Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1127 am EDT Sat Oct 10 2015
low pressure will move through the area today and tonight...
gradually lifting northeast away from the southeast coast Sunday and
Monday. A weak cold front will move through on Tuesday followed by
high pressure through Saturday.
Near term /through tonight/...
late this morning...no significant changes to the forecast or the
forecast reasoning. The band of showers and embedded thunderstorms
has progressed nicely across the forecast area from the west...but
infrared satellite shows that cloud tops are warming and the
reflectivity data shows that the convection has weakened. Forecast
rainfall totals have largely been left as they were...with some
slight decrease in a few areas. Still...1-2 inches with locally
higher amounts is possible. The ongoing Flash Flood Watch is well
placed and is still warranted given the lingering issues from the
recent flooding...and not much additional rainfall will be needed
to cause new flooding issues. Hopefully rainfall rates will remain
light to moderate...potentially easing flooding concerns.
Concerning the potential for severe weather...we have seen enough
breaks in the cloud cover to allow temperatures to warm up into
the upper 70s. The window is closing though...and destabilization
will be limited through the early afternoon. There is 30-35 knots
of deep layer shear...so the severe threat is non zero but
Tonight...our Flash Flood Watch will end later this evening as
forcing and convergence with the upper low ease off to the NE and
east overnight. The backside of the system should begin to affect the
region with plenty of low clouds and scattered lighter rains and
even some late night drizzle possible. Many areas will see lows in
the 60-62 degree range.
Short term /Sunday through Tuesday/...
guidance has definitely slowed down the upper low...now keeping it
over the area for much of Sunday before a stronger upper trough
kicks it out of the area by Monday. The surface low meanwhile will
linger along the SC coast through Sunday night...albeit weaker than
earlier in the weekend. Moderate wrap-around moisture and isentropic
ascent will support patchy rain showers over eastern areas on
Sunday. We also included some drizzle for Sunday morning given
stronger isentropic ascent in the warm conveyor belt. The best
chance for rain will shift over the coastal waters by Sunday night
but we increased sky cover through Monday morning given the slower
progression of the low. Extensive cloud cover and cold advection
will result in a cool day on Sunday with highs in the lower to
A quasi-zonal flow sets up on Monday with a weak cold front pushing
through on Tuesday. Increasing low-level thicknesses and westerly
flow will favor a warming trend during the period with high temperatures
around 80 by Tuesday.
Long term /Tuesday night through Friday/...
a quasi-zonal flow will be the main feature during the extended
range...maintaining high pressure at the surface. Dry weather
expected with seasonable temperatures in the 50s/70s.
Aviation /15z Saturday through Wednesday/...
VFR conditions will prevail at kchs and ksav this morning but
changes on the way as convection continues to congeal over east
central Georgia north into the csra and South Carolina midlands.
We have timed the convective precipitation including cumulonimbus/thunderstorms into the
terminals in the 16z-22z time frame. MVFR ceilings/visibilities are possible
in convection during this time frame. Later this afternoon into
early evening...the dry slot may briefly improve conditions before
a cold front drops quickly south tonight. In the wake of the
front...precipitation should become more drizzle or patchy light rain and
ceilings will lower to MVFR/IFR overnight with northerly surface winds
Extended aviation outlook...low ceilings possible into Sunday.
Mainly VFR conditions expected thereafter.
today...light and variable winds and seas 1 to 2 feet prevailing
within 20 nm. There is a small potential for some gusty thunderstorms
winds this afternoon as convection shifts off the coast.
Tonight...north to north-northeast winds will increase after
midnight with surface wind gusts to 20 knots developing late
behind the departing surface low. Seas will build to 2 to 4
North to north-northeast winds will persist on Sunday with a morning surge due to
the surface low remaining near the coast. There could be a brief
window of 25 knots gusts over the waters Sunday morning before the
gradient relaxes. Conditions settle down early next week though a
brief surge is expected out of the west-southwest on Tuesday as a
cold front pushes through.
this potential flash flooding event the next 24 hours will not
mirror the catastrophic event South Carolina recently endured...
precipitable waters and water vapor imagery do not exhibit a strong tropical tap
and moisture convergence will wane in comparison. Our Flash Flood
Watch this morning blankets a region reviving from the duress of
an extremely unusual long period of flooding and damage. While 1
to 1.5 inches of rain will probably not cause further extensive
flooding...locally higher amounts are certainly a decent
possibility and they would likely create additional problems.
Northern portions of the Charleston tri-County region are the
primary focus for the most persistent rains. The Flash Flood
Watch for east central South Carolina is in effect through this
The edisto...which is in major flood...and Ashley rivers have
crested...but water levels appear to remain nearly steady or will
only slowly fall through the weekend. The Santee river is expected
to crest over the weekend...reaching major flood stage. Rainfall
amounts exceeding one inch...with locally higher amounts today
should not significantly worsen flooding along area rivers/creeks...
but could slow water level recession.
high tides are expected to approach 7 feet mean lower low water
for Charleston Harbor for the next several days. Any expected
levels above 7 feet would require coastal flood advisories.
SC...Flash Flood Watch until 11 PM EDT this evening for scz043>045-