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000
AGNT40 KWNM 260856
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
456 AM EDT Wed Apr 26 2017

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

NCEP 06Z surface analysis places low pressure center near Ocean
City MD with 1002 mb pressure with an occluded front along the
mid Atlantic coast and warm front across the southern NT1 waters
and trailing cold front between 70W and 72W over outer NT2
waters. 0221Z ASCAT-B pass returned a swath of gales as high as
40 kt in advance of the cold front mainly over and to the south
of the Gulf Stream affectong ANZ910 and 920 but some 35 kt noted
to the north into ANZ915 where GOES satellite imagery and
lightning strike and density data indicate heavy convection. With
SREF SPC guidance indicating up to 20 pct probability of severe
thunderstorms in that area there may be locally more hazardous
wind conditions and rough seas with the thunderstorms. Elsewhere
winds are 15 to 25 kt from ASCAT and surface observations. These
conditions are in line with 00Z GFS 10m winds except closer to
first sigma level winds near and south of the Gulf Stream.

Low pressure which is vertically stacked with upper low will
continue to track north along the mid-Atlantic coast today with a
slow weakening trend and pass near Long Island this evening and
then pass over the eastern NT1 waters Thursday and dissipate.
With high confidence will continue the gale warnings over the
ANZ910 and 920 zones today with the gales ending early Wednesday
night in line with the weakening trend shown in the latest 00Z
models. The 00Z GFS has been consistent in handling this low and
the somewhat higher winds shown in 00Z GFS/GEM are preferred but
otherwise the 00Z models agree well with the low track through
Thursday.

Otherwise after the low dissipates the 00Z models generally
agree that a quiet weather pattern will develop and persist. With
minor timing differences the models all forecast a high pressure
ridge to build W across the southern NT2 waters Thu night
through Fri night, then persist Sat/Sat night while a weak cold
front pushes offshore across the NT1 waters with moderately
strong (generally up to 20 or 25 kt) associated gradients
developing. After Friday night will use a 50/50 blend of the 00Z
GFS and 00Z ECMWF as a compromise.

Seas...The latest NWW3 and ECMWF WAM are similar but 00Z ECMWF
WAM is a couple of feet higher in the northern NT2 waters near
the front. For the new forecast will use a 3:1 blend of the two
wave models weighted toward the ECMWF WAM initially but after 18Z
today and for the remainder of the forecast use a 50/50 blend of
the 00Z models.

.EXTRATROPICAL STORM SURGE GUIDANCE...Observed water levels
indicate the positive surge of 1 to 1.5 ft along the mid Atlantic
coast is best handled by the 00Z ESTOFS.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale today.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale today into tonight.

$$

.Forecaster Bancroft. Ocean Prediction Center.



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