered over the Red River Valley. However, low pressure is
expected to develop across the mid Atlantic states, with the trough
remaining across the Carolina foothills and Piedmont. The pressure
gradient across the forecast area will slowly steepen through the
day, with gusty conditions expected across the marine zones and near
the coast. Forecast soundings indicate that the LFC will range
between 4-5 kft, with shallow CIN lingering into the late afternoon.
A slight increase in llvl moisture convergence should support
scattered thunderstorms inland, with slight chcs generally east of I-
95. Maximum temperatures should reach around 90 over the beaches to
the mid 90s across inland GA.

Monday, NAM12 shows a mid level low and sfc low passing north of the
coastal zones during the morning hours. This feature appears much
weaker on the other short range models, I will favor the GFS. Using
a blend of temperature guidance, high temperatures are forecast to
range a degree or two below values reached Sun. Dewpoints are
expected to remain in the low to mid 70s as a sfc trough remains
west of the region. Given the temperatures and dewpoints, any CIN
should erode during the late afternoon hours. Short range guidance
indicates that a weak short wave is expected to ripple across the
forecast area during the late afternoon. PoPs should range from 40
percent over GA and around 30 near the coast and coastal waters.

Tuesday, the 12z GFS indicates that the axis of deep moisture will
shift across the CWA during the daylight hours. PW values are
forecast to increase to around 2.3 inches. Features across the
forecast area will remain limited to broad mid level trough with low
pressure across the southern Appalachian Mountains. Provided
generally weak forcing and tall and thin instability, I will keep
PoPs limited to high chc.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
An unsettled weather pattern will persist through mid week while
southwest flow advects deep moisture to the region ahead of a cold
front approaching from the north/northwest. The cold front should
struggle to push south into the region, likely stalling just north
of the area as its parent mid/upper lvl trough of low pressure
shifts off the Northeast coast. However, weak low pressure should
develop near the stalled boundary on Wednesday and persist through
late week until a second mid/upper lvl trough shifts over the
Northeast and gradually amplifies, forcing some mid-lvl energy over
the Mid Atlantic and Southeast states late week. The pattern
suggests at least chances of showers and thunderstorms each day with
greatest coverage occurring during peak diurnal heating hours. Temps
will generally peak around normal mid week, but could approach the
mid 90s by the upcoming weekend. Overnight lows will range in the
mid 70s.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
Mainly VFR through 00Z Monday. However, there is at least some
chance that SHRA/TSRA could impact KCHS and/or KSAV on Sunday,
with a risk of temporary flight restrictions.

Extended Aviation Outlook: Afternoon showers/thunderstorms could
provide brief flight restrictions each day. However, greatest
chances of flight restrictions should occur on Wednesday as a cold
front stalls just north of the area. Otherwise, VFR conditions will
prevail.

&&

.MARINE...
Tonight: We increased winds a bit from the previous forecast
based on recent trends and latest guidance, which shows a fairly
decent nocturnal low level jet and tightening of the gradient
between sub-tropical ridging and a lee side trough. SW winds
will be as high as 15 or 20 kt with higher gusts, before
dropping off some late as weak land breeze circulations develop.
While we will experience gusts at times near 25 kt, they do not
look to be frequent enough, nor will they encompass a large
enough area to raise a Small Craft Advisory. Seas will average
3 or 4 ft. Mariners should be prepared for isolated to scattered
showers/t-storms after midnight.

Sunday through Thursday, Broad high pressure will remain across the
western Atlantic as low pressure remain inland. Low pressure may
deepen across the mid Atlantic states late Sunday into Monday. A
steeper pressure gradient should support gusty SW winds Sunday and
Monday afternoon and evening, gusts may near 25 kts. Conditions are
expected to remain below Small Craft Advisory criteria. Low pressure
will gradually settle over the southern Appalachians Monday and
Tuesday, shifting over the GA/SC coastal waters by late Wednesday.
Winds should weaken and shift from the SE. Thunderstorm coverage is
forecast to increase each day through Wednesday.

&&

.TIDES/COASTAL FLOODING...
Tides will remain elevated into early next week due to the lunar
perigee and new moon. Additional coastal flooding advisories
are likely.

&&

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

&&

$$
NEAR TERM...
SHORT TERM...NED
LONG TERM...DPB
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...



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