Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
750 PM EDT Wednesday Sep 2 2015
a weak trough of low pressure will persist over the southeast
through late week. This weekend...high pressure will push a cold
front south of the region and will build into the area from the
north...and low pressure could develop off the southeast coast.
The region will then remain situated between inland high pressure
and offshore low pressure into the middle of next week.
Near term /until 6 am Thursday morning/...
scattered showers and a few thunderstorms will be possible over
southern areas early tonight as the remnants of Erika shift east-
northeast across much of southeast Georgia. A 500 mb shortwave will
then shift over the central Appalachians and toward the Middle
Atlantic States...steering the remnants of Erika off the southeast
coast. Expect most precipitation to occur along the southeast Georgia
coast where deep layered moisture and low level convergence is
maximized. Further north...the area will remain on the northern
fringes of the circulation pushing offshore...likely resulting in
a much dryer scenario through the night. After midnight...the
focus of most shower and thunderstorm activity will likely be over
the coastal waters. In general..overnight low temperatures will range in
the low to middle 70s under partly to mostly cloudy skies.
Short term /6 am Thursday morning through Saturday/...
Thursday and Thursday night...shortwave ridging will ripple through
the area through much of the day as a trough crosses the southern
Appalachians and approaches from the northwest. At the surface...a
trough will align along or near the coast and into the remnant
circulation of Erika. Models show the bulk of precipitation being
confined to areas along the coast...though there are considerable
differences in the amount of coverage. Model soundings aren/T very
noteworthy with cape values generally less than 1500 j/kg. So the
thinking is that coverage will be in the isolated to scattered
range...with a low end potential for a couple stronger storms thanks
to the presence of some middle-level dry air. Showers and storms will
dissipate in the evening with the loss of heating and the overnight
will largely be dry for land areas. Highs will run a few degrees
above normal...with values reaching the 91-93 degree range away from
the immediate coast. Overnight lows in the low to middle 70s.
Friday through Saturday...a rather complex pattern will begin to
take shape into the first half of the weekend. Much of the southeast
will be caught within a west/east oriented trough axis with a
building ridge across the central Continental U.S. And a large anticyclone
centered over New England. Friday will still see the weak surface
trough linger along the coast ahead of a backdoor cold front that
sinks in from the north late Friday night and into Saturday. Then
for much of Saturday the forecast area will be on the periphery of a
surprisingly well defined inland wedge. Convergence along the
periphery of The Wedge as a coastal trough tries to form will help
to drive improving coverage of showers and storms on Saturday
despite cooler temperatures. Friday will bring one more warm day
with highs into the low 90s...then Saturday will be in the middle to
Long term /Saturday night through Wednesday/...
a blocking pattern will prevail over the eastern Continental U.S./Western
Atlantic through this period. Onshore flow provided by the sprawling
surface high pressure to the north...moisture convergence enhanced
by a surface trough/low pressure just off the southeast coast and
low pressure aloft should translate to above normal probability of precipitation and
slightly below normal high temperatures. However...pockets of dry
air circulating into the region from the northeast will complicate
probability of precipitation forecasts. Thus...highest probability of precipitation are placed south southern
counties and over adjacent coastal waters through the period.
Toward the middle of next week...the upper low could retrograde
west of the region...and perhaps this will support slightly lower
probability of precipitation and a warming trend.
Aviation /00z Thursday through Monday/...
ksav...scattered showers and a few thunderstorms associated with the
remnants of Erika will slowly shift east-northeast and possibly
produce MVFR ceilings/visibilities at the terminal early tonight. Activity
is then expected to shift offshore by midnight. Otherwise...expect
VFR conditions to prevail through 00z Thursday.
Kchs...VFR conditions are expected to prevail through 00z Thursday.
However...we could see a few showers develop over the terminal early
tonight as the remnants of Erika shift offshore.
Extended aviation outlook...Thursday and Friday...chances for
brief flight restrictions with be driven by afternoon/evening
showers and thunderstorms. Increasing chances for restrictions
expected over the weekend as high pressure builds in from the
tonight...the remnants of Erika will slowly shift off the southeast
Georgia and South Carolina coast...bringing periods of showers and
thunderstorms over much of the coastal waters. A south to southwest
flow will be maintained over the waters...with speeds at or below 10
kts. Seas will continue to range between 1-2 feet across all waters.
Thursday through Monday...through the late week time period a weak
pressure pattern will prevail across the waters. This will keep
winds less than 10-15 knots...highest along the land/sea interface with
the diurnal sea breeze. Seas will be 1-2 feet. Then late Friday night
and into Saturday...strong high pressure will build in from the
north and a tightening northeast gradient will prevail. This setup
will result in increasing winds and seas. At this time it appears
winds will peak in the 15-20 knots range with seas perhaps as high as 4
feet beyond 20 nm. There is a low end potential for needing small
craft advisories late in the weekend and early next week.
Rip currents...an elevated risk for rip currents could develop again
next weekend into early next week.