Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
642 am EDT Monday may 4 2015
Atlantic high pressure will prevail today into Wednesday. An area
of low pressure is expected to develop off the East Coast of
Florida on Wednesday...and lift northward off the southeast coast
through Thursday. The low could then meander back toward the coast
through Friday...and linger nearby through the weekend.
Near term /until 6 PM this evening/...
early Monday morning...ongoing forecasts remained on track and
required only minor adjustments to hourly parameters...mainly
cloud cover to account for altocumulus over SC counties.
As onshore flow continues to push shallow marine moisture onshore
and a pool of vorticity noted on water vapor imagery becomes
cut-off over the region under a building upper ridge...high
resolution guidance suggests that isolated/diurnal showers will
develop and dissipate along the sea breeze this afternoon. While
non-zero probability of precipitation are reasonable...strong subsidence depicted by model
soundings suggests that even shallow showers with measurable
precipitation will remain hard-pressed to develop...although a
decent diurnal cumulus field should spread out under the
inversion. Thus...held probability of precipitation below 10 percent with no explicit
mention of precipitation this afternoon.
Otherwise...highs in the lower 80s will be common away from the
coast this afternoon...and a few inland spots could reach the middle
80s. Onshore winds will maintain cooler temperatures mainly
in the Lower/Middle 70s on area beaches/Sea Islands.
Short term /6 PM this evening through Thursday/...
expect another tranquil night as Atlantic high pressure ridges
onshore across the region. Lows should range from the middle 50s
inland to middle 60s on the coast. Patchy ground fog could once again
develop late. While fog has not yet been included within public
Tuesday and Tuesday night...the pattern aloft will
be quite nebulous and summerlike with the forecast area under a
ridge that extends north and northwest back toward the upper
Midwest. At the surface...high pressure over the Atlantic will
extend into Georgia and the Carolinas leading to a prevailing
easterly flow through the day. Overnight...winds will turn more
northeasterly with time as an area of low pressure begins to develop
to the south and coastal trough becomes better defined along the
coast. Land areas are expected to be dry on Tuesday...thanks to
plenty of middle/upper level dry air and warm middle levels. There may be
isolated showers over portions of the coastal waters within some
weak surface convergence...but any activity that develops will
remain offshore. Overnight...much of the column remains quite dry
and the developing low is still located well to the south.
Therefore...land areas are dry and the forecast is generally quiet.
Tuesday highs will be held in check thanks to the prevailing
easterly flow and temperatures are expected to top out in the low
80s away from the immediate coast. Overnight...look for upper 50s to
Wednesday through Thursday...the details of the forecast will
ultimately be determined by the eventual track and strength of a
surface low that develops off the Florida East Coast/northern
Bahamas. Overall model agreement has improved significantly...though
notable differences remain with regards to proximity to the coast
and potential impacts from the system. There has been a clear
westward trend...first in the location of surface low
development...second in the track proximity to the southeast coast.
The NAM/GFS/ECMWF offer three different solutions...with the GFS
being the furthest west and the European model (ecmwf) being the furthest east. In
fact...the 00z GFS takes the center of the low to just off the
Charleston County coast by late Thursday. It also remains unclear if
conditions will be supportive of this system transitioning to
subtropical or tropical...especially given the early may sea surface
temperatures in the upper 70s within the Gulf Stream and beyond. The
National Hurricane Center continues to advertise a 30 percent chance
of the system acquiring subtropical characteristics. Based on the
westward trend in the models...the forecast has been adjusted to
feature higher probability of precipitation mainly along the coast with cooler temperatures.
At the very least...it appears we will be dealing with an increased
risk of rip currents and the potential for some beach erosion with
the persistent northeast to northerly winds around the low. Stay
tuned for forecast updates especially as model agreement hopefully
Long term /Thursday night through Sunday/...
there continues to be much uncertainty in the long
term period thanks to the presence of the aforementioned surface low
near the coast. Though the models have come into better agreement
there are still significant differences that have a significant
impact on the forecast. Due to the westward trend in the model
guidance...probability of precipitation have been increased but confidence is still low
enough to keep probability of precipitation in the chance range at best. Other than the
GFS...the consensus is for the low to pull away to the east and
northeast through the weekend and early next week. This would allow
for a warming trend in temperatures with the possibility for diurnal
Aviation /12z Monday through Friday/...
VFR conditions will prevail at kchs and ksav through 06z Sunday.
Low probability for brief visibility restrictions in ground fog toward
the end of the 12z taf period.
Extended aviation outlook...VFR conditions will prevail through
Wednesday. Chances for flight restrictions then increase as an area
of low pressure tracks near the coast through the latter part of the
today...a weak pressure pattern and light/variable winds to start
will give way to onshore winds...enhanced to some degree by the
sea breeze this afternoon. Winds should remain less than 15
knots...accompanied by seas 2-4 feet...highest beyond 20 nm.
East winds less than 20 knots and seas 2-4 feet are expected tonight.
Tuesday through Friday...the main forecast problem revolves
around an area of low pressure expected to develop off the East
Coast of Florida on Wednesday and track north through the end of
the week...eventually drawing closer to the coast. Eventual marine
impacts will ultimately be determined by the proximity to the
coast and the strength of the system. Also...the National
Hurricane Center continues to advertise a 30 percent chance that
the system could acquire some subtropical characteristics late in
the week. Regardless...beginning Wednesday night and continuing
through Friday...the gradient will tighten across the coastal
waters leading to at least likely Small Craft Advisory winds.
Gales will be possible as well...depending on the proximity of the
low. Seas will ramp up during the latter part of the week thanks
to the strengthening winds and swell energy associated with the
low. Models have trended further westward recently...and further
trends will have to be monitored in the coming days.
Rip currents... an elevated risk for rip currents is expected
especially later this week and this weekend as a surface low moves
near the coast. Increasing swell energy and coastal winds associated
with the low will be the main causes for the increased rip current