Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
355 am EDT Friday Aug 1 2014
a trough of low pressure will linger near the southeast coast through
the middle of next week.
Near term /through tonight/...
pre-dawn...a very complex mesoscale for this time of year over
Georgia and the Carolinas. A layer of drier air atop the boundary
layer was moving through along and west of I-95 producing build-down
stratus which is resulting in some fog in some areas. A wedge
pattern was locked into north Georgia and upstate South Carolina
while the coastal trough was right along the beaches and barrier
islands for the most part. Moisture convergence was substantial
along coastal zones and the Santee Cooper lakes region. Scattered
showers and perhaps a few thunderstorms are anticipated east of I-95 in South
Carolina prior to dawn...some downpours likely across Charleston
Excessive rains fell across a chunk of the Charleston area
yesterday with flooding occurring around low tide which was a
fortunate outcome for the tide cycle. The stalled coastal trough
along with rich low level moisture fields and a series of short
waves through Georgia and the Carolinas should continue to provide
a risk of periodic convective rains across the forecast area
through tonight. This morning...high resolution models suggest the
coastal convergence will eventually result in repeating convection
along the South Carolina coast which should peak between middle
morning and middle afternoon before convergence shifts a bit further
inland. A Flood Watch issued last evening was trimmed to drop
Colleton County but retained until 15z given the latest model
trends and approaching late morning high tide. Morning radar trends
and 12z sounding analysis will dictate whether it will need an
extension into the afternoon hours.
After low clouds and fog dissipate over southeast Georgia...instability
will increase and scattered showers and thunderstorms should break out
this afternoon. Not every area inland from I-95 will see rain
today...but there could be some multi-cell clusters producing
locally heavy rain. Clouds will temper high temperatures again today with
upper 80s many places. A few 90s possible inland Georgia.
Tonight...the risk for heavy rains may redevelop along coastal South
Carolina as moisture convergence strengthens and moisture profiles
deepen. Scattered to perhaps numerous showers and isolated thunderstorms
expected to develop after midnight with models in good consensus
that we will need to watch for additional localized flooding.
Short term /Saturday through Monday/...
a very unsettled and wet pattern will dominate the southeast U.S.
Coast through Monday with deep/tropical moisture featuring precipitable waters
in excess of 2 inches advecting northeast out of the Gulf of
Mexico and interacting with a decaying coastal front. A broad
long-wave trough extending from a large cyclone over Canada
coupled with various waves of upper divergence associated with the
right entrance ageostrophic circulation of the upper jet will
provide favorable periods of upper forcing...while at the surface
low-level convergence along the coastal front and eventual mesoscale
boundary interactions will support well above normal rain chances
through the period. Likely probability of precipitation of 60-70 percent will be shown
each day with activity tapering off during the overnight hours.
However...a southerly flow regime atop the boundary layer will
support nocturnal shower/thunderstorm activity just about each night with
the best rain chances occurring over the coastal counties. There
will be a continued risk for locally heavy rainfall and the
potential for flooding with the 925-700 hpa flow remaining fairly
light. Highs each day will will generally maximum out in the upper 80s
with overnight lows in the Lower-Middle 70s.
Long term /Monday night through Thursday/...
drier...more seasonable weather will return by middle week as the front
meanders offshore and dissipates. Atlantic high pressure will
gradually reestablish itself across the region with more a more
typical convective pattern expected. Highs will be in the upper 80s
to lower 90s with lows in the Lower-Middle 70s. Tropical Storm Bertha
is forecast to remain offshore as it curves well off the southeast
Aviation /08z Friday through Tuesday/...
kchs...showers and possibly some thunderstorms are expected to
advect onshore and impact the terminal beginning nearing daybreak
if not sooner. In addition...given the earlier heavy rainfall
that occurred...some patchy ground fog and low stratus could
develop and result in flight restrictions. IFR ceilings lurk just off
the west in saturated low level air at 0530z. As a result...the
taf indicates tempo showers with MVFR conditions in the 10-14z
time frame. Then VFR conditions should prevail...although showers
with at least some thunderstorms in the vicinity could be an issue
through about 20z with any flight restrictions being temporary
during times of heavier rainfall. The showers/thunderstorms should
shift west of the terminal later in the afternoon as a coastal
trough moves inland.
Ksav...low clouds and fog have expanded over the terminal pre-dawn
as a pocket of drier air atop the boundary layer have made for an
unusual early August build down stratus event. The NAM model shows
the prime conditions for low clouds and fog will be fleeting and
it looks like the LIFR conditions should not last too long after
daybreak...perhaps bouncing sooner. Scattered showers and storms
expected late morning and this afternoon with thunderstorms in the vicinity a good bet at
some point. Tonight...chances are best for VFR conditions but
cannot rule out a late night shower or brief MVFR ceilings.
Extended aviation outlook...periods of IFR/MVFR likely through
Tuesday as numerous showers/thunderstorms impact the region.
onshore flow more southeasterly on the Atlantic side of a surface trough
along the coast through tonight. Speeds mainly below 15 knots with
seas 2 to 4 feet on average...highest beyond 20 nm off the coast.
Showers and thunderstorms will be periodic and mainly nocturnal...lowering
visibilities at times.
Saturday through Wednesday...pinning down wind direction will be
tricky over the weekend as a stationary front meanders very near the
coast. Speeds look to remain light however and should not pose a
challenge to mariners. Winds will return to a more climatologically
favored southerly regime by middle week as Atlantic high pressure takes
hold. Swells generated by Tropical Storm Bertha could impact the
waters Monday into Wednesday as it curves well offshore of the
southeast U.S. Coast. There is a possibility that combined seas
could reach Small Craft Advisory thresholds during this period...but
its a bit too far to really pin that down this far out. Seas will be
capped at 4-5 feet for now.
SC...Flash Flood Watch until 11 am EDT this morning for scz044-045-