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AGNT40 KWNM 250044

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
844 PM EDT Sat Mar 24 2018

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

Current Conditions...The 18z ncep surface analysis shows a strong
complex low E of the offshore waters, with associated troughing
over the nt1 and NE nt2 waters. The analysis shows a weak 1014
mb low over Rhode Island, and a cold front approaching the New
England coast from the NW. The analysis also indicates a high
pressure ridge across the central and southern nt2 waters. Latest
available ascat and ascat hi-res passes from this morning show 20
to 30 kt winds in NW flow over the outer nt2 waters, with 10 to
20 kt winds indicated elsewhere in the offshore waters.

Models/Forecast...The 12z medium range models are in good
agreement across the offshore waters for tonight through Tue, so
have above average confidence in the storm warnings for the outer
zones in the northern part of the nt2 area for Mon night, with
all of the models showing strong N to NE flow across the north
wall of the Gulf Stream. The representative 12z gfs 10m/30m
solution with the stability smart tool will be used for the wind
grids through Tue. The models are not in particularly good
agreement with the timing and orientation of the overall flow
pattern around the offshore waters for Tue night through Thu
night, but the 12z ecmwf looks like a good median model solution
and has good support from the latest wpc medium range forecaster
guidance. So the 12z ecmwf will be used for Tue night through the
rest of the forecast period.

Seas...The ecmwf wam and wavewatch both initialized reasonably
well over the offshore waters, and are in decent agreement for
tonight through Tue with some minor differences noted, so a
50/50 blend of the models will be used for the sea height grids
for this timeframe. Will then use the 12z ecmwf wam exclusively
for Tue night through the rest of the forecast period, in order
to be consistent with the preferred 12z ecmwf winds over this

Extratropical Storm Surge Guidance...The latest estofs and etss
guidance indicates a positive surge in the 1.0 to 1.5 ft range
along the mid Atlantic and SE coasts for later Sun through Mon,
with looks reasonable given the fairly strong NE flow showing up
in preferred gfs model for those regions.


The GOES visible satellite imagery indicates cold air cumulus in
the cold air advection across the offshore waters this afternoon
in the wake of a cold front E of the offshore waters. The
lightning density product shows some showers and tstms just E of
the nrn NT2 offshore waters which were over the area this morning,
mainly in the unstable environment over the Gulf Stream. The
convection is an indication of the low level mixing in the cold
advection with stronger winds aloft mixing down towards the
surface. The previous forecast had gales over NT2 for earlier
today, but the imagery indicates that they have passed just E of
the waters. In addition, the Ascat wind retrievals from 15Z
indicate up to 30 kt over the offshore waters, and gales off to
the E near the front. The 12Z GFS winds are initialized well
when compared with the data, and the model indicates that
majority of the strongest cold advection will pass E of the area
early tonight. In addition, the 12Z GFS indicates that a
developing low will move off the SE coast tonight, then develop E
of the NT2 waters over the next several days. The 12Z GFS and
ECMWF agree fairly well over the short range, so planning on
starting out with with the 12Z GFS in the update package.

The 12Z models indicate the aforementioned low pres will move
develop E of the area Sun into Tue. There are differences on the
track of the low, and the 12Z ECMWF is just W of the 12Z GFS in
the short range before taking the low north of the 12Z
GFS/UKMET/GEM models. However, the models generally agree on the
strong NE flow over the offshore waters, mainly over NT2. The
previous forecast had storm force winds in far NE NT2 by late Mon
night, with a large area of gales surrounding the storm force
winds. Confidence is above average on the storm force as the
strong NE flow over the Gulf Stream will more than likely be
above the guidance when considering the low wind bias the models
have had this season. As a result, planning on keeping previous
headlines for the next forecast package, but will switch to the
12Z ECMWF on 12Z Tue.

On Thu, the 12Z models all indicate another low pres system will
approach the coast, though the 12Z GFS is a bit faster than the
rest of the 12Z models and takes the low and associated cold
front offshore. There is quite a bit of spread in the guidance,
and the 12Z ECMWF is a decent compromise when compared with the
12Z UKMET and GEM. In addition, the 12Z GEFS members are showing
a stronger signal for a slower solution, and are more similar to
the 12Z ECMWF. Also, the WPC medium range guidance is also
favoring the slower solution, so will continue with the 12Z ECMWF
through the remainder of the forecast period.

Seas...The 00Z ECMWF WAM and Wavewatch have both initialized
reasonably well over the offshore waters, and remain in decent
agreement through the short range period. As a result, will start
out with a 50/50 blend of the two models. Will then transition to
the 12Z ECMWF WAM at 12Z Tue since will be using the 12Z ECMWF
winds at that point onward.

Extratropical Storm Surge Guidance...The 12Z ESTOFS and ETSS
backed off slightly on the surge along the mid Atlantic and SE
coasts for later Sun through Mon night, are now showing just over
1.0 ft as a maximum. The ESTOFS is slightly higher than the ETSS,
and seems slightly more reasonable in the NE flow expected along
the coast.


.NT1 New England Waters...
.ANZ900...Georges Bank east of 68W...
     Gale Monday.
     Gale Possible Monday night into Tuesday.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Sunday night into Monday.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Sunday night into Monday.
     Storm Possible Monday night.
     Gale Possible Tuesday into Tuesday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Sunday night into Monday.
     Storm Possible Monday night.
     Gale Possible Tuesday into Tuesday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale tonight.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale tonight into Sunday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale tonight into Sunday.


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