Scientific Forecaster Discussion
fxus62 kchs 300810
Area forecast discussion
National Weather Service Charleston SC
410 am EDT sun Apr 30 2017
Atlantic high pressure will give way to a cold front which will move
through Monday night into Tuesday. High pressure will then bring dry
weather through Wednesday night before rain chances return late in
the week as a stronger storm system affects the area. Drier high
pressure should then return by the end of the weekend.
Near term /through tonight/...
the mid level pattern will continue to feature a 593dm ridge centered
near Bermuda as a closed low deepens over the Great Plains.
Heights will begin to fall just west of the forecast area, but
heights should change very little overhead through this
afternoon. Sfc winds will remain from the south-southeast through the day.
However, the mixed layer should deepen by mid to late morning,
momentum Transfer resulting in gusts above 20 mph. Gusty
conditions will continue until an hour or two after sunset.
Otherwise, it should be another very warm and dry afternoon
across the forecast area. Nam12 soundings indicate that a
capping inversion will persist at 700 mb through this afternoon. I
will not include any mention of showers and thunderstorms in the
forecast, but I would anticipate that deep convection will
occur closer to the fall line. High temperatures are forecast to
range from the upper 80s across the inland counties to the
around 80 over the Sea Islands.
Tonight, a cold front is timed to push east of the Mississippi River
by late tonight. The pressure gradient between the approaching
front from the west and high pressure over the Atlantic will
result in steady south winds over land, with increasing flow
across the marine zones. A fairly wide band of showers and
thunderstorms is expected to persist along and ahead of the cold
front. A few weak showers may reach inland Georgia by sunrise
Monday, highlighted with schc pops. Temps should cool generally
to around 70 tonight.
Short term /Monday through Wednesday/...
a cold front will approach from the west Monday, but should be
weakening as it does so as the main upper low moves north into the
Great Lakes. Still think there is a good chance for showers and
thunderstorms, especially inland and during the afternoon. There
could be a few severe storms, mainly late in the afternoon west of I-
95. Also, it will be quite breezy as the pressure gradient and low-
level jet increase. Gusts to 30-35 mph are expected, especially near
Charleston. A lake Wind Advisory is possible for Lake Moultrie. Most
showers should be offshore shortly after midnight Tuesday with drier
air moving into the area and persisting through Wednesday.
Temperatures should stay near to above normal through the period.
Long term /Wednesday night through Saturday/...
mainly dry through Wednesday night before rain chances return with
the approach of a stronger storm system toward the end of the week.
Although there is a bit better agreement between the GFS and European model (ecmwf)
than 24 hours ago, there are still some timing differences so
forecast confidence is lower than normal. Conditions look at least
marginally favorable though for some stronger storms Thursday. Rain
chances should diminish by Friday night but we kept small rain
chances into Saturday given the uncertainty regarding the . Timing
of the cold front and strength/timing of the upper trough.
Temperatures should be near or above normal through at least
Thursday night before likely dropping below normal Friday through
Aviation /08z Sunday through Thursday/...
pre dawn restrictive ceilings will remain the primary forecast
challenge with the 6z tafs. Guidance ranges on the coverage and
duration of low stratus are quite high and forecast confidence
is low. However, given the lack of observed restriction and
upstream cirrus, I will favor VFR until 9z with MVFR through
sunrise. NAM forecast soundings show thickening of low level
saturation after sunrise, finding agreement with 5z lamp. I will
include a tempo group at kchs and ksav from 11z-15z for bkn015
ovc025. As the mixed layer deepens by late morning, momentum
Transfer should yield south wind gusts from 23 kts to 28 kts
through the afternoon. Winds are forecast to decouple by 2z.
Extended aviation outlook: ceiling and/or visibility restrictions
are likely Monday into Monday night due to low clouds, showers and
thunderstorms, then again Thursday/Thursday night, possibly lasting
into Friday. Breezy/gusty conditions also expected Monday, Thursday
and Friday afternoons.
a cold front will sweep across the Mississippi River by this
evening and tonight. South-southeast winds will become gusty today, reaching
just short of Small Craft Advisory levels over the chs Harbor
this afternoon. However, wave heights of six feet will spread
west into amz374 by this evening, reaching the near shore SC
waters following midnight. Wave heights and gusty winds will
spread across all marine zones, including the chs Harbor by
Monday through friday: advisory conditions expected Monday into
Monday night when winds could gust near 30 kt and seas could build
to 8 ft near the Gulf Stream. Could see an advisory linger Tuesday
morning, mainly across the offshore Georgia waters, but otherwise no
significant issues until Thursday when another cold front
approaches. Much uncertainty regarding the timing of the frontal
passage but conditions look poor either way through Friday.
Record high minimums for 30 april:
kchs: 69/2014 and previous years.
Ksav: 72/1953 and previous years.
Marine...Small Craft Advisory from 8 am Monday to midnight EDT Monday
night for amz330-354.
Small Craft Advisory from midnight tonight to noon EDT Tuesday
Small Craft Advisory from 8 PM this evening to noon EDT
Tuesday for amz374.