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AGNT40 KWNM 041328

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
830 AM EST SUN 4 DEC 2016

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

The latest models are in good agreement that the offshore
northwest winds should continue to diminish today as a high
pressure ridge moves east to the coast. Will then retain the
marginal gales associated with the developing surface low
forecast to move off the North Carolina coast near Cape Hatteras
early Monday...with slightly above average forecast confidence.
Like the 00Z run, the 06Z GFS continued to be further north than
other guidance including the 06Z GEFS mean with the track of the
next developing low across the NT2 waters Tuesday night and
Wednesday. Made some minor adjustments to the wind grids Monday
and Monday night to limit the gales to outer NT2 zones including
ANZ930 ANZ925 ANZ920 and ANZ910. Also expanded the areas of 25+
kt winds over the west side of the developing low Monday night
based on the 06Z 4km NAM. In the medium range the overnight
models generally trended weaker with the winds associated with
the strong arctic cold front expected to move off the coast
Thursday night. The 06Z GFS continued to be more progressive
than the 00Z ECMWF/UKMET and 06Z GEFS. However the timing
differences are less than they were with yesterdays runs.

Across the outer NT1 and northern NT2 waters significant wave
heights are 1 to 3 feet higher than the 06Z Wavewatch III this
morning. The previous wave height grids which were roughly a 75%
ECMWF WAM/25% WW3 blend look are in good agreement with the
lastest west Atlantic ship and buoy observations this morning. A
12Z Jason-2 altimeter overpass returned 12 to 16 feet just east
of the northern NT2 waters which again indicates the WW3
guidance is a few feet underdone.

...Previous Discussion...

Over the short term...Though it missed the bulk of the
earlier 0141Z high resolution ASCAT-B scatterometer pass
confirmed that gale force winds were present in the strong NW
gradient across the outer nrn NT2 waters. Per a consensus of the
new 00Z models believe this gradient will weaken enough to start
the today forecast period with subgale winds. Otherwise the 00Z
models remain in good agreement that a high pressure ridge will
build offshore today into tonight causing the NW gradient to
continue to gradually weaken with its associated winds
diminishing from W to E. So for this diminishing trend...since
the previously populated 12Z ECMWF boundary layer winds are very
similar to those forecast by the new 00Z GFS/UKMET/ECMWF...see
no reason to repopulate our forecast wind grids for

Then in general the 00Z models agree that a surface low will
develop off the Carolina coasts late tonight...then track ENE
across the central NT2 waters Monday/Monday night. In regards
the forecast track of this system...the 00Z GFS/GEM are both
slightly slower and further S than the 00Z NAM/UKMET/ECMWF
solutions. With the 00Z GEFS mean indicating the 00Z GFS may be
too slow...would favor the other more progressive models. In
regards to the forecast gradients associated with this
system...the stronger 00Z NAM/UKMET/ECMWF and even the weaker
00Z GFS/GEM solutions all to varying degrees forecast SW gale
force boundary layer winds to develop in advance of its
associated cold front. Therefore forecast confidence in
associated gales developing has increased. So will populate our
forecast wind grids with a 50/50 blend of the 00Z ECMWF/UKMET
boundary layer winds for Monday/Monday night...with some
additional edits in deference to the 00Z NAM/GFS which will
result in a larger area of associated gales over the NT2 waters.

In the long range...On Tuesday into Wednesday night...the 00Z
models continue to differ in regards to their exact forecast
tracks and strengths for another developing surface low to form
off the SE coast and then track NE across the NT2 waters. In
regards to the forecast track of this low...the 00Z NAM/GEM look
like fast outlier solutions. Versus its previous 18Z/12Z
runs...the 00Z GFS track is similar but slightly faster. Again
the 00Z GEFS mean indicates that the 00Z GFS may be too fast.
Therefore would favor the less progressive 00Z UKMET/ECMWF
forecast tracks which are also somewhat further S than the 00Z
GFS. So will transition to populating with the 00Z ECMWF
boundary layer winds for Tuesday through Wednesday night.

Then further out in the long range...The disparity between the
00Z global models increases significantly Thursday/Thursday
night. The 00Z GFS continues the trend of its previous
respective runs of forecasting a fairly strong surface low to
develop near Cape Cod Thursday afternoon...then pass quickly NE
across the NT1 waters later Thursday into Thursday night while
pulling a strong cold front offshore. Again the 00Z GEFS Mean
indicates that the 00Z GFS may be too progressive with this
system. The 00Z GEM looks completely out of phase for this
system and hence will be disregarded. The 00Z UKMET and moreso
the 00Z ECMWF are slower than the 00Z GFS in both forecasting an
attendant surface low to develop and track N and NE of the area
and then also slower with their associated cold fropas. For now
with very low forecast confidence will continue to populate with
the 00Z ECMWF boundary layer winds on Thursday/Thursday
night...with its winds boosted up 10-15 percent late Thurday
night to account for its usual low bias in cold air advection.
So due to the large model disparity will also cap these winds at
below gale force to hold off on forecasting any possible gale

Seas...A compromise between the slightly higher 00Z ECMWF WAM
and lower 00Z Wavewatch III would initialized the current seas
best. Since this compromise remains in line with the previously
populated wave grids...which were a 75/25 blend of the 12Z ECMWF
WAM/Wavewatch III...will not repopulate our wave grids for today
and tonight. Then overall since the 00Z ECMWF solution will be
favored...will transition to populating with all 00Z ECMWF WAM
forecast seas for Monday through Thursday night.

.Extratropical storm surge guidance...N/A.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Monday into Monday night.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Monday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Monday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras
     Gale Monday into Monday night.
     Gale Possible Tuesday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale Possible Tuesday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Monday.
     Gale Possible Tuesday into Tuesday night.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale Possible Tuesday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Possible Tuesday into Tuesday night.


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