Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
949 PM EDT Thursday Jul 31 2014
a trough of low pressure will linger near the southeast coast
through the middle of next week.
Near term /until 6 am Friday morning/...
the forecast is on track. Already seeing confluence bands in the
form of fine lines on kclx radar reflectivity data developing
south of Charleston County and stretching into the Georgia
offshore waters. In fact...showers have recently started to
develop near the downtown Charleston area along one of these bands.
The latest h3r is coming in line with the latest rap and 4km-NSSL
WRF showing convection re-firing later tonight. The Flash Flood
Watch remains in effect.
Water vapor imagery shows a pronounced impulse move across Alabama
and Georgia early this evening which is prognosticated to approach the
area from the southwest overnight. Showers/thunderstorms are already
redeveloping over central and western Georgia ahead of this
feature within a corridor of enhanced uvm. The latest set of
mesoscale models including the rap and 4km-WRF suggest additional
showers/thunderstorms will redevelop over the Atlantic and move inland
well after midnight. It appears several enhanced confluence zones
will setup off the Georgia coast in the vicinity of the coastal
trough with mean steering trajectories favoring the lower South
Carolina coast as 850-700 hpa winds veer with time. Precipitable waters surging
back near 2 inches...enhanced uvm associated with the approaching
shortwave and favorable wind profiles atop the hybrid wedge
suggest there will be a continue risk for heavy rainfall overnight
into Friday morning. Adjusted probability of precipitation up per going short term and
model trends...showing categorical probability of precipitation across the far north
through about 11 PM with 30-50 percent probability of precipitation after midnight--highest
along the lower South Carolina coast.
As much as 4-8 inches of rain has fallen over portions of
Charleston...Berkeley...Dorchester and Colleton counties over the
past 12 hours...most of which has fallen with the last 6 hours.
With the risk for additional heavy rain and flooding possible
overnight into Friday morning...a precautionary Flash Flood Watch
will be issued for the aforementioned counties until late Friday
Short term /6 am Friday morning through Sunday/...
Friday and Friday night...a deep southwest flow aloft will prevail
across the region between an upper trough extending from the Great
Lakes region through the middle/lower Mississippi Valley and a
strong upper ridge over the western Atlantic. This will push
precipitable water values upwards of 1.9 inches across the area. At
the surface...a well-defined coastal trough appears to shift just
inland during the day where it will linger through Friday night. The
deep moisture combined with convergence along/near the surface
trough will cause scattered to numerous showers and thunderstorms to
develop. It appears the greatest coverage will be offshore extending
northward into North Carolina...and also just inland from the coast
along the western periphery of the trough. Given weak storm motions
have added heavy rainfall to the forecast as localized flooding will
be possible. Extensive cloud cover and onshore flow will limit highs
to the upper 80s to around 90 across much of the area inland from
the coast. Lows Friday night should be in the lower to middle 70s.
Saturday through Sunday...unsettled conditions will continue through
the weekend. The upper trough to the west will amplify while strong
ridging persists over the western Atlantic. Meanwhile...a trough of
low pressure at the surface will linger near or just inland of the
coast. Deep southwest flow aloft will continue to cause precipitable
water values to rise...likely exceeding 2 inches across most of the
region. The abundant moisture...convergence near the trough and
several disturbances passing in the southwest flow aloft will result
in showers and thunderstorms becoming likely across most of the
area. The threat for heavy rainfall and localized flooding will
continue as well. Temperatures will be impacted by extensive
cloudiness and good coverage of showers/thunderstorms...thus do not
anticipate highs exceeding the upper 80s. Lows Saturday night should
be in the lower to middle 70s.
Long term /Sunday night through Thursday/...
thanks to the presence of the lingering deep trough aloft and deep
moisture across the area...the active period of weather will
continue into early next week. The surface trough in the area
finally begins to weaken Tuesday into Wednesday while Atlantic
ridging begins to push back in from the east. By the middle of the
week it appears that we will return to a much more typical diurnal
pattern of showers and thunderstorms. Also...temperatures will warm
back to near normal values late in the period.
Aviation /02z Friday through Tuesday/...
kchs...showers and possibly some thunderstorms are expected to
advect onshore and impact the terminal beginning late tonight. In
addition...given the earlier heavy rainfall that occurred...some
patchy ground fog could develop and result in flight restrictions.
As a result...the taf indicates prevailing showers with MVFR
conditions in the 09-16z 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...VFR conditions are generally expected to prevail at the
terminal through 00z Saturday. However...patchy ground fog could
result in temporary flight restrictions to MVFR levels in the 09-13z
time frame. Scattered showers and thunderstorms are expected Friday
afternoon...but given lower confidence in timing/coverage have left
mention out of the forecast at this time.
Extended aviation...a surface trough will linger through early next
week. An unsettled period is expected and there are increasing
chances for periodic flight restrictions due to showers and
thunderstorms during this time.
across SC waters...a gradient enhanced between low pressure to the
south and high pressure to the north will support elevated onshore
winds of 15-20 knots this evening...then the gradient and associated
winds could relax somewhat overnight. Across Georgia waters...a weaker
gradient closer to low pressure will support S/southeast winds mainly 15
knots or less. Seas will range from 2-4 feet across the waters through
Friday through Monday...a trough of low pressure will linger near or
just inland of the coast through early next week. Winds will
generally remain anywhere from east to south across the waters
during this time. While the gradient may tighten at times and allow
for modest wind increases...overall winds and seas are expected to
remain below advisory thresholds.
SC...Flash Flood Watch through Friday morning for scz043>045-049-