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000
AGNT40 KWNM 140302
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1002 PM EST Wed Dec 13 2017

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

Incoming ASCAT imagery showed an area of storm-force winds
extending into the eastern part of ANZ905. Sampling suggested
winds peaking a knot or two just shy of 50kt. Still, updated the
gridded and worded forecasts to show winds to 50kt for a period
there this evening. No changes to hazard headlines as storm
warnings were in effect there.

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PREVIOUS DISCUSSION...

The latest observations correlate well with the current gridded
and worded forecasts. Highest winds were located over the Gulf of
Maine with a ship report of winds of 37kt. Farther south toward
the mid-Atlantic, and closer to the coast, sustained winds were
mainly under gales with gusts above gale force, in the mid-to-
upper 30s knots. The latest IR satellite images showed comma
cloudiness with a mid-level wave in the southern end of the mid-
level trough, from the center of the closed low over or just
east of Maine, moving out of the northern offshore waters. IR
clouds were showing, albeit briefly over most of the offshore
waters, a more stable appearance coming off of the coast, in
advance of the clipper system moving toward the mid-Atlantic.

While there were no surface observations to verify, 925mb winds
from the GFS and ECMWF suggested some small potential for storm-
force winds just east of the offshore zone ANZ905. These strong
winds move east early this evening, and as the first area of
gales remaining over the northern waters moves northeast, renewed
gales will occur over much of the mid-Atlantic waters associated
with the clipper system. The GFS, ECMWF, and UKMET are all in
basic agreement, and there is enough guidance, either near-
surface based or at 925mb, to continue with the mention of
storm-force winds in ANZ905 and ANZ910 on Thursday. These
conditions should end Thursday night as 925mb winds fall into the
teens to 20s knots, before yet another surface low moves across
the mid-Atlantic waters late Friday and Friday night with some
gales along and ahead of the low well east of the Carolina coast
Friday night, and then farther north especially off of the Jersey
coast in the cold-air advection Saturday and Saturday night. The
ECMWF, with the surface low deepening just a little faster than
the GFS and closer to the far eastern mid-Atlantic zones near the
Gulf Stream, would indicate conditions close to storm-force there
late Friday night and early Saturday. This will be something to
monitor for future forecast issuances.

In addition, there are concerns about gales building in from the
southwest beginning across the southern waters Monday. The GFS
is weaker with any low pressure and more slack with the surface
gradient over the offshore waters Monday, while the ECMWF is much
stronger with developing low pressure off the New England coast
during the day Monday. The UKMET is in between, not nearly as
deep as the ECMWF would suggest, but certainly deeper than the
GFS and providing for a tightening surface gradient beginning
over the southern waters on Monday. Will review later model
trends for the potential for any gales then.

The latest, available observations noted seas were also in line
with the current gridded and worded forecasts. The lone exception
was well off the South Carolina coast toward the eastern edge of
the offshore waters. There, indications are the seas were a
couple of feet higher than forecast, with forecast adjustments
made.

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

The morning 1500Z ASCAT overpass indicated widespread gales
occurring over most of the offshore waters, except for the far
southern NT2 waters. These winds align well with the first sigma
layer winds from the 12Z GFS. Overall, the 12Z models are now in
very good agreement over the region tonight into early Sun. For
this time frame we will rely on the 12Z GFS winds and use the
smart tool which places the higher first sigma layer winds over
the unstable areas and slightly lower 10 meter winds over the
stable areas right into early Sun. We will make a few minor
edits to the grids and forecast in deference to nearby coastal
WFO and TAFB grids, and also to maintain most of the previous
hazards for the afternoon package.

Gales associated with the strong low pressure system moving N
from New Brunswick, Canada will diminish over the waters early
tonight. A fast-moving clipper-type low pressure system
currently over the midwest will pass E over southern NT1 waters,
or near 40N, Thursday morning. Gale to storm force winds are
likely with this low pressure system as it crosses the waters
later tonight into Thu. The next low pressure system will then
impact the waters Fri into Sat with another period of gale force
winds likely especially from central NT2 waters N into southern
NT1 waters. Any storm force winds with this system will likely
occur well E of the waters Fri night. High pressure will then
slowly build E toward the region Sat night with gales ending at
that time. The high will then pass E over the waters Sun, moving
off to the E of the area Sun night. For Sun into early Mon we
will rely on the previous set of grids which used a blend of the
12Z ECMWF and 00Z ECMWF from yesterday. For most of Mon through
the end of the period we will trend the forecast toward the WPC
medium range guidance, and adjust the forecast over the next few
days, as the 12Z guidance from today is in rather poor agreement
over the region early next week. We will boost these winds by
10-12 percent or so, as they typically run a little on the low
side especially near and S of the gulf stream.

Confidence in the forecast is above average tonight into the
weekend, and then below average by Sun night through Mon night
with the 12Z models showing a wide variety of solutions over the
region at that time, and beyond.

.Seas...The latest observations over the offshore waters indicate
that the previous grids and forecast have initialized the best
over the waters, with the 12Z ECMWF WAM a little too high and the
12Z Wavewatch running a little too low over the waters. For the
afternoon package, we will rely on the previous grids and
forecast for tonight, transition to a 50-50 blend of these two
wave models Thu into early Sun, fall back to the previous grids
and forecast later Sun and Sun night to match the winds, and then
populate with the 12Z Wavewatch, but make several manual
adjustments to fit the WPC wind grids as noted above by Mon and
Mon night over the region.

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

.WARNINGS...Preliminary.

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

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

$$

.Forecaster Figurskey/Figurskey/Mills. Ocean Prediction Center.



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