Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
143 PM EDT Tuesday may 5 2015
Atlantic high pressure will prevail today into Wednesday as an area of
low pressure starts to develop just north of the Bahamas. This low
will slowly drift northward on Thursday before beginning to push
back to the west toward the southeast coast Friday through Saturday.
The low will then linger in the vicinity through the remainder of
the weekend...and could persist into early next week.
Near term /until 6 PM this evening/...
no major changes made to the previous forecast. Atlantic high
pressure to continue to the north and east leading to onshore
winds which will keep coastal locales a bit cooler in the 70s.
Inland highs will be in the lower 80s. Onshore winds will be a bit
breezy/gusty as the sea breeze develops and pushes inland this
afternoon. Diurnal cumulus should remain scattered...and the plume
of cirrus streaming from the south should remain primarily
Some high resolution guidance depicts isolated showers pushing
onshore into far southern counties this afternoon/early evening...
but the probability for measurable rainfall should remain very
low as strong subsidence prevails.
Short term /6 PM this evening through Friday/...
expect another rain-free night with near normal low temperatures
ranging from the middle/upper 50s inland to the middle/upper 60s on the
coast. Model soundings suggest the potential for some low clouds
to develop late...perhaps impacting late-night temperature
trends. Will continue to assess.
Wednesday and Wednesday night...Atlantic high pressure will
continue to extend into the forecast area as an area of low
pressure begins to develop just north of the Bahamas. Wednesday
should be another quiet day as the model soundings and time
heights show considerable middle/upper level dry air as well as warm
middle levels that keep the atmosphere sufficiently capped. As
such...the forecast is dry for inland areas. There may be isolated
to scattered shower activity well out over the coastal
waters...but even this will be highly dependent on how far west
the low develops and what its initial track will be. Highs will
continue to be right around normal...with low 80s in most
locations away from the immediate coast. Overnight...not much
change and the forecast is dry thanks to the Atlantic low being so
far offshore according to the model consensus. Lows will fall into
the low 60s.
Thursday through Friday...attention for the latter part of the week
continues to be closely tied to the Atlantic low that develops and
its eventual movement back towards the southeast coast. Model run to
run consistency continues to lack and the model consensus is
considerably different than 24...or even 12 hours ago. With the 00z
guidance...the overall consensus is for the low to drift
northward...well off the coast...through Thursday and then push back
to the west toward the coast beginning Friday. This further east
consensus for initial development and movement would act to at the
very least delay any potential impacts until mainly the weekend. The
European model (ecmwf)/GFS are similar with a further offshore solution...while the
NAM is much stronger and closer to the coast. Model phase analyses
continue to show the potential for the low to take on more tropical
or subtropical characteristics as it develops and evolves. As
such...the National Hurricane Center continues to advertise a low
chance for the system to acquire subtropical characteristics. Given
the current model timing and strength of the system...it appears any
notable impacts will be delayed into the weekend. The forecast has
been adjusted to more closely match the current models...which
results in lower probability of precipitation...warmer temperatures...and diminished winds.
Please continue to monitor forecast updates for the latest
Long term /Friday night through Monday/...
the forecast for the weekend and early next week
remains highly dependent on the track and strength of the offshore
low pressure system...and especially its proximity to the coast. The
closer to the coast the low tracks...the more potential there is for
impacts such as rip currents...shallow coastal flooding...and high
surf. The current model consensus favors a slower and more offshore
solution...though the lack of run to run consistency with the models
prevents forecast confidence in being too high. By early next
week...forecast confidence becomes even lower due to the fact that
the models handle the system very differently. The GFS takes the low
inland then allows it to linger over the forecast area through the
end of the forecast. On the other hand...the European model (ecmwf) does not push the
low inland and actually lifts it out to the northeast by Tuesday.
Given the uncertainty...the forecast features a warming trend for
temperatures and probability of precipitation no higher than the chance range.
Aviation /18z Tuesday through Sunday/...
VFR to prevail through 18z Wednesday...although there is a very low
probability for brief flight restrictions in ground fog around
daybreak Wednesday and low clouds late in the period.
Extended aviation outlook...VFR conditions are expected to prevail
through Thursday. Then late in the week and during the weekend...the
low is expected to push westward toward the coast. Depending on how
close the low comes to the coast and its strength...there could be
increasing chances for flight restrictions especially during the
as the sea breeze circulation augments synoptic east/northeast
winds...east/northeast winds 5-15 knots this morning will increase
to 15 knots with gusts to 20 knots this afternoon. Seas 2-4 feet
will prevail...perhaps as high as 5 feet beyond 20 nm late.
As low pressure begins to develop well south of the waters...winds
will back toward the northeast tonight...averaging less than 20
knots. Seas of 3-5 feet are expected...highest beyond 20 nm.
Wednesday through Saturday...the forecast for the
coastal waters remains highly dependent on the
track...intensity...and proximity to the coast of the offshore low
that develops starting on Wednesday. Forecast models have shown poor
run to run consistency which results in a low confidence forecast.
The current model consensus seems to favor a weaker and further
offshore solution which would delay any potential impacts until the
weekend. Regardless...small craft advisories will likely be needed
starting late Wednesday as increasing swell energy drives seas
higher. Winds would begin to increase on Friday as the low pushes
back to the west and the pressure gradient tightens around the low.
Also of note...model guidance continues to show the possibility for
the system to take on some tropical or subtropical characteristics.
The National Hurricane Center gives the system a low chance to
develop...and mariners are urged to monitor forecast updates for the
Rip currents...increasing swell energy and breezy winds will make
for a slightly enhanced risk of rips today although we decided to
leave the risk in the low category for now...especially given the
much increased risk later this week as a surface low pressure
system moves near the coast. Increasing swell energy and coastal
winds associated with the low will be the main causes for the
increased rip current risk.