Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
955 PM EDT Wednesday Jul 1 2015
Atlantic high pressure will reside across the area while a trough
of low pressure persists inland.
Near term /until 6 am Thursday morning/...
low level convergence zone...what looks to be the inland trough
aided by outflow from decaying upstream mesoscale convective system has shifted into the
forecast area. This feature along with short wave energy aloft and
upper difluence will continue to spur scattered convection mainly
to the west of Savannah prior to midnight. We have adjusted probability of precipitation
into the 40-50 percentile from Pembroke and Statesboro to Tattnall
and Candler counties. This is surrounded by 20-30 probability of precipitation from
Beaufort to Savannah and Ludowici and northwest to Ridgeville and
Hampton. This scenario is well depicted by the 18z NAM. Dcapes are
still as high as 1200-1300 j/kg and MLCAPES of 2000-2500 j/kg.
Despite the nocturnal environment and increasing cinh...a few of
these storms will be strong to severe through 11 PM or midnight.
Strong downbursts of wind will be the main hazard...along with
frequent lightning and locally heavy rains.
Coverage will gradually diminish overnight...but will attempt to
shift into the Charleston metropolitan area as middle level perturbation
rides toward coastal SC. That along with some enhancement from
coastal convergence within a SW synoptic flow could trigger
isolated showers/T-storms from about 1-4 am.
Smokey conditions aloft originating from western Canadian
wildfires will persist through the night...giving the sky the
appearance of cirriform cloudiness and odd colors to the full
moon. But actual cloud cover will generally be partly to mostly
cloudy...the result of scattered to broken cumuliform clouds and
scattered to overcast middle/high level clouds.
On average lows will be in the lower and middle 70s...but a few
places could hit 70 where heavier rains occur.
Short term /6 am Thursday morning through Saturday/...
Thursday and Friday...a broad middle level trough of low pressure will
expand over much of the eastern Continental U.S. Through late week with a
series of 500 mb shortwaves that round its southern periphery over the
southeastern United States. At the surface...the area will remain
between a Piedmont trough inland and high pressure over the western
Atlantic. The setup will favor a southwest flow that builds deep
moisture to the area with precipitable waters characterized around 2.0 inches each
day. These moisture levels along with forcing along the base of the
low pressure trough centered well to our north will favor chances of
showers and thunderstorms over the area. A few thunderstorms could
also be strong or severe when 500 mb shortwave activity along with the
arrival of a 20-30 knots low level jet shifts over the area on Thursday.
We could see a similar risk of stronger to severe thunderstorms on
Friday as the Piedmont trough makes a slight shift toward the coast
in response to low pressure developing over the middle Atlantic region.
The main threat associated with stronger thunderstorms will be
damaging wind gusts.
Overall high temperatures will generally be in the lower 90s each day.
However...temperatures could be a few degrees cooler than currently
forecast on Thursday due to smoke from fires originating in Canada.
Overnight lows will range in the low to middle 70s away from the coast
Saturday...weak low pressure should remain over the Middle Atlantic
States into the weekend...resulting in stronger low level
convergence along a seabreeze across the Charleston tri-County.
Chances of showers and thunderstorms will remain in the forecast
for most areas...but expect most activity in northern areas along an
inland moving seabreeze. In general...high temperatures will be in the low
90s for most areas.
Long term /Saturday night through Wednesday/...
the end of the weekend will continue to feature an inland trough as
high pressure continues to build in from the Atlantic. Coverage of
showers and thunderstorms should be rather minimal on Sunday with
little in the way of strong forcing for ascent. By early next
week...models are trending a bit wetter as they indicate more in the
way of troughing aloft. This will not only support at least
scattered showers and thunderstorms each day...but could also result
in slightly cooler temperatures than what is currently forecast.
Aviation /02z Thursday through Monday/...
we have recently adjusted the ksav taf to include mention of thunderstorms in the vicinity
due to upstream convection reaching near the airfield from 03-05z.
If a direct impact were to occur than a period of flight
restrictions would be necessary. Stay tuned for possible
Odds favor the upstream convection falling apart before reaching
kchs tonight. But we/ll amend if necessary.
For Thursday...probabilities are sufficient enough to include thunderstorms in the vicinity
and cumulonimbus clouds during the afternoon and evening with stronger
vertical wind fields and middle level energy to arrive. Should direct
impacts occur then flight restrictions and a temporary burst of
high winds would occur...mainly in the 18-23z time frame.
Extended aviation outlook...brief flight restrictions are possible
in mainly afternoon/early evening isolated to scattered showers and
thunderstorms. Otherwise...VFR conditions are generally expected to
tonight...there remains a decent gradient between sub-tropical
ridging to the southeast and south and a pronounced trough inland to the
northwest and north. This along with nocturnal jetting will keep SW
winds elevated near 15 knots in the Atlantic and 10-15 knots in
Charleston Harbor for much of the night...before land breeze
influences clock winds around more to the west at lower speeds
late. Seas will average 1-3 feet near the immediate coast and 2-4 feet
further out...dropping about a foot late. A few storms that move
offshore will be strong or even severe...producing wind gusts in
excess of 35 or 40 knots.
Thursday through Monday...Atlantic high pressure will generally
dominate the weather pattern over the coastal waters with a
Piedmont trough situated inland. A tightened pressure gradient
between the two features will keep winds fairly elevated through
the period. In fact...it appears that conditions could reach
marginal Small Craft Advisory criteria over a portion of the
waters Friday afternoon into Friday night. Otherwise...sea breeze
circulations and nocturnal low level jetting will allow
south/southwest winds to peak near 15-20 kts...with 25 kts locally
at times in favored locations. Seas will be 2-4 feet on
average...but could reach 5 feet in the outer portions of the waters
during nocturnal surges.
Rip currents...astronomical influences from a full moon along with
swell of 2 feet with a period of 8-9 seconds will lead to an enhanced
period of rip currents on Thursday. A moderate risk of rip currents
is therefore possible along the Charleston County coast.