Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 171735
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
135 PM 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/...
Radar continues to show showers and thunderstorms developing
across our area. Surface observations underneath this
precipitation are about ten degrees cooler. Given the increasing
coverage and the rain cooled air, we cancelled the heat
advisory. A handful of places still not yet impacted by the
convection could have heat indices remain around 110 degrees,
but this will be short lived as cold pool boundaries and/or the
sea breeze move through. Most of the convection is across the
Charleston Tri-County and these seems to match up well with the
CAMs. Additionally, SPC mesoscale analysis indicates the
greatest instability here. Thermodynamics look typical for this
time of year for pulse convection.

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 THURSDAY/...
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 /18Z THURSDAY THROUGH TUESDAY/...
Mainly VFR outside isolated to scattered afternoon and evening
showers/thunderstorms. Stronger storms could generate brief
flight restrictions, but we`re opting to put VCTS in for now and
reserve TEMPOs based on radar trends.

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...None.
SC...None.
MARINE...None.

&&

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


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