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AGNT40 KWNM 221411

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1011 AM EDT Sat Sep 22 2018

.FORECAST DISCUSSION...Major features/winds/seas/significant
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.

Previous forecast appeared to do well with the winds, and their
gradual decrease this morning, over the Gulf of Maine. 11 UTC
observations at MISM1 and MDRM1 had gusts around 35 knots,
sustained in the upper 20s, while 44027 had a wind 21G27 knots
and a peak at 31 knots. These values have decreased between 11
UTC and 13 UTC, with the 13 UTC highest peak at MISM1 to 29
knots. Winds should continue to decrease into the teens knots
this afternoon, and wave heights that were around 11 feet in the
eastern part of ANZ800 early today should correspondingly
decrease as well, to around 6 feet by the end of the day. In the
near term, added a foot to the average waves off of the Carolina
and Delmarva coasts based on observations, with otherwise only
very minor adjustments to wind and waves mainly in and near the
Gulf of Maine.

A cold front was located from Newfoundland southwest across
southern New England and Delmarva, and this front should continue
to move east and south and be located off of the southern
Virginia and North Carolina coasts by the end of the day.
Mariners should expect scattered showers and isolated
thunderstorms to develop along the front, particularly as the
front moves south and interacts with the warmer waters around the
Gulf Stream, where the SPC mesoanalysis currently show, and the
GFS forecasts, positive CAPE during the afternoon and into
tonight. Isolated showers, already occurring in parts of ANZ830
southwest into ANZ835 based on the latest radar trends, should
develop across the southernmost zones during the day with a few
thunderstorms in an unstable air mass.

Primary concerns for hazards, outside of thunderstorms, currently
exist in the Tuesday night through Thursday time frame first over
the northern Gulf of Maine, and later over the Gulf Stream. On
Tuesday night into Wednesday, strong surface high pressure will
move off of the southeast coast of Newfoundland while a front
approaches from the west. As the gradient tightens, 950mb winds
on the GFS increase to 50 to 55 knots, and on model soundings
from the GFS and ECMWF, down to 996 mb sustained winds would be
34 knots. It is a stable profile, and the most aggressive
guidance of the quarter-degree ECMWF currently provides for
gales in the coastal areas north of ANZ800 by Tuesday night. This
will be something to watch in later forecasts, along with the
potential for gales by Thursday near the Gulf Stream particularly
in ANZ905 and ANZ910. Once again, the quarter-degree ECMWF is a
little more aggressive, with a more unstable air mass and 34
knots of available wind in the boundary layer while the GFS, with
a deeper inversion, would be at or just above 30 knots. It will
be close, and will watch later guidance trends for indicators of
more confidence.


The latest NCEP surface analysis shows a cold front over SE
Canada extending to the SW into New England and the Ohio Valley
region this morning with the isobars indicating strong
southwesterly flow ahead of the front in the NT1 offshore
waters. Current surface observations show up to 30 kt along the
coast in the Gulf of Maine, with the elevated anemometers at the
C-MAN stations at Matinicus Rock (MISM1) and Mount Desert Rock
(MDRM1) in the coastal waters indicating up to 34 kt at 0700
UTC. The 00Z models all indicate a strong low level jet just
ahead of the front this morning with GFS/ECMWF 925 mb winds over
the Gulf of Maine to 60 kt. Despite the shear, the higher winds
aloft are not expected to mix down as GFS/ECMWF soundings show an
inversion near the surface as a result of the cooler SSTs in the
Gulf of Maine. Both models indicate mixed layer winds just below
30 kt, and this seems reasonable with the stable boundary layer.
As a result, am planning on starting out with 30 kt for this
morning. The GFS and ECMWF both indicate that the front will pass
quickly SE through NT1 today with the higher winds expected to
move E of the offshore waters early today. In addition, agreement
with the timing of the models on the front has been consistently
good. As a result, am planning on starting out with the 00Z GFS
in the next forecast.

In the medium range, the 00Z models remain in good overall
agreement into Tue. The GFS/ECMWF both show a tightening
pressure gradient across the central and northern NT2 offshore
waters late Sun and Mon as low pressure SE of the area tracks W
and the stalled front over NT2 slowly weakens. The low and front
will then both interact with strong high pressure building over
New England. This is expected to cause an increase in the
easterly winds across the Gulf Stream. The 00Z ECMWF/UKMET both
indicate up to 25 kt, while the first sigma winds from the GFS
show up to 30 kt in the outer waters of northern NT2. This seems
a bit more reasonable considering the unstable environment, and
since the models agree fairly well on the timing am planning on
staying with the 00Z GFS through Tue. There is also a chance for
gales briefly developing Mon night across the Gulf Stream, but
confidence is low as a result of the poor support from the
guidance with model soundings only showing about 25 to 30 kt
mixed layer winds despite unstable lapse rates. As a result,
planning on keeping winds below warning criteria for this system.

On Wed, the 00Z GFS continues continues to run slower than the
rest of the 00Z guidance with the next cold front to move into
the area. The 00Z ECMWF is roughly 6 to 12 hours faster with the
front, and has been very consistent with the timing. The GFS has
been slower for the past few runs, but has been trending slightly
faster. Additionally, the 00Z UKMET/GEM and WPC medium range
guidance are all favoring the faster solution, so planning on
transitioning to the 00Z ECMWF on Wed since it is supported
better and also maintains continuity.

.SEAS...The 00Z ECMWF WAM is initialized slightly better than the
00Z Wavewatch in the Gulf of Maine with 10 ft reported at 0600
UTC this morning. However, the models agree fairly well through
Tue which is not surprising since the associated atmospheric
models are in good agreement over that time period. As a result,
am planning on starting out with the 00Z Wavewatch while editing
the wave heights to match current conditions. Will then
transition to the 00Z ECMWF WAM on Wed to match the preferred
trends in the 00Z ECMWF solution.

show a positive surge of up to 1.5 ft along the mid Atlc coast
on Tue in the moderately strong easterly flow indicated by the
00Z models. This seems reasonable given the intensity of the
winds, while the 00Z ETSS is about 50 percent weaker and seems a
bit underdone.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...


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