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AGNT40 KWNM 282354

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
754 PM EDT Sun May 28 2017

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

A low pressure center moving NE off the Virginia Capes has max
winds up to 20-25 kt in its vicinity. Associated seas have just
begun to build and are only in the 4-6 ft range which are being
handled slightly better by the 18Z Wavewatch III versus the 12Z
ECMWF WAM at the moment.

Over the short term, overall the latest models share very similar
forecast tracks for the surface low to continue ENE across the
Nrn NT2 waters tonight and Mon, then pass E of the waters Mon
night. In regards to the forecast gradients associated with this
system, would favor a 18Z/12Z GFS and 12Z ECMWF compromise. Per
the 18Z GFS, late Mon/early Mon night it looks close as to
whether the associated gradient will reach gale force across NT2
zone ANZ905. But with the model forecasting low level stability
to remain in place there, the previously populated 12Z GFS winds
of 25-30 kt still look representative. So will continue to hold
off on forecasting any associated gales with this system and will
just make some minor edits to the previously populated short term
winds and seas.

In the long range, in the wake of the low passing E of the area,
would again favor the less progressive and similar 18Z/12Z GFS
and 12Z ECMWF solutions, versus the faster 12Z/18Z NAM
and 12Z GEM/UKMET, for a weak cold front to approach from the W
Tue, then push slowly offshore into the Nrn waters late Tue
night into Thu with weak (generally 15-20 kt). associated
gradients. Then the 18Z GFS remains consistent with its previous
12Z run in forecasting a slightly stronger cold front to approach
from the NW Fri, then push offshore into the Nrn waters Fri
night with moderately strong (generally up to 20-25 kt)
associated gradients. The 18Z/12Z GFS forecast timing for this
fropa looks like a reasonable compromise between the slightly
more progressive 12Z ECMWF and slower 12Z UKMET/NAVGEM/GEM
solutions. So will stay the course with the previously populated
12Z GFS winds for the long range through Fri night.


Low pressure will move east, and then northeast along a front
over central and then northern NT2 offshore waters tonight into
Monday. The low is located over northeast North Carolina this
afternoon, with a nearly stationary front extending east from
the low over central NT2 waters. All 12Z models are now deeper
with this low as it moves east and northeast over the region
during the next day or two. We will remain close to the 12Z GFS
guidance over the waters for the next few days as it appears to
be in very good agreement with the 12Z ECMWF, 12Z Canadian GEM,
and 12Z UKMET models. It also has strong support from the 12Z
NAM. We will rely on the smart tool which will place the
stronger first sigma layer winds over unstable areas, and lower
10 meter winds over stable areas throughout the forecast
package. An area of gale force winds is now forecast to develop
just east of the northeastern NT2 waters by Tuesday morning. We
will closely monitor the latest model guidance to see if any
gales might need to be added to the offshore forecast. For the
afternoon package we will keep maximum winds at 30 KT for now,
with confidence levels near to above average.

The latest radar, satellite and lightning density data indicates
scattered thunderstorms occurring near the front, moving east
and northeastward toward northern NT2 waters. The stronger
thunderstorms appear to be developing over northeast North
Carolina and near the Delmarva at this time. The latest SREF
thunderstorm guidance indicates thunderstorms spreading
northeast from central into northern NT2 waters into Monday,
before moving east of the waters later Monday as the associated
low also pulls east of the region. Additional thunderstorms are
possible by Tuesday into Wednesday as a weak cold front and
potentially another low pressure area crosses the waters from
west to east. Local wind gusts exceeding gale force and very
rough seas may occur in or near any of the stronger

High pressure still appears as if it will build behind the front
Wednesday night into Thursday night. The next cold front will
approach from the west Friday, and then potentially begin to
move offshore Friday night. The 12Z GFS remains the favored
model later in the week, with the 12Z ECMWF a little faster
with the front, and the 12Z UKMET slower. The 12Z GEM is fairly
close to the timing indicated by the 12Z GFS which leads to even
more confidence is using the 12Z GFS guidance for the afternoon
forecast package and grids.

As far as seas are concerned, it appears as if both the 12Z
Wavewatch III and 12Z ECMWF WAM have initialized well over the
offshore waters based on the latest sea state analysis and
altimeter data. Both models are fairly similar throughout the
forecast period, with the WAM building seas a little faster
later Monday and Monday night as low pressure moves east of the
waters. It also builds seas a little faster by Friday night in
the south and southwest flow ahead of the cold front described
above. As the Wavewatch can tend to under forecast seas in these
situations we will use a 50/50 blend of the two wave models for
the afternoon package.

Extatropical Storm Surge Guidance: The latest ETSS guidance still
indicates a positive surge value approaching .3 to .5 foot
Monday into Monday night from Cape May to near Montauk Point.
ESTOFS output is running a little higher, closer to the .5 to 1
foot range. We will prefer something a little closer to the
higher ESTOFS model over the next 24-48 hours as model guidance
is indicating a good chance for a somewhat stronger low and
therefore stronger winds occurring over the offshore waters
tonight into Monday night, helping to potentially drive a
somewhat higher surge from the mid-Atlantic coast northward to
New England.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...


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