Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
1020 am EDT Monday may 4 2015
high pressure will continue to drift slowly offshore early to
mid-week...providing sunshine and seasonable may temperatures. A
coastal low is expected to form in the Bahamas mid-week...and
meander along the southeast coast through the end of the week and
into next weekend.
Near term /through tonight/...
as of 1000 am Monday...the atmospheric column is relatively dry
overall...however 12z soundings from mhx and chs do show a
layer or moisture just below 700 mb. This moisture is evident in
visible satellite imagery as a layer of scattered clouds moving
south-southwest around 700 mb-800 mb high pressure centered over clt. Weak middle-level
troffiness will slide across the area this afternoon while modest low-
level return flow sets up around offshore surface high. Hrrr
indicates some potential for light showers inland by early
afternoon ahead of the sea breeze...but moisture will be the
limiting factor. Do not plan to add probability of precipitation to the forecast at this
time but can not rule out a few brief...light showers across the
western 2/3 of the County Warning Area. No change needed to maximum temperature forecast...
with an expected range of middle-upper 70s at the beaches to low 80s
Continued on shore flow tonight around offshore high will support
a slow rise in dewpoints after a diurnal drop this afternoon in
deeper mixing. This will keep overnight lows a few degrees
higher than last night...but still down to 55 to 60 most places
with ideal radiational cooling conditions under clear skies and
near calm winds. May see some patchy shallow ground fog again
tonight as temperatures drop out but column will be very dry above the
Short term /Tuesday through Wednesday night/...
as of 3 am Monday...Bermuda high on Tuesday will retreat Wednesday
ahead of disturbance emerging from the Caribbean and the Bahamas.
Middle level ridging in place above the Bermuda high Tuesday will maintain
warm and dry conditions across the area with only patches of shallow
afternoon cumulus. Onshore flow will continue keeping temperatures near the
coast around climatology while inland areas warm up a bit.
Second half of the period starts to get interesting as the system
emerging from the Bahamas will try and coalesce into a well defined
low. The going will initially be slow and the lack of steering flow
aloft is likely to keep the system south of the area through the end
of the period. Still a lot of uncertainty regarding this system and
while it may lead to an increase in cloud cover...especially along
the coast Wednesday night...do not think it would directly affect the
region before the end of the period. Thus will maintain the dry
forecast with highs around climatology and lows a bit above climatology.
Long term /Thursday through Sunday/...
as of 3 am Monday...long term period will be dominated by the low
evolving off the southeast coast middle to late week. Guidance
continues to struggle with this system...mainly due to the lack of
any real steering flow aloft. However some solutions seem hard to
buy. For instance the low deepening and moving up the coast seems
the least likely outcome given the 500 mb ridging to the north. Do think
the low could gradually strengthen through the period.
Although the water temperatures within the Gulf Stream are around 80 and
shear should be weak to nonexistent there are several factors that
would limit strengthening. A lack of diffluence aloft early in the
period and the large swath of sea surface temperatures outside the Gulf
Stream in the 60s being the biggest. So while some development of
this system is expected a full blown tropical system seems to be a
long shot. Instead the system will probably acquire some tropical
characteristics but remain quite weak as it wanders over the waters
off the southeast coast. Worth noting a lot of the GFS ensembles
take the weak low into Florida or southern Georgia...something which a fair
number of the members have been consistently showing over the last
Locally impacts through the period will be northeast flow...with
some enhancement later in the week and an increase in low level
moisture. This should bring about an increase in precipitation
chances...especially along the coast...as the week wears on. Given
the low confidence plan to make only small changes to the inherited
forecast which will now carry chance pop through the period with temperatures
near to slightly above climatology.
Aviation /14z Monday through Friday/...
as of 06z...persistence forecast regarding aviation purposes.
Bermuda high pressure will continue to dominate weather conditions
through the period with mostly IFR conditions. Some models are
generating a little sea breeze activity across South Carolina but
feel any activity will be too isolated in nature to include in
forecast. Added a little br to flow for the overnight hours per
observations this morning.
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 1000 am Monday...expect isolated showers creeping westward
towards Cape Fear to diminish as they become influenced by the
downward component of the sea breeze circulation. Otherwise...east-southeast
flow will be the story for the remainder of the day due to surface
high pressure well off the NC coast. Wind speeds around 10 knots
are expected as the pressure gradient will remain benign through
Short term /Tuesday through Wednesday night/... as of 3 am
Monday...Bermuda high will maintain light easterly flow Tuesday into
Wednesday. During the second half of the period the high will retreat
northeast as weak low pressure emerges from the Bahamas. Winds
will back to northeast for Wednesday and Wednesday night with a slight bump is
speed. Confidence with respect to the amount of strengthening the
low will undergo and its location is rather low and thus forecast
confidence is also low. For now keeping wind speeds Wednesday and Wednesday
night confined to the 15 to 20 knots range but higher values are
possible. Seas 2 to 3 feet Tuesday into Wednesday should build later Wednesday as
northeast flow increases and coastal low starts to become a little
Long term /Thursday through Friday/...
as of 3 am Monday...much of the forecast depends on the details
of the hybrid tropical system meandering off the southeast coast
late this week. Weak steering flow aloft has so far befuddled
guidance and not confident clarity will descend upon the forecast
in the near term. That said feel a forecast of northeast flow in
the 15 to 20 knots range is a good compromise and is in line with
previous forecasts. Seas will build on the back of extended
northeast flow with some zones likely to exceed 6 feet in places on
Thursday and remain above 6 feet Friday.