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000
AGNT40 KWNM 232350
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
750 PM EDT Sun Apr 23 2017

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

A stationary front persists W to E across the S central NT2
waters which in combination with a high pressure ridge across the
Srn NT1 waters is maintaining a moderately strong (20-25 kt) Ely
gradient immediately N of the front. Max seas throughout the
waters are in this gradient and are likely in the 6-9 ft range
which are being handled reasonably well by the 18Z Wavewatch III
and 12Z ECMWF WAM models at the moment.

Over the short term, the latest models agree that the front will
lift slowly N as a warm front tonight into Mon while maintaining
the moderately strong Ely gradient to its immediate N. Then later
Mon in response to a closed upper low forecast to move towards
the SE coast, the models remain consistent in forecasting a
surface low to lift N into the far SW NT2 waters and merge with
another low off the S Carolina coast with ESE gales developing.
Then Mon night into Tue night the models generally agree that the
surface low will become vertically stacked with the upper low and
track slowly NE to off the mid Atlantic coast with the associated
gales spreading NE across the NT2 waters. Versus its previous
12Z run, the 18Z GFS has trended slower with its forecast track
of this low which brings it into better agreement with the 12Z
ECMWF track. The previous forecast package used a mix of 12Z GFS
winds for this system. So these winds still look representative,
but will now go ahead and make some additional generally minor
edits in deference to the 18Z GFS/12Z ECMWF.

In the long range, as the models forecast the upper low to
weaken, they all forecast the associated surface low to weaken as
it moves towards the Srn New England coast Wed night with
associated winds becoming subgale. Therefore will continue to
make some additional minor edits then to the previously
populated 12Z GFS winds in deference to the 18Z GFS/12Z ECMWF.
Then late Fri/Fri night, versus its previous 12Z run the 18Z GFS
has trended slower with a cold front moving into the NT1 waters
which brings it into better agreement with the previously
populated 12Z ECMWF solution. So do not plan on making any major
long range changes in the next offshore forecast package.


-----------------------------------------------------------------
PREVIOUS DISCUSSION...

A late season nor`easter will bring widespread high-end gales to
mainly the southern and central NT2 offshore waters Monday into
Wednesday night.

Over the short term, a nearly stationary front extends northeast
from near Charleston, South Carolina, with high pressure
centered just south of Long Island. Low pressure was centered
over far northern Georgia with another low over far southwest
Florida per the preliminary 18Z NCEP surface analysis. These low
pressure areas will combine near the South Carolina coast later
Monday, with the low then tracking north and northeast along or
just off the east coast Monday night through Wednesday night
with the low weakening near eastern Long Island Wednesday night.
Further north, the high will move east away from the waters
Monday as a cold front moves southeast toward eastern Maine. The
front will dissipate Monday night just north of the waters as
high pressure builds over southeast Canada. Overall, the 12Z
models are in very good agreement over the region through
Wednesday night. The 12Z GFS has remained quite consistent over
its past few cycles, and is in excellent agreement with the 12Z
ECMWF. As a result, we will rely on the 12Z GFS and continue to
use the smart tool which places the stronger first sigma layer
winds over the unstable areas and somewhat lower 10 meter winds
over stable areas tonight through Wednesday night over the
waters. Widespread gales appear likely over southern and central
NT2 waters Monday into Wednesday night, with above normal
confidence. We will manually trim back gales for Tuesday from
the far northwest NT2 waters, or zone ANZ820, for this package
which will keep the headlines consistent from the previous few
forecasts. We will continue to monitor future model guidance and
trends to see if the gales need to be expanded into northern NT1
zones over the next day or so. Otherwise, widespread shower and
thunderstorm activity will occur near the low as it tracks over
the region tomorrow through Wednesday night. Currently, most of
the scattered thunderstorms are located near the stationary
extending east-northeast from near Charleston. Wind gusts
exceeding gale force and very rough seas can be expected in or
near any of the heavier thunderstorms that develop over the
waters during the next few days.

Over the long term, the 12Z models remain in good agreement over
the waters Thursday into Thursday night. The low will weaken
near the New England coast Thursday into Thursday night as a
high pressure ridge builds west toward southern NT2 waters. We
will remain close to the 12Z GFS 10 meter winds for Thursday and
Thursday night as a result, with no headlines expected over the
waters during this time period. For Friday and Friday night the
12Z models begin to diverge. The 12Z GFS appears to be a little
too fast in bringing the next cold front east and southeast over
the waters Friday and Friday night, with the 12Z ECMWF closer to
the model consensus and ensemble mean forecasts. As a result, we
will trend the forecast toward the 12Z ECMWF for Friday and
Friday night over the region, with confidence levels near
average. This will result in the cold front passing southeast
over NT1 waters Friday night, with the ridge dominating from
southern NT2 waters eastward. This idea is also supported by the
latest medium range forecast from WPC.

Seas...Both the 12Z Wavewatch and ECMWF WAM are both initialized
well over the waters per the latest altimeter data and
observations. For the afternoon package we will continue to use a
50/50 blend of the two wave models tonight through Thursday
night, and then to match the wind forecast trend the forecast
toward a nearly 100 percent WAM model output for Friday and
Friday night.

.Extratropical Storm Surge Guidance...The 12Z ESTOFS model is
indicating a surge of about 1 to 2 feet so along the southeast
and Mid-Atlantic coast by Tuesday into Wednesday with the
persistent easterly fetch ahead of the nor`easter moving up the
coast. The 12Z ETSS is a bit lower with the surge. At this time
a compromise between these two surge models is preferred over
the region into the middle of the week, as we continue to rely on
the GFS model guidance for the wind forecast over the region.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Tuesday.
     Gale Possible Tuesday night into Wednesday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Tuesday.
     Gale Possible Tuesday night into Wednesday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale Tuesday.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale Monday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Monday night into Tuesday.
     Gale Possible Tuesday night.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
     Gale Monday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale Monday into Monday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Monday into Tuesday.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale Monday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Monday.

$$

.Forecaster Vukits/Mills. Ocean Prediction Center.



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