Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
944 PM EDT Tuesday Sep 23 2014
a high pressure wedge will build inland...while a coastal trough
persists offshore into middle week. The region will then remain
situated between high pressure to the north and a stationary front
to the south and east through early next week.
Near term /until 6 am Wednesday morning/...
the pattern at the surface and aloft is quite anomalous for late
September...and in fact Tuesday was the first day of autumn and
it sure felt like it for many locales. We only hit 72 at the
Charleston Airport...the coolest since may 2. A few parts of the
Charleston tri-County area even failed to even get out of the 60s!
The synoptic pattern features a nearly stationary middle/upper low
that is filling over the Carolinas and northern Georgia...while a wedge
of surface high pressure continues to build inland and a
sharpening coastal trough persists offshore. A solid north/NE
flow prevails on the periphery of the inland wedge and is
responsible for keeping skies mostly cloudy to overcast over many
sections. There has however been some clearing of skies across
interior southeast Georgia...but this will only be temporary as satellite
trends showing an increase in clouds to arrive again overnight.
Deep moisture streaming up the eastern and northern edges of the
low aloft...favorable middle/upper dynamics within the right rear
quadrant of the 250 mb jet and spokes of vorticity circulating
counterclockwise around the cyclone aloft will provide additional
forcing for ascent. When combined with convergence along the
coastal trough anchored out to the east of the Gulf Stream...this
will support our best rain chances tonight east of I-95. Thus we
have maintained the notable precipitation gradient across the
area...with chance to likely rainfall impacting Charleston
County...slight chance or chance most elsewhere east of I-95...but
with little or no probability of precipitation further inland.
The coastal trough will amplify further overnight and tighten the
gradient between the inland wedge...supporting moderate to strong
north/NE winds. Conditions will be breezy at times inland and
downright windy along the coast...gusting up to 25 to 30 miles per hour at
times. The combination of these cool trajectories and small cool
advection will allow temperatures to fall several degrees below
seasonal normals...ranging from the upper 50s inland to the middle
60s near the coast. These values will be the coolest air mass so
far this season.
Short term /6 am Wednesday morning through Friday/...
Wednesday...the core of the upper low will remain centered just
west of the forecast area...while a surface trough will persist over
or close to the coastal waters. Through at least the morning
hours...the upper low will continue to draw a north-S plume of deep
moisture over the coastal trough and into coastal counties
especially along the SC coast. Farther South/West the upper low...a
middle-level dry slot will persist closer to the upper low...and this
should greatly limit the coverage/persistence of any showers.
Thus...morning probability of precipitation are graduated from likely along the South
Carolina coast...where locally heavy rain is possible...to slight
chance inland. During the afternoon...23/12z guidance suggests that
the upper low will gradually unravel/weaken...and forcing for
ascent/associated probability of precipitation should diminish along the SC coast.
Otherwise...high temperatures are expected to remain below normal...ranging
from the Lower/Middle 70s north/near the SC coast to upper 70s/near 80f
Wednesday night through Friday...23/12z guidance agrees that the
upper low will continue to dissipate west of the region in favor of
an offshore upper ridge expanding over the region. The surface
trough will likely linger over or close to the coastal waters...and
moisture will remain sufficient for probability of precipitation tapered from chance along
the coast to slight chance inland...highest during the afternoon.
Meager instability justifies no mention of thunderstorms over land
through the period.
Temperatures should gradually moderate and high temperatures could approach
normal values as early as Friday.
Long term /Friday night through Tuesday/...
considerable disagreement between models in the long term though it
looks as though the expansive high will remain north of the area
through Sunday. Enough moisture and lift will exist for isolated to
scattered showers across the area. Potential for increasing rain
chances early next week if a front lifts north out of the Gulf per
the 00z European model (ecmwf). Temperatures return to near normal for late sept by the
Aviation /02z Wednesday through Sunday/...
we have followed a climatological based forecast for the airfields
within the cool air wedge and a continued feed of north/NE winds.
Since widespread low cloud decks are already in place we should
remain at MVFR or lower through the night and into Wednesday at
both terminals. While we are forecasting solid MVFR
conditions...IFR is certainly possible at times...and this is
shown by a few sites near kchs and ksav that are down under 1000
feet overcast this evening. Sporadic light to perhaps moderate rains
will impact kchs as a continued influx of moisture streams in from
off the Atlantic within a deep south/SW flow above about 8-10k feet.
Meanwhile...a tight gradient between inland high pressure and a
pronounced coastal trough will support gusty north/NE winds for
much of Wednesday. Although not enough to warrant the inclusion of
wind shear given the elevated surface winds...pilots are advised
that winds will be as high as 30 or 35 knots tonight into Wednesday
morning around 1-2k feet.
Extended aviation outlook...MVFR or lower ceilings possible at times
into the end of the week with a lingering wedge pattern and ample
low level moisture.
tonight...we have continued the previous forecast which shows
solid small craft advisories in place across the entire marine
community...courtesy of a strong north/NE pinching of the gradient
between formidable inland high pressure wedge and a well-defined
coastal trough east of the Gulf Stream. There is little overall
change in this scenario through the night...although land breeze
influences will attempt to hold winds more northerly closer to the
coast than further away from the shoreline. We have seen frequent
gusts up near or past 35 knots or gale force winds at buoy 41004 for
several hours since Tuesday afternoon...and while a few gusts
within our local waters could approach gales...the more frequent
gusts will be capped near 30 knots. Seas will be mainly short-period
and wind driven...averaging 3-5 feet near the immediate shore to as
much as 6 or 7 feet out near 20 nm offshore...but up to 8 or 9 feet
further out. Given the low stratus decks...visibility will become
somewhat restricted at times...but nothing worse than about 3 nm.
Through late week....strong high pressure will remain centered to
the north...a stationary front will remain south and east of the
area...and a coastal trough will meander over or close to the
waters. This pattern should support a prolonged period of elevated
northeast winds and associated elevated seas. However...23/12z
guidance has reduced winds/seas during the Wednesday-Friday time
frame...and this introduces forecast uncertainty. Perhaps highest
winds/seas will periodically surge into the region...as opposed to
the relatively steady state conditions depicted by the ongoing
forecast. For now...significant changes are not justified...and
small craft advisories will continue into Wednesday night for all
marine zones outside Charleston Harbor.
Next weekend...high pressure will remain to the north and the
stationary front will remain south/east of the waters. However...the
high pressure could expand across the region...perhaps supporting a
subtle relaxation of the surface pressure gradient and associated
Rip currents...local guidance yielded a borderline low/moderate risk
for rip currents on area beaches Wednesday. Given some uncertainty
regarding wind direction/speed...without a significant change in the
ongoing pattern...opted for a persistence forecast of moderate
risk for Wednesday. An elevated risk for rip currents will likely
continue through at least late this week...and perhaps even into
a prolonged northeast flow with moderate wind speeds will result in
elevated tide levels through the upcoming weekend. Although the
positive anomaly would need to be a little more than 1.0
feet...there is a moderate chance of the tide reaching 7 feet MLLW
along the South Carolina coast that would necessitate coastal flood
advisories around the times of high tide.
the downtown Charleston weather sensor at Waterfront Park /kcxm/
remains out of service...and hourly observations and daily climate
information will not be available until further notice. It is
unknown at this time when the sensor will be returned to service.
Marine...Small Craft Advisory until 5 am EDT Wednesday for amz330.
Small Craft Advisory until midnight EDT Wednesday night for