Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
717 am EDT Friday Oct 31 2014
high pressure will nudge into the area today between an
approaching cold front to the west and a weak area of low pressure
along a stalled front offshore. A powerful cold front will cross the
region on Saturday as cold Canadian high pressure builds in through
the rest of the weekend. This area of high pressure will then become
the dominant weather feature through the first half of next week.
Near term /through tonight/...
as of 645 am Friday...weak surface low along the stalled front just
offshore southeast from Winyah Bay...is prognosticated to track NE along the
offshore stalled frontal boundary today. This low is a result of a
weak middle-level short wave trough or vorticity lifting to the NE. Models indicate
both the surface and upper dynamics should track away from the forecast area
later this afternoon and early evening. Have stayed with a dry bias
given the recent 12-24 hour performance of the NAM and cumulative
MOS guidance overemphasizing the moisture/cloudiness/pops across
the forecast area. Will continue to illustrate clouds along the immediate
coast through midday today given the recent trends of the 11-3.9
micron infrared imagery. A brief lull will exist across the forecast area this
evening into the pre-dawn Sat hours. This will provide partial
clearing with winds possibly even decoupling for several hours.
Am reluctant to put in fog for this evening given the fast
approaching and amplifying southward digging upper closed low
that moves from the southern Great Lakes this morning to the
Carolinas by daybreak Saturday.
The incredible dynamics accompanying this upper system...along
with current local SSTs around 70 over the ilm SC waters to the
low to middle 70s off the NC waters south of hat...and a surface
baroclinic zone just offshore...will all combine to ignite a surface
low off the Carolina coasts late tonight...that will be poised to
possibly or likely bomb before leaving the NC waters to the NE
during Sat. Therefore...will indicate deteriorating conditions
especially across the western portions of the ilm County Warning Area by daybreak
Sat in the form of light rain with increasing winds. Later periods
may have to talk up a Wind Advisory for portions of the ilm County Warning Area.
As for temperatures this go-around...stayed closer to the milder GFS mav
MOS guidance which allows portions of the forecast area to observe 70 degree
maximum temperatures this afternoon before winter makes its 1st unofficial
appearance later this weekend.
Short term /Saturday through Sunday night/...
as of 3 am Friday...although model solutions are congealing on
Saturday there are still some minor differences. Such differences
with such a strong system however can translate to considerable
forecast uncertainties. For example, the GFS has shown a bit of a
warm sector/trowal briefly getting into mainly coastal NC zones at
12z Saturday...likely dictating what high temperatures are followed by
quite a drop off in the cold air advection. Agreement has also increased in the
portrayal of dry slot moving into southern zones to keep higher quantitative precipitation forecast over
northern zones. Areas further south should still see some light rain
given the strength of the forcing but should largely miss out on the
deep moisture. Tightly wound system so the wrap-around moisture
already underway by afternoon and once again more widespread and west
higher amounts possible NC. Winter-like cold with most places stuck
in the low 50s save for possible early day highs along the coast.
There still may be a window of opportunity for some graupel pellets
to reach the ground but they should be the exception and not the
rule. The cold air advection plunges Sat night lows down into the 30s. Some wind
and cloud cover prevent rad cooling. The upper wave moves offshore
by 12z Sunday allowing for strong dry air advection. Nearly full sun
expected with a diminishing breeze and temperatures once again well below
climatology. Sunday night will be a few degrees cooler than Saturday night
especially inland where winds may go calm. For a perfect rad night
the center of the high would need to have been closer.
Long term /Monday through Thursday/...
as of 3 am Friday...the original chilly high still directly
overhead on Monday. However between warming aloft as well as solar
insolation moderating the airmass high temperatures will end up
closer to but still shy of climatology. Building upper ridge to our west
spreads overhead by Tuesday allowing for some weak onshore return
flow and highs back in the 70s. Additional moderation and weak
advection should bring an even milder Wednesday and moisture return
will add some cloud cover. A weak upper disturbance moves through
Wednesday night into Thursday and tries to touch off a few showers.
Aviation /11z Friday through Tuesday/...
as of 12z...things are going to get interesting in about 24
hours...however fairly benign conditions until then. Showers off the
coast are associated with an old front. The NAM as a few showers
coming on to the coast this afternoon...but think the convection
will remain offshore. Mainly northerly winds today as high pressure
slowly retreats. Tonight...strong upper low with an unseasonably
cold core will approach the region from the northwest. Not much will
happen overnight as we get dry slotted and the gradient and cold air
advection will not commence until the end of the forecast period. A
spoke of moisture may clip the inland terminals with intermittent
MVFR conditions worst case scenario.
Extended outlook...MVFR/IFR conditions with showers through Sat.
Breezy to windy conditions to develop on Saturday and persist
into Sunday after low pressure develops off the Carolina
coasts...bombs...and accelerates to the NE. VFR for Monday and
Tuesday as the center of strong high pressure moves overhead.
near term /through tonight/...
as of 700 am Friday...weak surface low developing just offshore and southeast
from Winyah Bay...will track NE along the stalled offshore frontal
boundary today...reaching the offshore waters southeast of hat this
evening. The coastal waters could see widely scattered light rain
or showers this morning through early afternoon as this low lifts to
the NE. The lows main influence across the local waters will be a
slightly tightened surface pg as it tracks by...resulting in winds
slightly backing from a true NE direction to N-NNE...with speeds
in the 10-20 knots range. Look for a brief lull this evening as the
local waters will be located between 2 weather systems...the surface
low exiting to the NE...and the amplifying and deepening and
likely historic upper closed low dropping to the Carolinas from
the Great Lakes by daybreak Saturday. This will spur the beginning
of a surface low off the Carolina coasts early Saturday...that has
great potential to bomb as it begins to accelerate to the NE along
the offshore stalled frontal boundary. Gales will not be realized
this period but Small Craft Advisory conditions may quickly engulf the southern ilm
waters by or up to several hours after daybreak Saturday...depending
how quickly the surface pg tightens ahead of the approaching upper
system. The fantastic cold air advection surge will combine with a further
tightening surface pg during the next period...with subsequent gales
Short term /Saturday through Sunday night/...
as of 3 am Friday...Gale Warning likely in effect for most if not
all of the area for cold surge that should realize gales by late
Sat afternoon/evening and/or night. Its still about one forecast cycle
too early to raise the warning especially given the small
differences in how the models handle the overall evolution of the
cyclone. Winds will be northwest to west and thus strongly offshore
throughout which could yield a considerable wave shadow along land
up against the increasingly large and steep waves as one moves
away from the coast. Locally generated Swan data appears to be
handling this OK...although it will be interesting to see how the
possible deep mixing of wind gusts caused by cool air over warmer
SSTs manages to bolster seas. Strongest of the low level jetting
appears slated for Saturday night. Thereafter these winds along
with surface flow will begin to abate albeit very gradually at
first. This trend will be more noticeable on Sunday as the low
continues to both deepen and accelerate off to the northeast.
Long term /Monday through Tuesday/...
as of 3 am Friday...northerly winds really relax on Monday as high
pressure moves over the coast. Wind direction may veer slightly as
well while seas settle to just 2 feet or less. The actual center of
the high does not move much into Tuesday but it does take on an
east-west elongation that will further turn the flow move onshore.
Seas generally unchanged and minimal in size.
Marine...gale watch from Saturday morning through late Saturday night
Gale watch from Saturday afternoon through Sunday morning for