Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 171455
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1055 AM EDT Thu Aug 17 2017

.SYNOPSIS...
Atlantic high pressure will remain in control as troughing
persists inland through late this week. A cold front will
approach this weekend, likely falling apart early next week as
high pressure returns. Another cold front will approach during
the middle of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The 10 AM temperatures were running anywhere from 1-3 degrees
higher than forecasted in most locations. Opted to bump up the
high temperatures by 1-2 degrees since high temperatures should
be reached before afternoon convection begins. As a result, this
pushed heat indices to 110 degrees for the remaining inland
counties. Therefore, we extended the heat advisory to these
locations. They may be borderline whether they reach the 2 hour
minimum criteria before convection fires, but even one hour is
still impactful for those outside.

Otherwise, the expansive mid/upper level subtropical ridge will
persist across the region with another dose of hot and steamy
summer weather for the forecast area. Due to a general increase
in moisture between 700 MB and 500 MB, we will see PWATs build
over the area but with the ridging aloft, the main impetus for
diurnal convection will be a sluggish sea breeze and some broad
low level moisture convergence near an inland surface trough.
Thermodynamics look typical for this time of year for pulse
convective and POPs today closely maintained general
persistence, not far off typical climo numbers for this time of
year.

It is interesting to note that both the GFS/ECMWF show a good
coverage of convective coverage prior to 18Z and if this were to
occur, heat advisories would be cancelled early. We favored the
slightly later initiation with convective temps from forecast
soundings showing the mid 90s.

Tonight: Isolated evening or perhaps even spotty coastal nocturnal
convection is possible, as models show the upper ridge flattening
late. We played it mainly dry after midnight with another very
warm night with lows in the upper 70s most areas.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
Friday: The area will lie near the southern extent of the westerlies
aloft, as deep ridging persists across the region from the
Atlantic to the desert SW. The surface pattern features a well
pronounced Piedmont trough and the sub-tropical ridge axis
across FL. Despite weak subsidence capping, there is adequate
instability and moisture to generate scattered convection,
mainly in the afternoon and generally triggered by the sea
breeze and small-scale boundary interactions. PWat in excess of
2.0-2.2" and storm motion from W to E at only around 10-12 kt
will support locally heavy rain in persistent storms. The
overall thermodynamics aren`t overly impressive, but with DCAPE
greater than 1000 J/kg, strong wind gusts can occur in a few of
the tallest t-storms. Both the 925 mb and 850 mb temps remain
abnormally warm, supportive of another day with above normal
highs, at least in the lower and middle 90s prior to the onset
of convection. Associated heat indices should peak at 105- 108F,
so no Heat Advisory is currently anticipated.

Saturday: Despite a strong short wave sweeping through the Great
lakes, OH valley and upper midwest, deep ridging holds firmly in
place. The impulse aloft will push a cold front a little closer to
the area from the NW, but similar to the pattern aloft, strong
ridging will block the progress of the front from getting too close.
But due to its proximity, PWat greater than 2" and typical
instability, convective rain chances will reach at least 50%,
perhaps higher once boundary interactions, mergers, etc occur. There
is a little more shear (around 15 or 20 kt within the first 6km), so
a few multicellular clusters can occur. Moisture pooling in advance
of the front can result in a heavy rain threat, but storm motion is
a bit higher than Friday, so no widespread flooding would occur.
Again with the DCAPE in excess of 1000 J/kg we`ll need to be
concerned with some wet microburst potential a few of the storms.
Little change in low level temps and thickness values, but with a
higher rain chance, max temps won`t be as hot as recent days, but
still above normal.

Sunday: Deep sub-tropical ridging will dominate, as the cold front
fizzles out to the NW, as the sea breeze looks to be the main focus
for convection, aided by some forcing due to the RRQ of the upper
jet across eastern NC and SE VA. Plenty of moisture and typical
instability, will support at least chance PoP`s, and with less flow
than on Saturday and the high PWat`s there is yet again another
locally heavy rain potential. Not much change in low level temps and
thicknesses, so max temps will again reach above normal prior to
convection developing.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
Abnormally strong mid level ridging will prevail Monday and Tuesday
before broad long wave trough develops over a good portion of the
eastern states during the middle of the week. Simultaneously at the
surface the sub-tropical ridge will persist as a lee side trough
redevelops for Monday and Tuesday, but with diminishing heights
aloft and the formation of the large scale trough, a cold front will
attempt to approach late in the forecast period. Plenty of moisture
and modest thermodynamics will support at least scattered coverage
of showers and t-storms through the period, including Monday, the
"big" day of the total solar eclipse. Temps will remain above August
norms.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
Mainly VFR outside isolated to scattered showers/thunderstorms,
mainly this afternoon and early evening. We have a late day VCTS
at KCHS given persistent model trends from the NAM model and at
this time we have left KSAV without any convection and let radar
trends play out this afternoon before any further committals.

Extended Aviation Outlook: Temporary flight restrictions can occur
in scattered and mainly diurnal convection through early next
week.

&&

.MARINE...
The low level ridge axis remains well off to the south and
southeast of the waters through tonight and a typical warm
season regime of south to southwest synoptic flow will persist
over the waters. Speeds will be fairly light today with seas 2
ft or less near shore and 3 ft well offshore. Late day and
overnight surges should not amount to more than 15 kt on average
with seas still in the 2 to 3 ft range overall.

Friday through Monday: For the most part the sub-tropical Atlantic
ridge will maintain its hold on the coastal waters, blocking
upstream low pressure troughs and a cold front from ever getting
this far SE. At times there is enough tightening of the gradient and
a boost from the sea breeze and nocturnal low level jetting to
produce S or SW winds as high as 15 or 20 kt, but not quite enough
to reach advisory levels. Seas will top out at 3 or 4 ft. Mariners
should plan for at least isolated to scattered showers and t-storms
through the entire period.

&&

.TIDES/COASTAL FLOODING...
Astronomical influences will lead to a round of elevated tides early
this weekend into early next week. Only small tidal departures are
necessary and we could approach shallow coastal flooding levels,
especially over parts of the SC coast, including downtown
Charleston.

&&

.CLIMATE...
Record high minimums for 17 August...
KCHS 78/2010
KCXM 82/2010
KSAV 78/1995

Record high minimums for 18 August...
KCHS 78/2010
KCXM 82/1998
KSAV 78/2010

Record High minimums for 19 August...
KCHS 79/2010
KCXM 81/2009
KSAV 79/1878

&&

.EQUIPMENT...
The temperature sensor at the Downtown Charleston observation site
(KCXM) has failed. Technicians are working to resolve the problem.
Temperature and dewpoint data will not be available until the sensor
is replaced.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Heat Advisory until 7 PM EDT this evening for GAZ087-088-
     099>101-114>119-137>141.
SC...Heat Advisory until 7 PM EDT this evening for SCZ040-042>045-
     047>052.
MARINE...None.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...33
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...
CLIMATE...
EQUIPMENT...



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