Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
457 am EDT sun Aug 30 2015
a strong upper level disturbance will lift north through Georgia
and the Carolinas through tonight. On the periphery of Atlantic high
pressure...a weak trough of low pressure will meander over the
southeast most of this week. Late this week into next weekend...low
pressure will develop over the Atlantic while weak inland high
pressure expands south and east into the region.
Near term /through tonight/...
pre-dawn...quite a surge of deep moisture has been ongoing across southeast
Georgia overnight as low level flow has veered to southeast off the Georgia
coastal enhancing strong convergence along the Savannah River and
pushing showers and thunderstorms north into southern and coastal South
Carolina. A cluster of intense rainfall passing through eastern
Chatham County but it should move along and perhaps weakening
just a bit as it shifts into Jasper and Beaufort counties. By
dawn...chance probability of precipitation reaching into our far northwest zones while likely or
even higher chances south of I-16 into coastal South Carolina.
Through tonight...an anomalous middle and upper low over the western
Florida Panhandle at daybreak will lift NE and gradually open
into a wave as it heads toward the southern Appalachians. This
feature will result in an steady influx of deep moisture with
precipitable waters above 2 inches into southern SC and southeast Georgia. It also arrives
with decent forcing for ascent in response to a 40-45 jet maximum at
500 mb and steady upper divergence as it passes overhead. This
will cause at least scattered to numerous coverage of rainfall
with pockets of deep convection likely along with occasional
A complex mesoscale with unseasonably strong low level moisture
gradients last night has given way to rains marching north with some
thunder along coastal and southern zones. There should be a good
potential for locally heavy rainfall today...the GFS and recently
high resolution outputs from both the hrrr and NSSL arw models
suggest there could also be a lull in heavy rain potential north
of the Savannah River this afternoon between thunderstorms developing
ahead of the ejecting low across east central Georgia/csra and
coastal convergence continuing to the south of the Savannah
River. Models suggest steepening inland lapse rates along with at
least 25 knots of 0-6 km bulk shear across our inland zones this
afternoon and evening. Inland areas to west of I-95 could see a
window for isolated strong to severe thunderstorms as upper level forcing
increases ahead of very well defined drier slot between 850 mb and
500 mb. Would not be surprised to see convection organize later
today to the west of I-95 and we will have to monitor for isolated
severe weather potential.
Tonight...continued complexities as the upper low ejects by and
drying middle levels eventually shut down rains along and west of
I-95 but attention turns to heavy rain potential along coastal
South Carolina after midnight as strong upper divergence will
enhance lift while deep layered moisture and significant low level
moisture convergence enhances convective rains. We ramp probability of precipitation up
to categorical along the central South Carolina coast late.
Locally heavy rains...perhaps isolated excessive rainfall may
occur along Charleston County if the latest model guidance pans
Short term /Monday through Wednesday/...
Monday morning...the upper trough and associated forcing for ascent
will have advanced north/northeast of the region. However...focused
low/middle level moisture transport and an associated plume of precipitable waters 2
to 2.25 inches will continue to push onshore especially into South
Carolina counties. Given above-average model agreement regarding
this scenario...Monday morning features likely/categorical probability of precipitation
along/east of I-95...highest into SC counties including the
Charleston tri-County region. Given elevated precipitable waters and high
precipitation efficiency...introduced a mention of heavy rain into
areas along/east of I-95. Also...an elevated high tide will occur
between 900 and 1100 am...and if/where heavy rain is falling within
coastal counties localized flooding will be possible. This potential
included downtown Charleston.
Monday afternoon...probability of precipitation become less well defined due to the unknown
impact of morning showers/thunderstorms and due to the potential for
subtle drying to occur across northern/inland counties. Early
afternoon probability of precipitation are tapered from likely near the coast to chance
Otherwise...due to clouds and precipitation...Monday high temperatures in
the Lower/Middle 80s should be common...slightly below normal for late
Monday night...a pool of deep moisture will remain focused near the
coast and over coastal waters. Inland probability of precipitation decrease from chance to
slight chance with time inland...while probability of precipitation near the coast remain in
the 50-60 percent range...highest over central/southern coastal
counties. Locally excessive rainfall will remain possible near the
The scenario appears similar Tuesday through Wednesday. While
slightly drier air could infiltrate northern/inland counties...a
pool of enhanced moisture will remain trapped beneath a building
upper ridge...especially over southern counties and across adjacent
coastal waters. Probability of precipitation are distributed accordingly...with
highest/likely probability of precipitation across southern counties. High precipitable waters and weak
steering flow will continue to support the potential for locally
heavy rainfall. Temperatures will depend on cloud/precipitation coverage.
Highs in the middle 80s will remain common both days. However...
temperatures could remain in the lower 80s if/where clouds/showers
persist...while temperatures could recover into the upper 80s where
precipitation coverage is reduced...especially inland.
Long term /Wednesday night through Saturday/...
until late week...sufficient moisture will persist beneath an upper
ridge to support at least scattered showers/thunderstorms especially
during the midday/evening hours and along the coast/over adjacent
coastal waters during the late night/early morning hours. Late this
week into next weekend off the middle Atlantic coast near 37n/70w...an
amplifying upper trough will promote surface cyclogenesis...and this
system will evolve into deep layered/cut-off low pressure. This
system will drive the upper ridge well south of the region...but the
impacts on sensible weather remain uncertain. If the Atlantic trough
phases with the upper low along the Gulf Coast...an unsettled
pattern could develop over the region. However...if the systems
remain separate...somewhat drier air could spread into the region
from the north/northwest. Thus...probability of precipitation remain pretty close to
normal. Temperatures should remain close to normal through the
period...and forecast temperatures remain below 90f through next
weekend. However...the probability for warmer temperatures into the lower
90s could increase late week into next weekend especially if drier
air circulates into the region.
Aviation /08z Sunday through Thursday/...
the overall picture is a swing to gradually deteriorating
conditions with arrival of showers and storms before dawn at
both ksav and kchs. Showers and a few storms numerous through the
afternoon hours as deep moisture and dynamic lifting processes
resulting in widespread coverage. Periods of IFR conditions are
possible during bouts of heavier rains and/or thunderstorms today...
especially from around midday to early evening.
Extended aviation outlook through Thursday...periods of flight
restrictions appear likely through early week...especially in
showers/thunderstorms. VFR should become more common middle to late
onshore flow will increase across Georgia waters later today and
tonight...perhaps 15-20 knots with some higher gusts possible from
Grays Reef and beyond. This could assist seas building to Small
Craft Advisory levels over Georgia waters from 20-60 nm later
tonight. Widespread rains and occasional thunderstorms over the waters
through tonight will lower visibilities below 3 nm at times and produce
locally gusty winds...especially tonight. We have held off on
issuing an Small Craft Advisory for outer waters just yet until we get a better
handle on convection patterns which may disrupt winds but
locally enhance waves.
Persistent southeast winds could usher a 6 feet swell into Georgia coastal waters
beyond 20-40 nm Monday. This scenario remains uncertain...thus will
not yet raise a Small Craft Advisory. Otherwise...southeast winds
should remain capped at 15-20 knots and seas should remain 3-5 feet
through Tuesday. Middle to late week...a weakening onshore pressure
gradient will translate to lighter winds...frequently 10 knots or
less...and seas should subside to 1-3 feet...highest beyond 20 nm.
Meanwhile...showers/thunderstorms will remain common over coastal
waters through this week. Stronger thunderstorms could produce gusty
winds...frequent lightning and poor visibility in very heavy
rainfall. Mariners should remain alert for special marine warnings
and marine weather statements.
Mariners are advised that scattered to numerous showers and
thunderstorms will occur each of the next several days and nights.
Some of these storms will produce frequent lightning...heavy rains
reducing visibilities to near zero at times and wind gusts up to
at least 30 knots with briefly higher seas. Remember to monitor our
forecasts and your National oceanic and atmospheric administration weather radio.
Conditions won't be much different Sunday with onshore
winds...small swells and astronomical influences from the perigee
and full moon. This will result in at least a moderate risk of rip
currents at area beaches.
While we start to lose the astronomical influences as we go through
the week...there is still an onshore wind component and possible
small swells to generate a possible elevated risk during at least the
early part of the week.
Rip currents...a moderate risk of rip currents may be upgraded to
high risk at Tybee Island later this morning...otherwise an
elevated risk for rip currents could persist into early this week.
through tonight...astronomical influences due to the perigean
Spring tide plus onshore winds will maintain elevated high tide
levels along the coast. While we will be close to possible
shallow coastal flooding in Charleston Harbor with the morning
high tide today...a better chance for shallow salt water flooding
exists with the evening high tide. Coastal flood advisories will
likely be required for at least parts of the SC coast...and we/ll
be close to the advisory criteria further south...including the
Savannah area. Also of note...periods of showers/thunderstorms
with heavy rain are expected. If heavy rain falls in coastal
communities...including downtown Charleston and/or downtown
Savannah around the times of high tide...more significant flooding
Monday and Tuesday...lingering astronomical influences and
persistent onshore flow could produce a couple of additional cycles
of shallow salt water flooding with the nighttime high
tide...especially along the South Carolina coast. Also of concern...
periods of heavy rain are possible along the coast early this
week...and localized flooding is possible especially if heavy rain
falls around the times of high tide.