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AGNT40 KWNM 200157
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
957 PM EDT Thu Apr 19 2018

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

The latest NCEP surface analysis indicates strong low pressure
just E of the NT1 offshore waters, with a cold front extending
back to the SW across the NT2 waters. The lightning density
product indicates showers and tstms along the front, mainly in
the vicinity of the Gulf Stream, and is good indication of the
deep mixing in the unstable environment over the Gulf Stream. The
Ascat wind retrievals from 15Z this morning indicated a few gales
ahead of the front as it was moving offshore, and the 18Z GFS
indicates gales continuing ahead of the front to the E of the
offshore waters. The GFS also indicates a second area of gales in
the cold advection behind the front, mainly over the Gulf Stream.
The rest of the 12Z models are also showing this enhanced area of
higher winds, so confidence with gales over the nrn NT2 offshore
waters is slightly above average. Will continue previous
headlines as a result, and allow warnings to expire in the
offshore waters by early Fri morning.

The 12Z/18Z GFS and 12Z ECMWF/UKMET remain in decent agreement
throughout the remainder of the forecast period, and the models
all indicate a developing low will move off the SE coast late Mon
night into Tue. The GFS remains slightly faster than the rest of
the models with this low, and also with the gale winds ahead of
the associated warm front over the NT2 waters. The GFS is also
stronger than the rest of the models, so previous preference was
with the 12Z ECMWF as a result of the better overall agreement.
The preference has not changed from the previous forecast, as the
GFS continues to be an outlier solution. As afar as the intensity
is concerned, the 12Z models are all showing at least marginal
gale force winds along of the warm front, so confidence is about
average on the prospect of gales developing with this system. Am
planning on continuing previous gale headlines for Mon and Tue,
while keeping winds below the GFS. Otherwise, not planning on any
major changes for the remainder of the update package.

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

Low pressure continues moving quickly NE this afternoon,
analyzed to 999 mb and centered at 40.2N 68.6W at 18z. The low
continues to pull a strong cold front into the offshores,
evidenced in Goes-16 visible imagery this afternoon, draped SW
from the low center to near Cape Hatteras. Gales are likely
occurring across several offshore zones at the moment,
coincident with strengthening low level jet ahead of the front
atop the warmer waters of the Gulf Stream. Widespread gales are
still expected this evening and overnight poleward of the front
in cold air advection, and with destabilization and steep lapse
rates over the warmer Gulf Stream, enough mixing will occur to
easily support gales. No major changes in the near term portion
of the forecast; confidence remains well above average, and gales
should end west to east by 15z Fri.

Simulated reflectivity fields from NAM/HRRR suggest a squall
line will likely develop over the outer NT2 waters by 21z this
afternoon along the cold front, stretching from the eastern
portions of ANZ905 SW across the offshores and the Gulf Stream.
Wind gusts to storm force are possible within areas of strong
convection and thunderstorms.

High pressure builds over the W Atlantic Fri night through Sun,
producing a period of relatively light winds and flat seas over
the offshores. The ridge slowly moves E Mon into Tue night,
giving way to low pressure moving along the SE coast mentioned
below.

Sun night through Tue night, southern and central waters: 12z
model suite has come into much better agreement with respect to
the timing and location of a surface low that moves E across the
Deep South Sun and Sun night, moves NE into the southern zones
Mon and Mon night, then lifts NE along the coast Tue and Tue
night. Most notably, 12z non-NCEP guidance has somewhat
surprisingly trended faster versus their previous runs and
ensemble means. The GFS still remains on the faster side of the
consensus, however, so plan on using the ECMWF as the basis for
the wind grids in the upcoming package. This continues to be well
supported by the UKMET/GEM/GEFS mean. With each global model
continuing to signal gales in the east and southeasterly flow
ahead of the low - and they have been for several days - will
initiate gale headlines across the OPC forecast waters in the
upcoming package. Gales forecast to develop Mon afternoon across
ANZ835, spread east to ANZ935 overnight Mon, then lift NE across
Baltimore Canyon Tue and Tue night. Confidence is increasing in
this event, especially now that the upper trough is inland across
southern CA and better sampled.

Seas: both the WW3 and WAM were performing well compared to
ongoing obs at the 18z sea state analysis. Plan on using an even
blend of the guidance into late Sat, before transitioning to all
WAM Sun and beyond where OPC preference is with the ECMWF.
Forecast seas build to nearing 20 feet across the SW waters in
response to prolonged strong E-SE winds and gales.

Extratropical Storm Surge Guidance: no significant positive
surge events are expected during the next few days. While too
early to pinpoint specifics, with strong onshore flow Sun night
into early next week north of the approaching low pressure,
positive surge will affect portions of the SE coast Sun night
through Tue night. Forecast specifics will become clearer by Fri
or Sat as the event nears.

.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 into Friday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale tonight into Friday.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale Possible Tuesday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     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 into Tuesday night.
.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 Monday into Tuesday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Possible Monday night into Tuesday.

$$

.Forecaster Kells/Collins. Ocean Prediction Center.



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