Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
735 PM EDT Monday may 4 2015
high pressure extending into the area will continue to drift
farther offshore into mid-week...providing seasonable may
weather. A coastal low is expected to form in the Bahamas
on Wednesday...drifting north before meandering along the
southeast coast from Thursday into next weekend.
Near term /until 6 am Tuesday morning/...
as of 730 PM Monday...cumulus field has faded with the loss of daytime
heating. Satellite loops show that we may see some thin cirrus
pass overhead at times...but overall we will see a clear overnight
period. Light S-southeast winds will become light and variable before
decoupling this evening. This will be a good setup for
radiational cooling and temperatures should fall into the middle 50s most
locations. Fog of patchy to areal extent will be possible after
midnight...especially inland where the temperature-dewpoint depression
spread will be smallest.
Short term /6 am Tuesday morning through Wednesday night/...
as of 3 PM Monday...early Summer pattern with Bermuda-like high
centered offshore...providing a balmy early may circulation of
air climatologically indicative of the season. Moderate cumulus
build-ups seems likely during the afternoon near and west of the
the afternoon sea-breeze...but arid air above 10 thousand feet
through Wednesday afternoon will keep measurable rain probabilities
below mentionable levels. Maximums either side of 80 degree warmest
over the interior...and minimums upper 50s to low 60s early Wednesday
and 60-65 early Thursday.
A coastal low approaching from the south will bring a slight
chance of rain showers favored along the coast Wednesday night into
early Thursday...as low-level wind direction and windspeed
convergence increases offshore.
Long term /Thursday through Monday/...
as of 3 PM Monday...all eyes remain on a low pressure system which
looks likely to develop and strengthen off the southeast coast late
this week. NHC has issued a second special tropical weather outlook
to address this potential...and while it is becoming more likely a
system will develop...there is still very little confidence into how
it will move once forming.
An area of moisture already evident across the Caribbean/Bahamas
today will consolidate and lift slowly north to off the Georgia/SC coast
at the start of the extended. Initially...this system will feed of
baroclinicity as an et-cyclone...but guidance suggests it will
transition to a warm core system by late week and possibly take on
tropical or sub-tropical characteristics. This is important because
the environment is not entirely favorable for barotropic (without
temperature gradients) strengthening thanks to weak diffluence
almost exclusively on the east side...and water temperatures well below the
80f threshold once you get outside the Gulf Stream. This suggests
that a combination of barotropic and baroclinic features will
determine the strength...hence the likely hybrid-low. The guidance
still diverges considerably on the low track thanks to weak steering
flow...but as mentioned by the previous forecaster...a track due
north seems impossible with the upper ridge in that direction.
Thus...a retrograding west or SW track is most plausible and seems to
be supported by ensembles.
Still...this creates a very tricky forecast with a meandering and
possibly strengthening low for several days off the coast. The 12z
GFS brings a pretty strong low into central SC...but this is a new
solution from even the last run which brought it into the Georgia coast.
The European model (ecmwf) continues to keep the system spinning just offshore into
the weekend. While it is impossible to determine with any specificity
the track at this time range...will continue to show unsettled
weather Thursday-sun with the best chance for rain/wind being late
Thursday into Saturday. Although the system should begin to weaken
thereafter...continued schc/low chance pop is warranted right through
the end of the period before a trough approaches for early next week
which will act to finally kick out the low.
Temperatures through the period should be above climatology...but of course any
significant rain/clouds could hinder warming and will also be
dependent on this low track. Additionally...E/NE winds off cool
shelf waters will keep the coastal areas much cooler than well
inland...so a large temperature gradient across the County Warning Area is possible each
afternoon through the period.
Aviation /00z Tuesday through Saturday/...
as of 00z...much the same as yesterday with high pressure offshore
controlling the weather. There is another chance for fog tonight
mainly the usual suspects...cre and fog. Pockets of IFR are possible
but should be short lived. Tuesday...continued light southeast flow
with cumulus forming by middle morning but moving quickly inland with the
Extended outlook...pre-dawn morning fog is possible each day that
could result in MVFR/IFR conditions. Unsettled weather Thursday
night through Saturday with a subtropical low.
near term /through tonight/...
as of 730 PM Monday...latest observation show a light southeast flow of around 10
kts with 2 feet seas over the waters this evening. This southeast flow
will persist overnight as high pressure lingers over the western
Atlantic...with seas of 2 to 3 feet throughout the waters.
Short term /Tuesday through Wednesday night/...
as of 3 PM Monday...manageable marine conditions much of the
period with increasing sea heights mainly late Wednesday as
offshore southeast-east winds begin to ramp up in response to a Bahama
coastal low drifting north. Otherwise...Bermuda high like weather
with 2-3 foot seas and an increase in wind gusts near shore in the
afternoon to early evening due to the sea breeze circulation. No
thunderstorms expected this period...but seas may increase to 4-5 feet late
Wednesday night accompanied by increasing marine showers.
Long term /Thursday through Saturday/...
as of 3 PM Monday...potential continues for an extended period of
poor marine conditions thanks to a low pressure system that is
forecast to develop and potentially take on tropical
characteristics Thu-Sat. There is still considerable uncertainty
into the strength and track of this low...but it looks likely that
a system will develop and then meander in the area through the
period. At this time...it appears the best course of action is a blend of
guidance...and this creates east/NE winds of 15-20 kts
Thu/Fri...before easing Saturday...hopefully. This extended period
of east/NE fetch combined with swell generation from the low
pressure...will drive seas up to 4-8 feet Thursday and Friday...before
falling on Saturday. Of course this can change...and there is
potential for much higher seas as the wnawave is showing 16 feet
just outside of the coastal waters Friday. Either way...a long-
duration strong Small Craft Advisory is becoming more likely for
much of the period.