Marine Interpretation Message
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AGNT40 KWNM 130205

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
905 PM EST TUE 12 DEC 2017

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

NOAA buoys across the NT2 offshore waters from Cape Hatteras
southward are confirming the gales both east of the cold front
as well as within the strong cold air advection behind the
front. Buoy 41002 about 225 nm south of Cape Hatteras has been
reporting gale force gusts since early this afternoon, while
Diamond Shoals 41025 has reported some sustained gales. At 00Z
the cold front extended from near Nantucket south southwest
across outer NT2 offshore zones, and will continue to track east
across the waters tonight and should pass east of the NT2 after
midnight. The coldest airmass of the season thus far will then
usher its way offshore tonight and Wed, with widespread offshore
gales continuing. Versus last nights runs, the latest models
have backed off slightly with the strength of the clipper low
forecast to exit the Mid Atlantic coast near Delaware
Bay/Southern New Jersey early Thu. During the medium range, the
models have come into much better agreement with the next upper
level trough which will be moving to the eastern seaboard
Fri/Sat. That is, the GFS has trended weaker and more toward the
consistent ECMWF/UKMET. We will not be making any significant
changes to the previous OPC grids with this evenings updates.
The 18Z Wavewatch III and 12Z ECMWF WAM were equally well
initialized with the west Atlantic wave heights at 00Z. The
previous blended wave height grids still appear reasonable.

...Previous Discussion...

A complex low pressure system over the eastern Great Lakes and
western New England will move NE tonight with an associated
strong cold front sweeping E and SE over the offshore waters. A
1523Z ASCAT overpass indicated a large area of SW winds up to 30
kt or so over NT2 waters and S and SE winds to 25 kts over NT1
waters. There were a couple of 35 kt barbs near the edge of the
pass over central NT2 waters which may have been impacted by a
few thunderstorms in that region this morning.

Over the short term, we will refresh the grids and forecast
using the 12Z GFS tonight into Thu night (using the stability
tool with places the stronger first-sigma layer winds over the
unstable waters and slightly lower 10 meter winds over the
stable waters) as it fits the ongoing forecast and latest
observations rather well, and is in good agreement with non-GFS
models over the next few days. This will lead to gale force
winds developing over all offshore waters tonight into Wed, with
storm force winds developing over far NE NT2 waters later
tonight as the strong front passes E over these waters which are
near the north wall of the gulf stream. Thunderstorms have
become fairly widespread this afternoon and extend from NE NT2
waters or just S of Georges Banks SW over the outer portion of
the central NT2 waters, E of Cape Hatteras. Thunderstorms will
remain likely over this region into tonight near and ahead of
the strong cold front. Caution for strong wind gusts near and
exceeding gale to storm force and very rough seas in and near
the stronger thunderstorms tonight. Conditions will only improve
briefly Wed night ahead of the next low pressure system that
will impact the waters Thu as it tracks E along 40N or so. This
system will allow gales to return to central and northern NT2
waters Thu into Thu night, with a period of storm force winds
over NE NT2 waters once again by 18Z Thu or so. The 12Z GFS
backed-off on these winds slightly so we have manually forced
them back into the forecast as the 12Z GFS may be a little
underdone regarding winds over these waters at that time.
Confidence levels are somewhat above average over the region
tonight through Thu night with decent model agreement through
this period.

For Fri through Sat night, we will use a 50-50 blend of the 12Z
GFS stronger first sigma layer winds and 12Z ECMWF guidance over
the region, as yet another developing low passes NE over the
region. This blend will also tend to maintain gales as was noted
in the previous few OPC forecasts, with confidence levels near
average with gales developing over only NT2 waters Fri night
into Sat night. For Sun and Sun night, we will transition the
offshore forecast toward the 12Z ECMWF guidance as high pressure
moves E over the waters Sun, passing E of the waters Sun night
as this solution appears to fit rather well with the latest
medium range guidance from WPC. The next cold front will
approach the region from the W Sun night. For now, we will keep
winds below gale force for Sun night as this is the very end of
the forecast package, and we would prefer better model agreement
before adding any hazards to the forecast.

.Seas...The latest observations indicate the the 12Z Wavewatch
may be a little too low initially, while the 12Z ECMWF WAM is a
little too high. The previous grids and forecast appear to be
close to the initial conditions over the waters this afternoon.
For the afternoon package we will rely on the previous grids and
forecast for tonight into Thu night, transition to a 50/50 blend
of the Wavewatch and WAM by Fri into Sat night, and then use a
75 percent WAM/25 percent Wavewatch blend thereafter, as we
lowered the winds from the ECMWF guidance late in the period,
and this blend will lower the sea heights forecast somewhat
which will better match the wind forecast.

.EXTRATROPICAL STORM SURGE GUIDANCE...No significant issues with
the latest surge guidance is noted at this time.


.NT1 New England Waters...
.ANZ800...Gulf of Maine...
     Gale Wednesday into Wednesday night.
.ANZ805...Georges Bank west of 68W...
     Gale Wednesday into Wednesday night.
.ANZ900...Georges Bank east of 68W...
     Gale Wednesday into Wednesday night.
.ANZ810...South of New England...
     Gale tonight into Wednesday night.
.ANZ815...South of Long Island...
     Gale tonight into Wednesday night.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale tonight into Wednesday.
     Gale Thursday.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale tonight into Thursday.
     Gale Possible Saturday.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale tonight into Thursday.
     Gale Possible Friday night into Saturday night.
.ANZ905...The Great South Channel to the Hague Line...
     Storm tonight.
     Gale Wednesday into Wednesday night.
     Storm Thursday.
     Gale Possible Thursday night.
     Gale Possible Saturday into Saturday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Storm tonight.
     Gale Wednesday into Wednesday night.
     Storm Thursday.
     Gale Possible Thursday night.
     Gale Possible Saturday into Saturday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles
     Gale tonight into Thursday.
.ANZ828...Inner Waters from Cape Charles Light to Currituck
Beach Light...
     Gale tonight into Thursday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras
     Gale tonight into Thursday.
     Gale Possible Friday night into Saturday.
.ANZ830...Inner Waters from Currituck Beach Light to Cape
     Gale tonight into Thursday.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale tonight.
     Gale Wednesday night into Thursday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale tonight into Thursday.
     Gale Possible Friday night.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale tonight.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale tonight.
     Gale Possible Friday night.


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