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627
AGNT40 KWNM 231936
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
336 PM EDT Mon Oct 23 2017

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

The latest GOES water vapor satellite imagery shows a vigorous
shortwave trough inland over the SE CONUS moving to the E. The
latest NCEP surface analysis indicates the associated surface
low pres system just E of the upper trough, and the Ascat from
14Z indicated winds up to 20 kt in the SE flow ahead of the
approaching system in the srn and central NT2 offshore waters.
However, the Ascat missed areas right along the SE coast, and
recent surface reports show winds up to 20 kt and up to 25 kt
just W of the offshore waters. The 12Z GFS/ECMWF 10m/surface
winds are initialized well when compared with the current data,
and the models indicate the winds will increase in the offshore
waters tonight into Tue night as the front is expected to
approach to the coast and increase the pressure gradient from the
interaction with high pres E of the offshore waters. The 12Z
models are all indicating at least marginal gales by late Tue
night, mainly in the unstable environment over the Gulf Stream.
The models are also indicating a small area of gales extending
into NT1 by Wed as both the 12Z GFS/ECMWF show a strong low level
jet setting up ahead of the front with 925 mb winds in the 60 to
65 kt range. The vertical wind speed shear should induce some
turbulent mixing, and should be enough to mix gales down to the
surface for at least a brief period. Confidence in gales for NT2
is high as a result of the less stable environment, and about
average for NT1 as the strong low level jet should be enough to
break through the stable boundary layer over the cold shelf
waters with SSTs currently in the upper 50s to low 60s. For the
wind grids, am planning on starting out with the 12Z GFS 10m
winds into 12Z Wed, but will use first sigma winds in unstable
areas to account for the deeper mixed boundary layer.

In the medium range, the 12Z models all indicate the
aforementioned front will pass E of the offshore waters by Thu
night. The 12Z GFS has trended a little faster with the timing,
while the 12Z ECMWF is a tad slower. The models also differ on
the timing of a low developing along the front to the E of the
area, and the 12Z GFS becomes a northerly outlier with the track.
The 12Z ECMWF is supported a bit better by the rest of the 12Z
guidance, so planning on transitioning to the 12Z ECMWF at 12Z
Wed. The models also indicate another low, possibly a tropical
cyclone, will move into the srn waters on Sat. However, there is
quite a bit of spread on the track, and not much of a consensus
between the models. As a result, will continue with the 12Z ECMWF
through the remainder of the forecast period, but confidence is
somewhat low.

Seas...The 12Z Wavewatch and ECMWF WAM both initialized well
over the offshore waters, and are in good agreement through Tue
night. As a result, am planning on using a 50/50 blend of the two
solutions as a compromise. Will then transition to the 12Z ECMWF
WAM to reflect the trends of the preferred 12Z ECMWF winds.

.Extratropical Storm Surge Guidance...N/A.

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ800...Gulf of Maine...
     Gale Wednesday.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Possible Thursday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Wednesday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Tuesday night.

$$

.Forecaster Kells. Ocean Prediction Center.



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