Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
150 PM EDT Thursday Jul 2 2015
Atlantic high pressure will prevail as a trough of low pressure
lingers inland through the middle of next week.
Near term /through tonight/...
this afternoon and evening...a broad middle level trough of low pressure
will expand over much of the eastern Continental U.S. With a series of 500 mb
shortwaves that round its southern base 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 west/southwest flow that builds deep moisture to the area
with precipitable waters characterized around 2.0 inches this afternoon. These
moisture levels along with forcing along the base of the low
pressure trough centered to our north will favor a period of showers
and thunderstorms over much of the area this afternoon and evening.
The bulk of precipitation activity should come in two rounds...the first
being associated with a seabreeze along the southeast South Carolina
and southeast Georgia coastline early this afternoon. Thunderstorms
should remain pulse type in nature for the next couple hours...but
could produce isolated strong and/or damaging wind gusts. The second
round of precipitation will likely be associated with a 500 mb shortwave that
shifts over the area later this afternoon into the evening. Expect
precipitation coverage and intensity to peak while this shortwave begins
to shift over the area late afternoon into the evening. A few of
the hires models suggest activity will be in the form of a line of
showers and thunderstorms that push eastward into the area during
middle/late afternoon through evening hours. Sbcapes ranging between
1500-2000 j/kg along with 20-30 knots low level winds will support some
stronger and/or severe thunderstorms later this afternoon into the
evening...especially when activity encounters the seabreeze. The
main threat associated with these strong or severe thunderstorms
will be damaging wind gusts.
Tonight...convection will linger late this evening...but should
gradually taper off from west to east with the loss of daytime
heating early tonight. Low temperatures will range in the low to middle 70s.
Short term /Friday through Sunday/...
Friday and Friday night...a broad upper level trough will persist
aloft...while at the surface Atlantic high pressure will prevail as
a Piedmont trough lingers just inland. The resulting deep southwest
flow will allow abundant moisture to advect into the region...with
precipitable water values rising to just above 2 inches. The
moisture combined with short wave energy aloft rounding the base of
the upper trough will be more than sufficient to generate showers
and thunderstorms across the area. The coverage should be best
across the Charleston tri-County area due to its closer proximity to
the Piedmont trough and greater energy aloft...with the forecast
reflecting showers and thunderstorms likely here...with a diminishing
chance farther south into southeast Georgia. The chance for showers
and thunderstorms will persist well into the evening...before
diminishing overnight. There is the potential for some of the
thunderstorms to become strong or severe...especially if any
organization can occur...with the primary threat being damaging
winds. Highs should be in the lower 90s with lows in the lower to
middle 70s inland from the immediate coast.
Saturday and Sunday...the synoptic pattern changes little...but
there appears to be a bit less in the way of short wave energy aloft
and more of a westerly component to the low and middle level flow.
While this should result in less coverage of showers and
thunderstorms...will still maintain a low chance in the forecast
given the proximity of the Piedmont trough and to account for some
low level convergence along the sea breeze. Highs should be in the
lower to middle 90s on Saturday and around 90 or in the lower 90s on
Sunday. Lows should remain in the lower to middle 70s.
Long term /Sunday night through Wednesday/...
medium range guidance is in fairly good agreement through the long
term period. An upper level trough will deepen over the southeast
states and persist through mid-week. Meanwhile at the surface...
Atlantic high pressure will prevail as a Piedmont trough persists
inland. This pattern will favor chances of mainly diurnally driven
showers and thunderstorms each day...with the greatest coverage
likely to occur Monday and Tuesday. Temperatures will generally be
near normal...with highs around 90 or in the lower 90s and lows in
the lower to middle 70s.
Aviation /18z Thursday through Tuesday/...
VFR ceilings/visibilities should prevail at both terminals for the next hour
or two. Showers/thunderstorms will then be possible later this
afternoon into evening hours...producing a period of flight
restrictions at either terminal. Have maintained a period of MVFR
visibilities associated with thunderstorm activity between 21-01z. In
addition...gusty winds upwards of 20 knots are expected through much
of the afternoon. VFR ceilings/visibilities will likely return at both
terminals overnight and persist into late morning hours on Friday.
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
this afternoon...Atlantic high pressure will dominate the weather
pattern over the coastal waters with a Piedmont trough situated
inland. In general...south/southwest winds will remain at or below
15-20 kts...highest near a seabreeze circulation and during
nocturnal low level jetting. Seas will range between 2-4 feet. Winds
and seas will vary within thunderstorms...which are currently
ongoing near a seabreeze along the coast early this afternoon. A
few of these thunderstorms could be severe with strong wind gusts
being the primary concern.
Friday through Tuesday...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...winds will be a bit stronger along the Charleston
County coast this afternoon than previous days. This combined with
modest swell and astronomical influences will contribute to a
moderate risk of rip currents through this afternoon.