AGNT40 KWNM 230133
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
833 PM EST Wed Nov 22 2017

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

GOES satellite imagery and lightning data show a strengthening
low pressure system centered near Georges Bank this evening. The
low will continue to move NE away from the waters tonight into
Thu. Weak high pressure will build offshore in it's wake later
tonight through Thu. The next low pressure system will then
impact the NT2 waters Thu night through Sat, followed by the next
cold front moving offshore. Overall, we will not make major
changes to the ongoing forecast for the evening update, except to
adjust the grids and forecast for current conditions and to fit
nearby coastal WFO and TAFB forecasts. The current gale warnings
are expected to end over the far NE NT2 and far SE NT1 waters
around midnight or so, with storm force winds E of these waters
associated with the strengthening low pressure area.

For sea heights we will also not make major changes to the
ongoing OPC grids and forecast at this time, and adjust the
previous grids and forecast to fit current conditions and nearby
coastal WFO and TAFB forecasts prior to forecast issuance this
evening. Per the 00Z RA1 OPC sea state analysis, sea heights
range from 22 feet over NT2 waters near the gale warning areas,
to near 3 feet closer to the coast of Maine and the SE U.S.
coast.

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PREVIOUS DISCUSSION...

The GOES infrared and visible satellite data indicate developing
low pres along a frontal boundary across the W Atlc this
afternoon, and the lightning density product indicates showers
and tstms along the front with the majority of the convective
activity in the unstable environment over the Gulf Stream.
Previous weather grids had mentions of showers and tstms, so
planning on maintaining them while making minor adjustments for
current conditions. As far as the winds are concerned, the Ascat
from 1355 UTC this morning indicated a few gales near the front,
but the overpasses largely missed the offshore waters including
just E of the front where the 12Z GFS/ECMWF both indicated the
strongest winds over the Gulf Stream. Also, current surface
reports only indicate up to 20 kt in the offshore waters with nor
reports in the vicinity of the front. The 12Z GFS/ECMWF/UKMET
are all initialized fairly well on the synoptic features, and all
indicate the aforementioned developing low pres will intensify
tonight as it moves NE across the Gulf Stream. The 12Z GFS 10m
winds indicated 55 kt with the first sigma winds up to 60 kt at
18Z. The 12Z ECMWF also indicated 50 kt at 18Z, while the UKMET
only showed about 40 kt. However, model lapse rates and stability
indices indicate an unstable environment over the Gulf Stream,
while model 925 mb GFS/ECMWF winds increase up to 75 kt tonight.
This development of stronger low level winds is expected N of the
Gulf Stream, so confidence with hurricane force mixing down to
the surface is just below average. As a result, planning on
staying with storm warnings for early tonight, and then drop
all warnings in the offshore waters later tonight as the models
agree well on taking the low quickly off to the NE. Will stay
with previous wind and wave height grids for tonight which seem
representative.

The 12Z models then indicate another low currently organizing in
the Gulf of Mexico will slowly develop off the SE coast Fri into
Fri night, and move NE across the area Fri night and Sat. The
GFS/ECMWF were not in good agreement on the track and intensity
of the low and associated winds in previous model runs, and there
are still some minor differences as it moves into srn NT2. The
12Z ECMWF is a little W of the 12Z GFS/UKMET track with the low,
but is E of the 12Z GEM. The 12Z ECMWF did trend a little faster
from its 00Z run, and is supported by the 12Z GEFS members. The
previous forecast favored a previous run of the ECMWF, so
planning on populating with the 12Z ECMWF starting Thu. There is
also some question on the intensity of the winds. The previous
forecast went up to 25 kt. However, the 12Z ECMWF indicates 30
kt in NE flow over the Gulf Stream by Fri, and this seems
reasonable. There is also a question of whether or not gales will
develop further out on Sun in the cold advection behind the
front. However, confidence is low as a result of the poor
agreement on the track further out in the medium range. As a
result, planning on capping winds at 30 kt in the offshore waters
through the remainder of the forecast period.

Seas...The 12Z ECMWF WAM is initialized slightly better when
compared with the current data in the offshore waters. However,
will start out with the previous wave height grids for tonight
to better match the wind grids. Will then transition to the 12Z
ECMWF WAM Thu and continue with it through the remainder of the
forecast period to reflect trends of the preferred 12Z ECMWF
winds.

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


.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ900...Georges Bank east of 68W...
Gale tonight.

.NT2 Mid-Atlantic Waters...
.ANZ905...The Great South Channel to the Hague Line...
Gale tonight.

$$

.Forecaster Mills/Kells. Ocean Prediction Center.