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000
AGNT40 KWNM 191442
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1042 AM 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.

Low pressure has moved offshore this morning, with buoy 44066
east of Long Beach, NJ, now reporting a N wind of 15g20 and a
pressure down to 1002.4 mb. Warm front remains draped NE across
the northern portions of NT2. Upper trough is still 3-6 hours
from moving offshore, coincident with strengthening low level jet
ahead of the cold front and atop the warmer waters of the Gulf
Stream. Gales expected to develop soon, likely within the next 3
hours initially south of the cold front that will be pulled
offshore momentarily. Widespread gales then expected overnight
poleward of the front in cold air advection, and with
destabilization and steep lapse rates over the warmer Gulf
Stream, enough mixing should occur to easily support gales. OPC
preference remains with a blended solution between the GFS first
sigma layer winds with boosted ECMWF output; confidence remains
well above average, and should end west to east around 15z Fri.

Overall 06z and early arriving 12z models remain in good
agreement with timing and coverage of the hazard winds today.
However, out of deference to the 06z 3km NAM Nest and to a lesser
extent the 12z HRRR, a small convective cluster could form along
the Chesapeake Bay by 18z, then continue SE across zone 828 in
the evening. Will increase grids to 35 kt to reflect this
possibility. Will also include ANZ825 in the gale warnings in
cold air advection this evening based on regional guidance.

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.

Sun night into Mon night, far southern waters: 00z/06z GFS
remains much faster and stronger compared to all non-NCEP
guidance and 00z EPS and 00z/06z GEFS means. Similar to yesterday
and overnight, OPC preference still remains with the ECMWF.
Gales are a distinct possibility in the onshore easterly flow NE
of the low center, however too much uncertainty remains with
respect to the timing and location of the surface low to
introduce headlines at this time. Will re-evaluate with full
suite of upcoming 12z models, and should they at least remain
consistent, could envision adding headlines into the grids later
this afternoon for far southern zones ANZ835 and ANZ935.


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

The GOES water vapor imagery indicates a shortwave trough inland
over the Ohio valley region, and the latest NCEP surface
analysis shows a complex low center and a strong surface front
to the E of the trough moving E towards the offshore waters.
Current surface reports show up to 25 kt in the offshore waters
in the southerly flow ahead of the front, and the 00Z models all
agree on the timing of the front as it is forecast to move
through the W Atlc today into Fri though trending slightly S
with the surface low. The GFS has been consistent with developing
gales ahead of the front associated with a strong level jet
setting up just ahead of it. The 00Z GFS/ECMWF are both
indicating 55 kt winds at 925 mb for later today into tonight
over the outer NT2 offshore waters in the vicinity of the Gulf
Stream. However, the models have backed off slightly on the
intensity of the surface winds, but confidence remains well above
average on them as a result of the strong low level winds with
the deeper mixed layer over the Gulf Stream. The models also
indicate a second area of gales in the cold advection behind the
front as the NW flow over the Gulf Stream is again expected to
allow for a deeper mixed layer. The GFS has backed off slightly
with this area of gales, but feeling at this time that they may
be a little underdone, especially with the models struggling with
winds in cold advection. However, not completely discarding the
00Z guidance, so planning on starting out with a 50/50 blend of
the previous grids with the 00Z GFS 10m winds, substituting first
sigma winds in unstable areas. This will maintain previous
headlines, while incorporating trends in the 00Z guidance.

The 00Z models then remain in decent overall agreement into Sun,
before differences between the 00Z GFS and ECMWF start
developing late Sun with an area of low pressure that all models
show approaching the SE coast. The GFS had been consistently
faster in previous runs than the ECMWF, but was an outlier
solution when compared with the rest of the global models.
However, the 00Z GFS has trended a little slower, and is now
closer to the ECMWF with the track of the low moving off the SE
coast. Despite that, the GFS remains a little faster than the
consensus of the 00Z guidance, including the available ensemble
guidance and the WPC medium range forecast. The GFS is also
stronger with the winds ahead of the low and associated warm
front, and indicates gales in the srn NT2 offshore waters by
Mon. The 00Z ECMWF/UKMET are both weaker with the winds, but do
indicate a few marginal gales a little later than what is shown
by the GFS. However, confidence with gales over NT2 is low at
this point with the models still having issues on the timing and
intensity. As a result, am planning on holding off for right now
on any headlines on Mon, preferring to see if the models come
into better agreement on this system. For the winds grids, am
planning on transitioning to the 00Z ECMWF starting 12Z Sun, as
it has better support from the rest of the 00Z models.

Seas...Both the 00Z Wavewatch and ECMWF WAM initialized within a
ft of the current data in the W Atlc, and remain in good
agreement into Sun. As a result, will populate wave height grids
with a 50/50 blend the two models into 12Z Sun. Will then
primarily use the 00Z ECMWF WAM thereafter since will be using
the 00Z ECMWF for the winds.

Extratropical Storm Surge Guidance: no significant positive
surge events are expected during the next few days. However, the
00Z ESTOFS is indicating a positive surge greater than 2 ft along
the SE coast Mon into Tue in the strong easterly flow ahead of
the developing low indicated by the 00Z GFS. However, there is a
high amount of uncertainty on the track and intensity of this
low, so confidence in the guidance is somewhat low since the GFS
is used for the forcing in the surge guidance.


.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ815...South of Long Island...
     Gale tonight.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale today into tonight.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale tonight.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale today into 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 today into Friday.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale today.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale today.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale today into tonight.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
     Gale today.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale today.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale today.

$$

.Forecaster Collins/Kells. Ocean Prediction Center.


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