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000
AGNT40 KWNM 190137
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
837 PM EST Sun Feb 18 2018

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

GOES infrared satellite imagery indicates an intense hurricane
force low pressure system to the E of the offshore waters, and
cold cumulus extending to the E into the outer NT2 offshore
waters in the cold advection. Current surface observations
indicate winds to 60 E of the area, though the 12Z/18Z guidance
indicates hurricane force winds in the SW semicircle of the low.
However, the strongest winds moved E of the offshore waters
earlier this afternoon, and the guidance indicates winds down to
25 kt in the outer offshore waters. The models all indicate that
the winds will continue decreasing overnight as the strong low
continues to move away from the offshore waters, and high pres
builds over the area. The previous forecast used a blend of the
GFS/ECMWF solution, and this still seems reasonable. As a
result, will only be making minor changes over the short range in
the update package.

In the medium range, the models all indicate another frontal
system will move offshore and late Wed and Thu. There have been
some slight timing differences on the front as it drifts S over
the area, and the ECMWF has been fairly consistent on the timing.
Also, the GFS has looked a bit overdone with the SW flow ahead
of the front over the colder NT1 waters, while the ECMWF has been
weaker and seemed a bit more reasonable. The previous forecast
used a 50/50 blend of the GFS and the ECMWF, and this seems to be
a decent compromise for the slight timing and intensity
differences between the two solutions. As a result, planning on
continuing with that blend in the update package, and will keep
all winds below warning criteria throughout the forecast period
with above average confidence.

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

Current Conditions...The 12z ncep surface analysis shows a
complex low with its mean center about 220 nm SE of Cape Cod,
which is expected to move E and rapidly intensify. A cold front
extends SW from the low into the Charleston region, with a warm
front extending SE from the low. Latest available ascat and
ascat hi-res passes from a few hours ago show 25 to 35 kt winds
over the nt1 and northern nt2 waters, with 35 to 45 kt winds in
the E semicircle of the low E of the offshore waters.

Models/Forecast...The medium range models are in good overall
agreement across the offshore waters during the forecast period
regarding the flow pattern and timing of the main weather
features. The 00z ecmwf appears to have had the best handle on
the conditions associated with the rapidly intensifying low as it
was moving across the offshore waters, based on strong upper
level support evident with the system. The ecmwf/gfs have been in
the best agreement through the forecast period but with a few
intensity differences, so will use a 50/50 blend of the 00z
ecmwf/gfs for the wind grids for this evening through the rest of
the forecast period. Confidence is above average for a hurricane
force wind warning well E of the offshore waters this evening.

Seas...The ecmwf wam and wavewatch have both been initializing
reasonably well over the offshore waters. A representative 50/50
blend of the 00Z ecmwf wam/wavewatch will be used for this
evening through the rest of the forecast period, in order to
match up with a 50/50 blend of the 00z ecmwf/gfs being used for
the wind grids.

Extratropical Storm Surge Guidance...N/A.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Kells/Scovil. Ocean Prediction Center.


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