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AGNT40 KWNM 260226
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1026 PM EDT Tue Apr 25 2017

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

NCEP preliminary surface analysis places slow-moving low pressure
center over far southeast VA with pressure 1002 mb. Short
occusion os over Delarva then becoming a warm front extending
from near Ocean City MD to ANZ915 and a cold front extending
southeast through ANZ925 and far east portion of ANZ930. Current
surface observations show winds around 25 kt north of the
occlusion and warm front and 10 to 15 kt far northern Gulf of
Maine. Previous ASCAT passes from around 15Z showed gales mainly
from the Gulf Stream southward over the central outer zones of
NT2 but north of ANZ930. The highest winds are in the south to
southeast flow ahead of the cold front. Current wind grids are
populated with the 12Z GFS but modified with the smart tool
placing the first sigma level winds over the more unstable areas
such as south from the Gulf Stream and the lower 10m winds over
the more stable areas with lower sea surface temperatures to the
north. These winds appear representative when compared to current
observations and prior ASCAT winds. Lightning strike and density
data indicate scattered thunderstorms mostly over the west
portions of the central and northern NT2 waters mainly near and
east of 1000 fms.

Low pressure which is vertically stacked with upper low will
continue to track north along the mid-Atlantic coast tonight
toward Long Island later Wednesday with a weakening trend. With
high confidence will continue the gale warnings over the outer
central and northern NT2 waters mainly from the Gulf Stream
southward overnight and Wednesday with the gales ending early
Wednesday night in line with the weakening trend shown in the
latest models. The models lose the low over the NT1 waters
Wednesday night. With the 18Z GFS showing little change will
leave the current grids based on the 12Z GFS unchanged. Based on
the earlier scatterometer passes will favor the slightly higher
12Z GFS/GEM/ECMWF solutions versus the 12Z UKMET in the near
term. Otherwise after the low dissipates the 12Z models and 18Z
GFS/NAM generally agree that a quiet weather pattern will develop
and persist. With minor timing differences the models all
forecast a high pressure ridge to build W across the southern
NT2 waters Thu night through Fri night, then persist Sat/Sat
night while a weak cold front pushes offshore across the NT1
waters with moderately strong (generally up to 20 or 25 kt)
associated gradients developing. After Friday night as a
compromise with the stronger UKMET and GEM coundary layer winds
will use the 12Z ECMWF.

Seas...The latest NWW3 and ECMWF WAM are similar but are a couple
of feet too low over the central and northern NT2 waters. For the
evening update used the previous grids based on the 12Z NWW3
through Friday night with edits to boost the seas 10 percent over
the central and northern NT2 waters initially to better fit the
observed conditions. After Friday night with ECMWF preferred for
winds, repopulated grids with 12Z ECMWF WAM.
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PREVIOUS DISCUSSION...

Gale force winds are still prevalent over the central waters
with seas peaking at 17 ft. The satellite images show cyclonic
circulation with center near the western edge of the southern
waters. The clouds with cold tops are mainly over the northern
half with the density lightning map showing intense lightning
over the central waters and eastern portion of the southern
waters. The latest observations including a scatterometer pass
about 1408Z show higher winds over the central region in the gale
force range. Winds are mainly from the southeast and east over
the northern half while they are mostly from the west and south
west over the southern waters. At 1800Z the NCEP map still has
low pressure 1002 MB near the NC coast with fronts into the
central and southern waters. Another front extends from Hudson
canyon into the Baltimore canyon.

In the upper levels the models GFS/CMC/ECMWFHR/UKMETHR indicate
an upper level closed low with energy embedded in it over the
western portion of the southern waters while a weak ridge
oriented NW-SE with little energy is over the northern waters. in
the short term, models agree on moving the low northeast along
the western edge of the forecast region. The low will then move
through the north waters and continue to the northeast of the
region as high pressure builds from the south. In the extended
period the models agree on maintaining an upper level ridge with
little energy across most of the region. As such, the pressure
gradient will remain relaxed over the southern waters and then
expand to the northern waters.

The models GFS/CMC/ECMWFHR/UKMETHR/NAM/NOGAPS have initialized
well the 18Z surface observations with just minor differences on
the low pressure over the southern waters mainly on the central
pressure value within 2 MB and for position within 30 NM.
Otherwise, the models have been consistent in the previous runs
and they continue to be in good agreement on low pressure moving
northeast into the north waters and then exit and continue moving
northeast away from the north waters. Will not make major
changes to the previous forecast and so will continue with mostly
GFS.

.SEAS...The seas are largest over the central waters with peak
at 17 ft. The wave models NWW3 and ECMWFWAVE fit well the 18Z
observed seas pattern. Both wave models have been consistent in
the previous runs and will therefore just go with NWW3 for the
seas. In the short term the largest seas will shift northeast
into the Baltimore Canyon and will move northeast of the region
in the extended period. Seas will continue to subside over the
southern waters to less than 8 ft.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale tonight.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale tonight into Wednesday night.

$$

.Forecaster Bancroft/Musonda. Ocean Prediction Center.



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