• Currently

    Reported at Hilton Head Automatic Weather Observing / Reporting System, SC
    12:50 PM EST SUN AUG 20 2017
  • 90°F32°C
  • Partly Cloudy
    Partly Cloudy
  • Feels Like:105°F41°CDew Point:79°F26°CHumidity:71% Winds:SSE 6 mph 10 kph
    Pressure:30.09 in1019 hPaVisibility:10.00 miles16090.00 meters
  • Area Forecast Discussion

151 PM EDT Sun Aug 20 2017

.SYNOPSIS...
A weak stationary front will gradually dissipate across the
region early this week. A stronger cold front is expected to
pass over the region on Thursday. Slightly cooler and drier
conditions are expected through the rest of the week into next
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
No changes with this update.

Rest of today: A TUTT low will continue to slowly progress W-NW
through the Gulf of Mexico, allowing for an expansion of strong
sub-tropical ridging to spread across the local area, keeping
short wave energy off to the N. A weak stationary front located
not too far to the N-NW of the CWFA will meander nearby
throughout the day, as Atlantic high pressure starts to build
in. At this time, the synoptic flow backs around to the SE,
causing even higher PWATs to arrive from the Atlantic. The sea
breeze and the proximity to the front looks to be the main focus
for convection during the afternoon, with coverage similar to
yesterday (mainly in the 20-30% range). We`re not too concerned
with any severe potential due to warm mid levels, but dry air
in roughly the 750-450 mb layer will cause DCAPE values to peak
around 1,000-1,200 J/kg, enough for locally gusty winds. PWATs
around 110-120% of normal and weak steering flow will cause a
few of the storms to produce heavy downpours with minor
flooding concerns.

The onshore trajectories and an early start to the sea breeze
will keep our coastal counties no higher than 90-94F, but still
into the middle 90s further inland. Heat indices will peak at
100-105F, maybe briefly a little higher near the US-17 corridor
if dew points can climb above 80F.

Tonight: A short wave located near the northern Bahamas from
this morning will lift N-NW with a weakness in the sub-tropical
ridge, bringing with it a further increase in moisture from the
ocean. A deepening SE flow that extends through about 500 mb,
better isentropic ascent and larger scale forcing for ascent
with the short wave will produce an increasing chance of showers
and t-storms arriving overnight, probably after 2 or 3 am. By
daybreak Monday at least widely scattered to scattered activity
will be encompassing the coastal counties. Although the steering
flow is more progressive, given the abundance of moisture there
will be a heavy rain and localized flooding threat. Temps will
remain well above average for this time of year, which has been
the case most nights this month. Not completely convinced of
fog late at night since SREF probabilities are low and we may
not reach our cross-over temps. But MOS guidance is showing fog
along the NW tier of counties after 09Z, so will make
adjustments in later forecasts if necessary.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Monday: Short range guidance shows a broad mid level low will track
across the western Gulf of Mexico as the center of ridge shifts near
the SC coast during the daylight hours Monday. A weak mid level
inverted trough is expected to run between the Gulf low NE across FL
off the GA coast. GFS and ECMWF indicate that a band of enhanced
cloud cover will develop within the region of the weak mid level
trough. Simulated IR from the GFS indicates that the cloud cover
associated with the H5 trough will expand between the Southeast
coast and Bermuda between 15z through 0Z today. It will be very
interesting to observe later today if the mid and high cloud cover
develops as guidance suggests. Otherwise, Mon will be a day where PW
values increase to around 2 inches with tall and thin instability.
This environment should support a healthy Cu field with some degree
of cirrus level moisture. Overall, cloud cover and deep convection
will be a factor across the forecast area during the solar eclipse.
However, several gaps in the cloud cover should occur, more common
north of the Edisto River Basin. Scattered thunderstorms are
forecast across SE GA and west of I-95. Elsewhere, isolated
thunderstorms are possible. Guidance supports high temperatures
ranging from the low 90s across the mid Savannah River valley to the
upper 80s near the beaches.

Tuesday and Wednesday: Mid level ridge is forecast to remain
centered over the forecast area on Tuesday and will gradually weaken
as trough ripples over the Mid West on Wednesday. Forecast soundings
show an inversion at H75 Tuesday afternoon, decreasing on Wednesday.
I will indicate low Chc PoPs across the land zones on Tuesday,
increase to solid Chc PoPs on Wednesday. High temperatures are
forecast to peak across the inland Savannah River Valley Weds
afternoon, reaching the mid to upper 90s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
A cold front should push across the forecast area on Thursday,
likely supporting a band of thunderstorms. PoPs on Thursday have
been increase into the likely range across most of the forecast
area. 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 is low. Medium range
guidance indicates that the sfc high center will shift over New
England, ridging SW across the Carolinas and GA. In addition,
both the GFS and ECMWF show a coastal low developing off the
GA/SC coast by late this week. Guidance supports prolonged NE
winds, 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 5 through 7 period.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Mainly VFR. Convection along the sea breeze will bring a risk
of SHRA or TSRA to the airports. Direct impacts could lead to
brief flight restrictions. For now, we`re opting to go with VCTS
for the next few hours. However, we may need to amend to add
TEMPOs. This will be highly dependent on radar trends. The
convection will quickly dissipate around sunset. Models hint at
the potential for some fog overnight, but confidence is too low
to include in the current TAFs. Another round of SHRA and TSRA
are expected to develop Monday morning and we started trending
that way in the TAFs.

Extended Aviation Outlook: Thunderstorms may result in short periods
of flight restrictions, greatest potential during the afternoon and
early evening. Patchy fog is possible over wet soil Wednesday
morning.

&&

.MARINE...
Today: A stationary front will meander inland over the SE, as
high pressure becomes more dominate from the E. The result is
for winds backing around from SW to SE through the day at no
higher than 10 or 15 kt. Even with a small ~9 second swell,
combined seas will be no more than 2 or 3 ft.

Tonight: The stationary front becomes ill-defined as sub-
tropical high pressure continues to expand west, producing SE
and S winds around 10 kt. Little change in seas.

Monday through Friday: Western Atlantic high pressure will yield a
weak pressure gradient over the coastal waters through mid week. A
cold front should sweep across the forecast area on Thursday. Sfc
winds are forecast to remain light from the SE through 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 on Friday. In addition, a coastal low may
develop off the GA and SC coast late Friday. Winds should shift from
the NE, increasing to around 10 kts by Friday afternoon. Four foot
wave are forecast across portions of AMZ350 and possibly beyond 20
nm offshore.

Rip Currents...The combination of swell, onshore flow, and
astronomical influences have led to a favorable environment for
rip currents. Multiple rip currents have been reported at
several beaches today. The moderate risk continues through this
evening.

&&

.TIDES/COASTAL FLOODING...
Astronomical influences will continue to produce elevated tides
through Tuesday. Only small tidal departures are necessary and
we could approach shallow coastal flooding levels with the
evening high tides along parts of the SC coast. Coastal Flood
Advisories might be needed.

&&

.CLIMATE...
Record High Minimums for August 20...
KCHS...78 set in 2010 and previous.
KCXM...80 set in 2009 and previous.
KSAV...82 set in 1878.

&&

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

&&

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

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...
CLIMATE...
EQUIPMENT...