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597
AGNT40 KWNM 220124
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
824 PM EST Tue Nov 21 2017

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

We will mainly update the ongoing forecast so that our grids fit
current conditions and nearby coastal WFO and TAFB grids and
forecasts for the evening update, with no major changes expected
for the OPC evening update. High pressure is moving E away from
the waters with low pressure still likely to develop and move NE
over the outer NT2 waters Wed as a strong cold front moves
offshore. The 18Z GFS is a little stronger and just a little W
of the 12Z cycle regarding the previously mentioned low. For
now, we will not make any major changes, and evaluate a full set
of 00Z models once they arrive tonight into early Wed morning.

The latest satellite, radar and lightning imagery indicate
scattered to numerous showers and thunderstorms over southern
NT2 waters, or off the Georgia and South Carolina coasts, all
lifting north and northeastward. The latest satellite, lightning
and radar trends show this activity continuing to overspread
southern into central NT2 waters, with the latest SREF
thunderstorm guidance indicating a decent chance for
thunderstorms over mainly NT2 waters tonight through Wed, then
diminishing Wed night as the front associated with low pressure
tracking NE over the region pushes E of the waters. Caution for
local wind gusts exceeding gale force and very rough seas in and
near any of the stronger thunderstorms tonight through Wed.

As far as sea heights are concerned, again we will adjust grids
and the forecast to fit conditions noted just prior to forecast
issuance, and also attempt to fit the previous grids and forecast
to nearby coastal WFO and TAFB grids. Otherwise, no major changes
are anticipated to the previous forecast this evening, with seas
heights running a little higher than the previous forecast over
NT1 waters where sea heights near 11 feet are noted over the
eastern Gulf of Maine and seas to near 9 feet S of Long Island
per the latest observations and 00Z RA1 OPC sea state analysis.

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

This mornings 1415Z and 1500Z Ascat overpasses returned
southwest winds to 25 kt extending offshore from near Cape May
northeast to Martha`s Vineyard as well as across the Gulf of
Maine. The 12Z models were generally well initialized with these
winds, and are in good agreement that these winds will expand
eastward across the NT1 and northern NT2 zones tonight as a cold
front slowly approaches the coast through early Wed. The 12Z GFS
and 12Z ECMWF then remained stronger than the 12Z UKMET/NAM/CMC
with the developing surface low expected to track northeast
across the outer NT2 zones tonight through Wed evening. Versus
its previous run, the 12Z GFS trended slightly faster with this
feature, but also was further northwest with the track of the
low and now is very close to the 12Z ECMWF which has shown
remarkable run to run consistency with the low. The 12Z GFS is
also faster than the 12Z ECMWF in developing the associated gale
and storm force winds. The 12Z UKMET as well as the 12Z GEFS
support the slower timing. Will update the wind grids with the
12Z ECMWF through Wed night and plan to maintain continuity with
the wind hazards, limiting them to the outer NT2 zones as well
as ANZ900 Georges Bank. We have well above average forecast
confidence that warnings will be limited to the outer zones,
well east from any Mid Atlantic coastal zones. The 12Z GFS/ECMWF
then both show a weaker low will move northeast across the NT2
waters south of Cape Fear late Thu into Fri, with the 12Z GFS
slightly faster than the 12Z ECMWF.
Per the ECMWF/UKMET will limit the winds associated with this
feature to 25 kt.

The GFS then lacks any run to run consistency with the next
developing low forecast to move northeast across the outer NT2
waters Fri night through Sat night, and continued to flip flop
between producing gale force winds across these zones. The 12Z
GFS was again stronger, similar to the 00Z run. The ECMWF has
been consistently weaker and is supported by the 12Z UKMET.
Given the difference in the models we are reluctant to go with
any winds higher than 30 kt, and will populate the wind grids
with the 12Z ECMWF through Sat night. The 12Z ECMWF is much more
amplified than the remaining guidance with the sharp upper level
trough moving through New England Sun and passing offshore Sun
night. At the surface, and to varying degrees, the models all
indicate strong northwest winds will develop behind the cold
front, with the stronger ECMWF even showing some gales across
the NT1 and northern NT2 waters. To tone down these winds and
account for some of the forecast uncertainty will use an even
blend of the past two runs of the ECMWF for the wind grids which
will limit winds to 25 or 30 kt Sun/Sun night.

.Seas...Gulf of Maine buoy 44037 and buoy 44025 south of Long
Island both reported 8 ft at 18Z, which is about 1 ft higher
than the 12Z Wavewatch III and 12Z ECMWF WAM. The 12Z ECMWF WAM
then appears slightly too high with the wave heights associated
with the developing storm Wed/Wed night. So will use a 75/25 12Z
ECMWF WAM/12Z WW3 blend to tone down these seas slightly Wed/Wed
night. Thereafter am favoring an even blend of the above
guidance.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a.


.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ900...Georges Bank east of 68W...
     Gale Wednesday night.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Wednesday.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Wednesday.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Wednesday.
     Storm Wednesday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Storm Wednesday.
     Gale Wednesday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Wednesday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale tonight into Wednesday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale tonight.

$$

.Forecaster Mills/Clark. Ocean Prediction Center.



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