Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
444 am EDT Sat Aug 23 2014
Atlantic high pressure will prevail today as a weak trough of low
pressure lingers near the area. A backdoor cold front will slowly
push south through the area late tonight and Sunday followed by
cooler and drier high pressure through the middle of next week.
Meanwhile low pressure near the Bahamas will slowly move
northward...likely remaining well offshore.
Near term /through tonight/...
today...the eastern periphery of a high amplitude middle and upper level
ridge centered over the lower Mississippi Valley will continue to
extend over the region today...but heights should begin to lower
late in the day as a trough extends across New England and farther
down the middle-Atlantic coast. At the surface...Atlantic high
pressure will prevail while a weak trough lingers near the
area...but then as heights begin to lower a backdoor cold front
will advance toward the area from the north late.
Isolated to scattered showers and thunderstorms are expected to
develop this afternoon across much of southeast South
Carolina...with the greatest coverage across the Charleston tri-
County area as the backdoor cold front nears. Model soundings show
rather impressive instability this morning...with cape values
around 3200 j/kg...dcape values around 1000 j/kg and Li values
down to -6c. Once the subsidence inversion breaks...it appears
storms could quickly build in this environment...with some of them
becoming strong or severe through the afternoon. As a
result...have added mention of the potential for a few severe
storms in the hazardous weather outlook. Farther south across
southeast Georgia...a strong subsidence inversion appears to hold
through most of the afternoon and have kept a rain-free forecast
going through the day.
It is going to be a scorcher for one last day...as the influence
of strong subsidence...a downslope flow and compressional heating
ahead of the backdoor front will drive afternoon temperatures to
97-101 inland of the immediate coast. The sea breeze should remain
pinned along the Georgia coast...but appears to make better inland
progress along the South Carolina coast than previous days. Dew
points will likely not mix out significantly today and during
peak heating should range from around 70 across interior southeast
Georgia to the middle and upper 70s along the coast. Given this
combination of heat and humidity...heat index values should be
around 110 degrees in many areas and have issued a heat advisory
from noon until 8 PM for the entire forecast area.
Tonight...the backdoor cold front will make slow progress
southward through the area. The potential for showers and perhaps
a few thunderstorms will shift southward with the front and the
forecast will indicate isolated to scattered convection...with the
greatest chances in the evening. Temperatures will be mild once
again...with lows in the middle to upper 70s across much of the area.
Short term /Sunday through Tuesday/...
Sunday...a cold front will push through from the north causing some
showers and thunderstorms...especially over southeast Georgia where
moisture will be deeper. Highs around 90 near Charleston increasing
to the lower to middle 90s south and west of there...warmest closer to
the Altamaha river. Heat index values across southeast Georgia could
peak around 105-108 degrees. It will also become breezy...mainly near
the coast as the pressure gradient increases. Noticably drier air
will filter in Sunday night making for more comfortable conditions
with lows ranging from the middle/upper 60s north and west to middle to
upper 70s along the coast...warmest in Georgia. Some showers will be
possible...mainly along the Georgia coast where coastal convergence
and moisture will be maximized. Breezy north/northeast winds
expected...especially along the coast.
Monday/Tuesday...a cooler and drier air mass will generally persist
due to high pressure to the north. Meanwhile...low pressure near the
Bahamas will slowly trek northward but should stay well offshore.
However it should be noted that there is quite a bit of uncertainty
with this system given that it has not even developed yet. Temperatures
will generally be near to even slightly below normal...probably
coolest on Monday. Some showers could push inland from time to time
given the abundant low-level moisture/convergence.
Long term /Tuesday night through Friday/...
high pressure looks to remain in control inland through middle week but
low pressure is expected to track northward off the southeast US
coast...probably in the form of a tropical cyclone as per the NHC
tropical weather outlook. This system should stay far enough
offshore to not be much of a concern locally but folks should
monitor it just in case. Much will likely depend on its strength as
that will determine which steering currents affect it.
Otherwise...the brisk north/northeast flow will cause some showers
which could push onshore just about anytime through middle week. By
more uncertainty for late week as much depends on the timing of the
low pulling away and a possible cold front approaching from the
Aviation /08z Saturday through Wednesday/...
VFR conditions expected to prevail at kchs and ksav through 06z
Sunday. Increasing probabilities for showers and thunderstorms to
develop along/ahead of an approaching front from the north later
today. Have added vicinity thunderstorms at kchs from 18-23z but
just limited ksav to vicinity showers after 23z given greater
uncertainty in coverage of thunderstorms during the later part of
the forecast period.
Extended aviation outlook...MVFR ceilings and some showers/thunderstorms are
possible Sunday as a cold front pushes through. VFR conditions
should then prevail Sunday evening through middle week...but it should
be breezy each afternoon as mostly northeast winds develop behind
today and tonight...Atlantic high pressure and a weak trough of low
pressure inland will maintain a south-southwest flow through much
of the day. A backdoor cold front will then begin to drop
southward across the waters tonight...with winds gradually backing
east or northeast. Wind speeds should remain below 15 knots however
as the gradient remains rather weak. Expect seas to persist in the
1-2 feet range through tonight.
Sunday through Thursday...conditions will go downhill Sunday behind
a cold front. Increasing north/northeast winds will lead to building
seas with advisory conditions likely spreading southward later
Sunday. The forecast gets a bit uncertain toward middle week as much
depends on the strength and track of a possible tropical cyclone
offshore. Mariners are thus urged to stay tuned to the latest
forecasts. At this time the system should stay weak enough and far
enough offshore to have limited impacts locally. Otherwise...the
brisk northeast winds will also cause an increase in tide levels
higher than the normal astronomical tide...possibly reaching close
to advisory levels during the evening high tide cycles through early
week...mainly along the SC coast.
record maximum temperatures...
ksav hit 100 degrees today...the first time since July 26 2012.
kchs...........98 set in 1983
kcxm...........97 set in 1987
ksav...........99 set in 1987
Last time 100 degrees or higher...
kchs..........June 21 2011
kcxm..........June 20 2011
Georgia...heat advisory from noon today to 8 PM EDT this evening for
SC...heat advisory from noon today to 8 PM EDT this evening for