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000
AGNT40 KWNM 261348
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
948 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.

The latest NCEP surface analysis indicates strong low pres just
east of the Delmarva peninsula with a cold front extending over
SW NT1 and nrn NT2. The GOES visible satellite imagery along with
the lightning density product shows an area of showers and tstms
along the frontal boundary, and the Ascat wind retrievals from
02Z late last night indicated up to 40 kt in the southerly flow
ahead of the front. However, Current surface reports indicate
winds in excess of 50 kt in the convection with the highest over
the Gulf Stream over the nrn outer NT2 waters. In addition, an
07Z Jason-2 pass indicated winds near 40 kt to the west. The 06Z
GFS winds are initialized low when compared with the data, and
only indicate about 40 kt. Confidence is above average in storm
force winds currently in NT2 as a 56 kt ship report in the
vicinity of the highest winds over NE NT2 has a +6 kt bias looks
good, so planning on putting up a storm warning for early today.
Will also maintain mention of showers and tstms across the
offshore waters with this system as it is actively generating
convection. However, the models all indicate the system will move
north into New England today while weakening. This trend was
indicated in the previous forecast, so not planning on making any
significant adjustments as the guidance is in good agreement and
the GFS was used in the previous forecast.

Otherwise, the models are in reasonably good agreement throughout
the remainder of the forecast period and indicate a pair of
weaker cold fronts will move through the area during the period.
The GFS and ECMWF have only minor timing differences on these
features, and the UKMET/GEM are both in decent agreement with the
GFS/ECMWF solutions. Winds are expected to remain below gale warning
criteria for the remainder of the forecast period with above
average confidence, so not planning on having any headlines other
than those related to the current system for today into early
tonight. Also not planning on making any major changes from the
previous forecast which was a blend of the 00Z GFS/ECMWF
solution.

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

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 affecting 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 Kells/Bancroft. Ocean Prediction Center.


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