Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
716 PM EDT Wednesday Jul 30 2014
a coastal trough will settle over the area through late this week
and will persist into the middle of next week.
Near term /until 6 am Thursday morning/...
this evening: various convective outflows and other mesoscale
boundaries continue to generate shallow convection early this
evening. One well defined outflow extending from Perkins to Rocky
Ford-Oaky- Pocotaligo-Jacksonboro at 30/2250z will likely be the
primary focus for renewed convection over the next 1-2 hours
before activity wanes with the loss of insolation. Adjusted probability of precipitation
to match the expected evolution of the mesoscale pattern through
the remainder of the evening hours.
Overnight: low-level speed convergence will increase along the
coast after midnight as a weak wave of low pressure meanders near
the Jacksonville Florida area and inland wind fields drop off in
response to a decoupled boundary layer. This synoptic setup could
support the formation of late night or early morning shower/thunderstorm
activity along the coast...especially given the broad cyclonic
flow aloft. Increased probability of precipitation to 30 percent across the coastal areas
to account for this...especially along the barrier and Sea
Islands. Lows from the upper 60s inland to the middle-upper 70s
immediate coast and downtown Charleston look on track.
Short term /6 am Thursday morning through Saturday/...
shortwave energy dropping into the longwave upper trough on Thursday
will cause it to dig toward the central Gulf Coast. As a result middle
and upper level winds will back to southwesterly...eliminating the
cap and increasing precipitable waters . Extensive cloudiness and the lack of
low-level ridging will keep high temperatures from climbing much higher
than 90 degrees in most locations. This in turn will limit
instability parameters with mixed layer cape around 1000 j/kg in the
afternoon. The coastal trough will begin shifting toward the coast
during the day...and a sea breeze will setup by afternoon.
Convergence associated with these two features should help trigger
isolated showers and thunderstorms.
Friday into Saturday we expect a significant increase in shower and
thunderstorm coverage. The upper trough will deepen and a series of
shortwaves will move into central SC. Precipitable waters increase above 2 inches
while the surface trough strengthens and moves inland. Additionally
a 90 knots jet streak developing across New England will be located
such that some upper level divergence will overspread the southeast
United States. We are showing high end chance probability of precipitation Friday through
Saturday though at some point likely probability of precipitation may be justified for
Saturday. Heavy rainfall with some localized flooding will be the
main hazard due to such a moist airmass combined with ample forcing
and steering flow around 10 knots. Highs will be in the upper 80s.
Long term /Saturday night through Wednesday/...
the unsettled pattern will continue for the latter half of the
weekend and at least into early next week. The trough will remain in
the vicinity of the forecast area with a trough aloft and precipitable waters in
excess of 2 inches. All these factors add up to an active period
with relatively high rain chances. Given the time range probability of precipitation have
been kept in the high chance range...but if the models continue
their current trends it is likely that higher probability of precipitation will eventually
be warranted. Locally heavy rainfall will remain a concern through
early next week with rounds of convection anticipated. Temperatures
will be below normal...running about 3-5 degrees below normal for
highs into the middle week time period.
Aviation /23z Wednesday through Monday/...
lingering shower activity is largely expected to diminish overnight.
Another round of showers is possible on Thursday...so have
maintained mention of vcsh in the taf. Will wait until later
issuances to include specifics when timing/coverage is better
defined. Outside any impacts from showers...VFR is expected to
Extended aviation...a surface trough will drift into the area late
this week and through the weekend. An active period is expected
and occasional flight restrictions are possible during this time.
tonight: a cyclonic pressure gradient somewhat enhanced between
high pressure to the north and low pressure to the south will
support east winds 10-15 knots with a few gusts as high as 20 knots
tonight. Seas of 2-3 feet will prevail...although seas could build
to as high as 3-4 feet beyond 20 nm late.
Thursday through Sunday...a coastal trough will drift over the
waters and eventually inland by early next week. East to southeast winds will
prevail Thursday into Friday...swinging around to southerly on
Saturday once the trough shifts inland. The gradient may tighten at
times though winds/seas will remain below advisory thresholds.