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AGNT40 KWNM 252004

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
404 PM EDT Tue Jul 25 2017

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

With a frontal boundary located from off of the Carolina coast
northeast to east of Nova Scotia, the primary hazard for the
near-term part of the forecast will be scattered-to-numerous
showers and thunderstorms in vicinity of the stalling, weakening
front. To the north, in advance of the next front and in warm air
advection, there should be some areas of low visibility over
northern waters by Thursday. Into Friday, the surface gradient in
general should be slack, resulting in winds well below hazards.

As has been consistently noted recently, the primary forecast
challenge comes toward the end of the week and into the weekend.
The latest guidance offers departures from previous runs. In
this case, the GFS and the ECMWF both have a closed 500mb low
moving southeast over southeast Canada, but lag a trailing low at
that level moving southeast out of the Great Lakes toward the
mid- Atlantic. The result is low pressure that develops and moves
off of the coast more slowly and farther south. The NAM is
somewhat in agreement with this, while the UKMET is even slower
aloft and keeps low pressure much farther north along the coast
and into the far northern waters late in the weekend.

GFS ensemble clustering is quite tight into Saturday, with
surface low pressure emerging off of Delmarva Saturday morning,
but then with good spread thereafter with a consensus track a
little north of east. The deterministic ECMWF is similar. If the
GFS and ECMWF verify, or verify close, it would seem two areas of
gales are quite possible. The first would be in the tight gradient
to the north of low pressure, off the coast from southern MA to
off of DE, primarily ANZ810, 815, and 820, into 915 before the
low starts to fill a little and move northeast. The other area
would be near the Gulf Stream from ANZ910 southwest to ANZ930,
where 850mb winds on both the GFS and ECMWF increase to 35 to
45kt. Given the guidance variability of late, confidence in the
timing and location of gales is only modest, but confidence is
growing that an area, or areas, of gales will occur Saturday into
Sunday. Some refinement is possible in any headlines over time.
Overall, for this forecast leaned toward the ECMWF winds with
modifications, particularly Saturday into Sunday.

.SEAS...Based on the wind forecast above, leaned toward the ECMWF
WAM wave output, modified especially for expected conditions
Saturday into Sunday. The primary increase in waves from that
guidance was in vicinity of the Gulf Stream Saturday and Saturday
night, in the axis of higher southwest winds. For this forecast,
waves should have a maximum near 15 feet east of the Delmarva
coast. As with the wind forecast described above, modifications
are possible if the variability in the guidance continues.



.NT1 New England Waters...
.ANZ810...South of New England...
     Gale Possible Saturday.
.ANZ815...South of Long Island...
     Gale Possible Saturday.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale Possible Saturday.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Possible Saturday into Saturday night.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Possible Saturday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Possible Saturday into Saturday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Possible Saturday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Possible Saturday.


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