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AGNT40 KWNM 241941

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
341 PM EDT Mon Apr 24 2017

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

The preliminary 18Z NCEP surface analysis shows a complex low
pressure system impacting especially southern and central NT2
waters this afternoon, with one low pressure area centered near
Hilton Head Island, South Carolina and a second center near 31N
76W. As a result, widespread gale force winds are developing
over southern NT2 waters at this time.

These two low pressure centers are still forecast to merge
south of Cape Hatteras later tonight or early Tuesday.
Widespread gales are still forecast over most of the southern
NT2 spreading north into the central NT2 waters tonight into
Wednesday evening, with the gales ending from south to north
over the waters beginning Tuesday. Further north, high pressure
will weaken and move east away from NT1 waters tonight into
Tuesday as a warm front lifts slowly north toward these waters.
The low is forecast to slowly weaken Tuesday into Wednesday as
it tracks north-northeastward from near Cape Hatteras Tuesday to
just south of Long Island later Wednesday. The low will then
likely dissipate near Cape Cod early Thursday as a high pressure
ridge expands westward into southern NT2 waters. Widespread
areas of rain, showers and thunderstorms are occurring from
southwestern NT1 waters southward over most of the NT2 waters
late today. The stronger thunderstorms are currently being
detected over southern NT2 waters moving north and eastward.
Wind gusts exceeding 40 to 50 kt, and very rough seas can occur
in or near any of the stronger thunderstorms over the waters
through tonight, and into the middle of the week.

A ship report at 18Z, about 90 NM off the North Carolina coast,
indicated winds around 50 KT. A QC check shows his bias at 6 kt
too high, with thunderstorms in the vicinity of the ship as
well.  Overall, the 12Z models remain in very good agreement
over the waters into early Thursday. We will once again use the
smart tool which places the somewhat lower 10 meter winds over
stable areas, and stronger first sigma layer winds over the
unstable areas, which matches up with the warmer SSTs over and
the east of the gulf stream, for the afternoon package through
Wednesday night. Therefore, there will be no change to the
previous warnings for the afternoon package during the next few
days, with confidence levels remaining above average.

Over the longer term, the main forecast issue continues to be
the timing of the next cold front forecast to reach especially
NT1 waters later in the week or next weekend. Once again the GFS
guidance appears to be too fast in bringing the front east and
southeast into the offshore waters Friday night, with the 12Z
ECMWF about 18-24 hours slower with the passage of the front,
which is also closer to the model consensus, ensemble mean and
WPC medium range forecast. Therefore, we will rely on the 12Z
GFS for Thursday into Thursday night it remains similar to the
non-GFS guidance during this time frame. For Friday through
Saturday night we will trend the forecast toward the 12Z ECMWF
guidance. No warnings appear likely over the offshore waters
Thursday through Saturday night, with confidence levels near

Seas...The 12Z WNA Wavewatch guidance appears to be running
closer to the latest observations over the offshore waters
verses the too high 12Z ECMWF WAM guidance. As we will rely on
the GFS winds for the afternoon package from tonight through
Thursday night we will also remain close to the 12Z Wavewatch
model output. From Friday through Saturday night, as we trend
the forecast toward the ECMWF winds, we will also trend the
forecast toward the WAM for sea heights.

.Extratropical Storm Surge Guidance...As the strong Ely gradient
continues to develop N of the low pressure center tonight, and
then shift northward up the mid Atlantic coast Tuesday into
Wednesday, the 12Z ESTOFS continues to forecast a somewhat more
significant surge to develop on the coast than forecast by the
12Z ETSS. Overall, we continue to favor the somewhat higher
ESTOFS guidance over the region as we prefer to the GFS model
winds for the afternoon package through Wednesday.


.NT1 New England Waters...

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


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