591
AGNT40 KWNM 180125
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
925 PM EDT Sun Jun 17 2018

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

Very tranquil conditions persist across the offshore waters this
evening. Localized area of winds to around 20 kt currently
across inner waters 828 and 830 associated with a weak surface
trough moving offshore. Otherwise, winds are generally 15 kt or
less. Light winds will continue overnight as surface high near
36N 70W slowly shifts farther SE of the area, allowing return SW
flow to build across the W Atlantic ahead of an approaching cold
front on Monday.

No changes are needed to the grids in the upcoming evening
update. See previous discussion below for forecast details.

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PREVIOUS DISCUSSION...

This mornings Ascat overpasses covered the offshore waters east
of about 74W and indicated winds were 10 kt or less, except
across the eastern Gulf of Maine where the southeast winds were
up to 15 to 20 kt. The 12Z models were well initialized with
these generally light winds, and are in above average agreement
across the west Atlantic for the next few days. Over the near
term, the models are consistent in moving a low pressure trough
or weak frontal boundary off the North Carolina coast, which the
NAM Nest/NMM/ARW/HRRR all indicate should enhance the south to
southwest winds to 20 kt or so across the waters off Cape
Hatteras tonight. Incorporated the 12Z NAM Nest and 12Z NMM into
the existing grids through 18Z Monday to reflect these details
in the forecast winds.

An upper level trough is expected to approach Quebec and
northern New England Mon into Tue, and support a relatively
strong cold front that will move off the New England coast early
Tue. The 12z models remain in well above average agreement with
the timing of this front and are also consistent in indicating
that the increasing southwest return flow will reach 30 kt
across the Gulf of Maine and far northern reaches of Georges
Bank Mon evening.
Despite the strong low level jet depicted in regional and global
guidance - generally 45 to 55 kt - poor mixing over the cooler
waters will likely inhibit any gales from making it to the
surface. In fact, the 12Z NAM Nest soundings maintain the strong
inversion with the gales limited to about 50m above the surface.
We would not be surprised to see some gale force gusts at the
elevated stations along the Maine coast, but such observations
are unlikely to represent the surface winds. We used a blend of
the 12Z NAM Nest and 12Z NMM for the wind grids Mon evening
through Tue evening.

The cold front should stall near Currituck Beach Light Wed, then
the models diverge somewhat with a developing surface low along
the front Wed night/Thu. As has been the case for the last
several cycles, the GFS continues to move the front further
south than the ECMWF/UKMET Thu, and is further south than the
latter models with the low. We will be maintaining forecast
continuity and using a blend of the 00Z/12Z ECMWF for the wind
grids Thu through the remainder of the period. The 12Z ECMWF
appears more reasonable with the stronger east to northeast
winds developing poleward of the front across the Mid Atlantic
waters Thu night into Fri night.

Seas: Both the multi-1 WW3 and ECMWF WAM appear well initialized
with wave heights across the offshore and coastal waters this
afternoon. Will use a blend of this guidance in the short term,
then generally follow same line of thinking as the wind grids;
will use the WAM beginning late Mon night. Highest seas to about
11 ft expected Mon night in 30 kt winds across the Gulf of Maine.

Extratropical Storm Surge Guidance: no significant positive
surge events are expected during the next several days.


.WARNINGS...Preliminary.

.NT1 New England Waters...
None.

.NT2 Mid-Atlantic Waters...
None.

$$

.Forecaster Collins/Clark. Ocean Prediction Center.