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AGNT40 KWNM 040000
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
700 PM EST SAT DEC 3 2016

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

The strong NW gradient persists across the NT1 and nrn/central NT2
waters with gale force winds still likely occurring across the
outer nrn NT2 waters pretty much as previously forecasted. Max
seas in this gradient are likely in the 10-15 ft range...highest
across the SE most NT1 and NE NT2 waters...which are being handled
slightly better by the higher 12Z ECMWF WAM model versus the 18Z
Wavewatch III at the moment.

Over the short term...The latest models continue to forecast a
high pressure ridge to build offshore tonight into Sunday night
causing the NW gradient to gradually weaken with its associated
winds diminishing from W to E. So see no reason to make any
significant changes to the previously populated 12Z ECMWF boundary
layer winds then.

Then late Sunday night into Monday night the 12Z ECMWF continues
to look like a reasonable compromise solution (timing-wise) for a
surface low forming off the SE coast and then tracking ENE across
the central NT2 waters. Per the 18Z NAM/GFS am tempted to expand
the previously forecasted gales associated with this system to
include NT2 zones ANZ833 and ANZ930 early Monday...but for now
will hold off and see later if the 00Z models present any better
justification for this.

In the long range...On Tuesday into Wednesday night...the latest
models continue to differ in their exact forecast tracks and
strengths for another developing surface low forming off the SE
coast and then moving NE across the NT2 waters. In general the
18Z/12Z GFS and 12Z ECMWF solutions are similar for this system
Tuesday into Wednesday...but then the 12Z ECMWF becomes less
progressive than the GFS late Wednesday/Wednesday night. The 12Z
NAVGEM/UKMET both look like too progressive outlier solutions for
this system. So with so much disparity still in place for this
system...will continue to use the previous official wind/wave
grids and will just make some additional edits in deference to the
12Z ECMWF.

Then further out in the long range...The 18Z GFS continues the
trend of its previous 12Z run of forecasting a fairly strong
surface low to develop near Cape Cod Thursday afternoon...then
track NE across the NT1 waters Thursday night while pulling a
strong cold front offshore...which is significantly faster than
forecast by the 12Z NAVGEM/UKMET/ECMWF. Therefore due to this
large forecast disparity...with very low forecast confidence...
will continue to use the previously slowed down 12Z GFS solution
and will hold off on forecasting any associated possible gales.





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

The 1420Z and 1500Z Ascat overpasses revealed an expansive area of
30 to 35 kt winds extending from south of Long Island and New
England south and southeast across the outer NT2 waters north of
Baltimore Canyon. Earlier passes indicated the gales extend as far
east as 58W. The 12Z models are in good agreement across the west
Atlantic for the next few days. The northwest gradient should
gradually relax tonight with gales shifting east of the offshore
waters by early Saturday and will continue with the gale warnings
through tonight across the outer NT2 waters north of Baltimore
Canyon as well as across the southern portions of Georges Bank.
Based on its better initialization with the winds and because it
is better supported by the higher resolution HRRR and 4km NAM am
favoring the 12Z ECMWF through Sunday. For wind grids, boosted
the ECMWF winds due to its low bias in strong cold air advection
as will continue offshore through Sunday. The 12Z models are then
consistent in moving a high pressure ridge offshore late Sunday
and Sunday night.

Models then all indicate a developing surface low will move off
the Mid Atlantic coast near Cape Hatteras Monday, but differ with
track and timing of low as it crosses the NT2 waters later Monday
and Monday night. Over the past several runs the ECMWF has shown
the most run to run consistency and with the timing has offered a
compromise between the slower 12Z GFS and more progressive 12Z
UKMET. Versus its previous run the 12Z UKMET trended even more
progressive and is further north than the ECMWF or GFS. More
uncertainty lies in whether surface low will deepen sufficiently
to get any gales across the NT2 waters late Monday into Monday
night. In addition to the 12Z ECMWF, both the 12Z GFS and 12Z NAM
are indicating some marginal gales, so with about average
forecast confidence will add gales but per the ECMWF and 4km NAM
will limit them to the ANZ925 zone. Through Monday night OPC
preference contiues with the 12Z ECMWF. The model solutions further
diverge with the next developing low forecast to track northeast
along the Carolina coast Tuesday and Tuesday night. Given the
detail differences between models with multiple surface low
developments and timing differences between GFS and ECMWF prefer
to retain the previous OPC grids which were based on the 00Z ECMWF
and included possible gales across the southern NT2 waters Tuesday
into Tuesday night.

Later in the week the guidance all shows a strong arctic will
approach the coast but there are significant timing differences
with the 12Z GFS and 12Z CMC 12 to 18 hours faster than the 12Z
ECMWF or 12Z UKMET. At this time am favoring the timing of the 12Z
GEFS mean which offers a compromise between the two model camps.
For grids am choosing to time shift the 12Z GFS about 6 to 9 hours
slower Thursday and Thursday night so is more in line with 12Z
GEFS mean. With the forecast uncertainty high with timing of the
front am choosing to keep winds below gale Thursday and Thursday
night.

Seas...The latest ship and buoy observations as well as a 1230Z
altimeter overpass suggest that the 12Z Wavewatch III is several
feet underdone across the northern NT2 waters while the 12Z ECMWF
WAM is slighly too high. To best match observations will be using
a blend with 75% ECMWF WAM and 25% WW3 through Sunday night...then
a more conservative 50/50 blend Monday through Wednesday. To be
consistent with the time shifted GFS winds late in the week will
use a similar time shift with the 12Z WW3 beginning Wednesday
night.

.Extratropical storm surge guidance...In the current strong
offshore flow the 12Z ESTOFS continues to forecast a more
significant negative surge along the coast from Delaware Bay to
Long Island Sound tonight than forecast by the 12Z ETSS model.
With the ESTOF typically overdone with the negative surge in these
situations would prefer the ETSS.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale tonight.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale tonight.
.ANZ905...The Great South Channel to the Hague Line...
     Gale tonight.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale tonight.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Monday.
     Gale Possible Monday night.
     Gale Possible Tuesday night.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
     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 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.

$$

.Forecaster Vukits/Clark. Ocean Prediction Center.



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