Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 211449

Area Forecast Discussion
National Weather Service Charleston SC
1049 AM EDT Mon Aug 21 2017

A weak stationary front will gradually dissipate across the
region today. High pressure will extend across the area Tuesday
and Wednesday with a Piedmont trough developing inland. A
stronger cold front is expected to pass over the region on
Thursday. A wave of low pressure could form on the front well
offshore Friday. Slightly cooler and drier conditions are then
expected into early next week.


Late this morning: Well eclipse day has arrived but we are sorry
to say that there has been no improvement to the viewing
forecast. We actually started off the morning with mostly clear
skies for locations away from the coast, with isolated
convection along the coast and over the coastal waters. However,
as can be seen in the morning sounding, plentiful low level
moisture combined with rapid heating has led to a rapid
expansion of low level convective clouds and resulted in mostly
cloudy skies. The surface pattern is pretty complex with low
level northeast flow prevailing due to the presence of high
pressure just to the north in almost a subtle cold air damming
setup. Aloft, an expansive ridge extends from the tropical
Atlantic across the southern Plains and into northern Mexico,
but there is a subtle shortwave across north Florida that is an
extension of an upper low over the Gulf of Mexico. Therefore,
the impact of some larger scale lift combined with low level
convergence along the coast is helping to kick off showers and
thunderstorms along the coast and the adjacent coastal waters.
Thus far the most vigorous convection has been closer to the
Altamaha and forecast models seem to be handling this reasonably
well. We are easily above convective temperatures (85 degrees)
as observations show current values already into the upper 80s
in many areas. Showers and storms will be capable of developing
at any time just about anywhere through the morning. As we
transition to the afternoon the focus will likely become further
inland as the sea breeze pushes inland and the flow turns more
southeasterly. Mostly cloudy skies will prevail through eclipse
time, but there will be some periodic breaks. High temperatures
are tough as we are already in the upper 80s, but cloud cover
will likely prevent most areas from rising too much higher. Some
low 90s expected inland, but most areas will be around 90.

Tonight: Convection far inland will fade this evening, as the
short wave lifts further inland. There will be some reformation
of convection over the Atlantic, some of which will make a run
for the coast late at night. There certainly could be some late
night fog in locations where it rained, but since various layers
of clouds will persist we are not showing any in the forecast at
this time. Lows generally in the mid and upper 70s, or still
well above normal.


Tuesday and Wednesday: Short term guidance indicates that the center
of a broad H5 ridge will remain around the forecast area on Tuesday.
The ridge should shift southward over the Gulf Coast as a longwave
trough ripples towards the Appalachians by Wednesday afternoon.
Forecast soundings show a fairly significant inversion around H85
across the CWA, with CIN of 10 J/kg or more well into the afternoon.
However, the pressure pattern should support steady onshore flow,
setting the stage for sea breeze development during the mid day. I
will forecast chc PoPs over the coastal counties by the early
afternoon, spreading east during the rest of the afternoon into the
early evening. High temperatures are forecast to range from the
upper 80s across the beaches to the low to mid 90s across the inland
counties. On Wednesday, forecast soundings indicate that the
inversion will lift to around H75 and inhibition should decrease.
Flow should shift from the SE to S during the daylight hours
Wednesday. A slow moving sea breeze will likely serve as a focus for
moisture convergence, supporting at least sct storms over the
forecast area during the afternoon. High temperatures are forecast
to range 1 to 2 degrees above values reached on Tuesday.

Thursday: Guidance has been consistent with the timing of a cold
front across the CWA during the daylight hours Thursday. The axis of
the H5 trough is expected to slide over the region during the
afternoon. The arrival of the trough over the slowing front will
likely support a frontal wave to develop, the GFS and ECMWF indicate
over the mouth of the Savannah River by 0z Fri. Given the passage of
the mid level trough and sfc front, PW above 2 inches, and
widespread weak to moderate instability, I will forecast likely
PoPs. H85 CAA is not expected until Thursday night, highs should
range around 90 degrees for most areas Thursday afternoon.


Friday through Monday: High pressure sourced from Canada will remain
centered well north of the region through the rest of the week.
Confidence in the day 5 through 7 forecast remains low. Medium range
guidance indicates that the sfc high center will shift over New
England, ridging SW across the Carolinas into GA. In addition, both
the GFS and ECMWF show a coastal low developing off the GA/SC coast
by late this week, with run to run trends shifting east. Guidance
supports prolonged NE winds across the CWA, yielding afternoon
dewpoints in the upper 60s inland by the weekend. I will favor the
marine and near shore zones for Chc PoPs through much of the day 4
through 6 period. I will keep the forecast dry for day 7, indicating
the best chance for drier air to arrive on the west side of the
departing coastal low.


VFR conditions will prevail through late morning, before deeper
moisture and a short wave moves through during the late morning
and afternoon hours from off the Atlantic. This will bring us a
good chance of SHRA and TSRA from about 15-21Z, and the risk of
occasional MVFR conditions. Heavy rains can occur in any of the
convection due to an abundance of moisture and weak flow,
potential resulting in IFR weather at times. VFR conditions will
return this evening into early tonight. Low to moderate risk
for late night fog/stratus, depending upon how much clearing
occurs and how much rainfall happens today.

Extended Aviation Outlook: Thunderstorms may result in short periods
of flight restrictions, greatest potential during the afternoon and
early evening.


Today and tonight: Deep layered sub-tropical ridging with a
slackened surface pressure pattern will be the main feature
across the area, interrupted by a weak inverted trough that
moves into the local waters today, then dissipates tonight.
Outside of scattered convection ( which will cause some locally
heavy rains, frequent lightning, and maybe a few strong winds
gusts) there are no concerns regarding the synoptic winds (which
hold under 12kt) and the resulting seas of 2-3 ft.

Tuesday through Saturday: Western Atlantic high pressure will yield
a weak pressure gradient over the coastal waters into the mid week.
A cold front should sweep across the forecast area on Thursday. Sfc
winds are forecast to remain light from the SE on Tuesday, then
veering from the south-southwest Wednesday and Thursday. Wave
heights are expected to generally range between 2-3 feet Monday
through Thursday. Large high pressure centered over the Great Lakes
will build over the region Friday into Saturday. In addition, a
coastal low may develop off the GA and SC coast by Friday and may
slowly depart on Saturday. Winds should shift from the NE,
increasing to 10 to 15 kts by Friday afternoon. Four foot wave are
forecast across portions of AMZ350 and possibly beyond 20 nm
offshore. On Saturday, NE winds are forecast to strengthen to 15 to
20 kts, gusts between 20 to 25 kts. Wave heights are expected to
range from 4 to 6 feet with 20 NM, reaching 7 feet across AMZ374.

Rip Currents...The combination of small swell, onshore flow,
and astronomical influences will lead to an enhanced environment
for rip currents today. Considering the fact that we had
numerous rip currents reported yesterday at Isle of Palms and
Hilton Head, the cuts in the sandbar could still be there. Thus
we have maintained a Moderate Risk through the evening.

Waterspouts: We will keep watch on possible spouts today due to
favorable conditions with light winds, ample moisture and
instability. We have not issued any Marine Weather Statements
just yet since the SPC Non-supercell Tornado Parameter is still
not favorable, nor are there any well defined boundaries.


Astronomical influences and onshore winds (albeit light) will
continue to produce elevated tides through Tuesday. Only small
tidal departures are necessary and we should reach shallow
coastal flooding levels with the evening high tides along parts
of the SC coast. Coastal Flood Advisories will likely be needed.
For now it looks like conditions will stay just below any
advisory thresholds on the GA coast and in Jasper County.


The KCLX radar continues to be down due to lighting strikes.
Parts are on order but the radar is not expected to return to
service until Tuesday afternoon at the earliest.

The NWR transmitter, WXM-93 at Cross, SC will be off the air
until further notice. An off site tech will be at the site
later today.

The temperature and dew point sensors at the Downtown Charleston
observation site (KCXM) could periodically fail. Technicians
plan on fixing the problem.





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