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AGNT40 KWNM 280227

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1027 PM EDT Thu Jul 27 2017

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

Gale warnings starting Saturday night will be retained as strong
low pressure will move east across the central waters. Geocolor
satellite images show a series of cloud bands inland approaching
the region from the northwest with just few clouds over most of
the central and southern regions. Lightning density map show
dense lightning east of the region moving southeast. The SREF
models has just PROB 20 for TSTMS threat over the far southern
waters. Highest winds are only 20 kt over maine waters while 5 to
15 kt winds have been observed elsewhere. The 00Z NCEP weather
map has low pressure 1014 mb just east of the Baltimore Canyon
with a stationary front over the far southern waters. Weak high
pressure 1018 mb near 44N47W extends a very weak ridge into the
northeastern waters. An inland front stretches southwest from low
pressure over Canada and passes just northwest of mid Atlantic
as it curves southwest to pass across the upper/lower
Mississippi. Pressure gradient is fairly relaxed.

Seas are relatively small with a peak at only 5 ft over the
eastern Baltimore Canyon and they range between 2 and 5 ft with
smallest over the western portion. Wave models NWW3 and ECMWFWAVE
fit well the observed seas pattern and continue to be in good
agreement on building seas to 15 ft in areas of gale warnings.
Will stick to a blend of both models. In the short term, seas
will remain below 8 ft then will build to peak at 15 ft.

In the upper levels, models GFS/CMC/ECMWFHR/UKMETHR show a trough
with some energy to the northwest of the region that will move
across the north waters. A pocket of energy over Canada just
north of the great lakes will move southeast and when it gets
near the waters will spread as its associated trough stretches
southwest and eventually pass east across the waters. Models
have very slight differences on the timing of this system as it
moves across the waters. Models have also minor differences on
the extent of the trough. GFS ends the trough farther south than
the other models. Otherwise, the models generally agree on
strengthening the trough and that will result in a tight pressure
gradient over the north and central waters.


The 1430Z and 1520Z Ascat overpasses across the western portions
of the offshore waters returned highest winds to 20 kt off the
Maine coast, with winds elsewhere 15 kt or less. Over the near
term the 12Z models are in good agreement across the west
Atlantic and present no major forecast problems. Both the weak
high pressure ridge extending northeastward from near Cape
Hatteras and the weak stationary front across the waters south
of Cape Hatteras will dissipate tonight. The models are also
consistent in moving a cold front off the New England coast
early Friday. The front should stall across the NT1 waters later
Friday. Over the past several runs, the models have come into
better agreement with an amplifying upper shortwave which is
forecast to move southeast across the Great Lakes and through
the Ohio valley tonight through Friday night before closing off
near the Mid Atlantic coast Saturday and Saturday night. The
models have generally trended slower and stronger with this
system. At the surface, both the 12Z GFS and 12Z ECMWF indicate
a pair of surface lows will develop with the first moving
northeast near Georges Bank Saturday night, while the second
becomes vertically stacked and remains quasi-stationary off the
mid Atlantic coast late Saturday through Monday. The preferred
12Z ECMWF is slightly deeper than the 12Z GFS. Also, the latest
ensemble guidance suggests that the 12Z GFS may retrograde the
surface low too far southwest Sunday night/Monday. The timing
differences with how quickly the upper low along the mid
Atlantic coast will weaken this weekend and eventually lift
northeast next week have narrowed with the 12Z models. The 12Z
GFS and 12Z ECMWF are in better agreement with the low position
Monday night/Tuesday night than they are Sunday/Monday. As for
the gales, based on slower trend we had previously extended the
north to northeast gales from south of New England to off the
Delmarva further out in time (through Monday), and this still
looks reasonable. So overall the gale hazards will remain mostly
consistent. By Tue/Tue night, the 12Z models finally show the
weakening upper low will open into a trough and along with the
associated surface feature, will lift northeast.

.Seas...The 12Z Wavewatch III and 12Z ECMWF WAM were about 1 ft
low near Georges Bank, but otherwise are well initialized across
the west Atlantic this afternoon. For the forecast wave height
grids, with the 12Z ECMWF being the basis for the wind grids,
used a 4:1 12Z ECMWF to 12Z Wavewatch blend through the period.

.Extratropical Storm Surge Guidance...N/A.


.NT1 New England Waters...
.ANZ810...South of New England...
     Gale Saturday.
     Gale Possible Saturday night.
.ANZ815...South of Long Island...
     Gale Saturday.
     Gale Possible Saturday night into Monday.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale Saturday.
     Gale Possible Saturday night into Monday.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Possible Sunday into Monday.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Saturday.
     Gale Possible Saturday night into Sunday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Possible Saturday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale Possible Saturday night into Monday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Saturday.
     Gale Possible Saturday night into Sunday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Saturday.
     Gale Possible Saturday night.


.Forecaster Musonda/Clark. Ocean Prediction Center. is the U.S. government's official web portal to all federal, state and local government web resources and services.