Marine Interpretation Message
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AGNT40 KWNM 230142
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
942 PM EDT Sun Apr 22 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 shows a high pres ridge across
the offshore waters, and a developing low pres system currently
inland over the southern CONUS. The GOES-16 infrared satellite
imagery shows the system moving E towards the NT2 offshore
waters, and the lightning density product along with the
infrared imagery indicate showers and tstms ahead of the
associated cold front. The convective activity would indicate
that the surface front is fairly strong, and there is ample
moisture and instability ahead of the front as it moves toward
the coast. The 18Z GFS indicates that the front will approach the
area tonight and Mon, before it moves offshore Mon night. The
12Z ECMWF and 12Z/18Z GFS are showing some differences with the
handling of the low center or centers, but those differences
mainly impact inland areas. However, the greatest hazard imposed
by this system to mariners is the interaction of the associated
front with the ridge over the area. The GFS indicates gales will
develop between the front and the ridge, and the rest of the 12Z
models are all in decent agreement on the increase in the
pressure gradient and subsequent winds. However, the GFS appears
to be a bit stronger than the rest of the 12Z/18Z guidance, and
indicates a bullseye of higher winds up to 50 kt to the SE of
Cape Hatteras over the Gulf Stream by 18Z Tue. The 12Z
NAM/ECMWF/UKMET/GEM are all weaker, with all showing marginal
gales. Thinking at this time that the GFS is having some feedback
issues, especially with the latest SREF guidance showing a high
probability that the current convective activity will follow
along with the front as it moves offshore. The previous forecast
used the 12Z GFS which is in decent agreement with the rest of
the guidance on the timing. However, winds were capped at 45 kt,
which seems to be a decent compromise. Confidence is high with at
least marginal gales developing ahead of the front as a result
of the good agreement. As a result, am planning on continuing
along the lines of the previous forecast in the short range with
only minor edits.

The models continue slowly diverging on the timing and track of
the low center over the next several days as it turns N and stays
inland. However, the biggest impact on the offshore waters will
continue to be the front as it slowly moves E across the area.
The models all indicate that the front will pass E of the
offshore waters by Thu, with the GFS being slightly faster than
the ECMWF/UKMET solutions. The previous forecast trended more
towards the 12Z ECMWF in the medium range, and this still seems
to be the best course of action with the bulk of the guidance
favoring the ECMWF. The strong southerly flow ahead of the front
is expected to generate the highest winds with this system,
mainly from the strong low level jet expected to set up ahead of
it which will increase vertical shear and induce mixing. The
previous forecast kept the gales up into Thu, and at this time
confidence remains about average on them so will leave previous
headlines as-is. Otherwise, not planning on making any major
changes from the previous package.

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

Current Conditions...The 12z ncep surface analysis shows a low
pressure trough approaching the Gulf of Maine from the N, with a
high pressure ridge over the offshore waters.

Models/Forecast...A low pressure trough will pass SE over the
nt1 area this evening and tonight. The ridge will build SE over
the offshore waters tonight and Mon, then shift E over the nt1
and northern nt2 waters Mon night through Tue night. Low pressure
will move E across the Gulf Coast states through Mon, track NE
along the SE Coast Mon night through Tue night, pass N over the
mid Atlantic waters Wed, move NE and inland over New England Wed
night and Thu, then move off to the NE Thu night. Interaction
between the low and ridge will result in gale force E to SE flow
over the offshore waters for the Mon night through Wed timeframe.

12z medium range models are in good agreement over the offshore
waters for tonight through Tue night, so the representative 12z
gfs 10m/30m solution with the stability smart tool will be used
for the wind grids through Tue night. The 12z gfs is faster and
less inland than the other models with the low for Wed through
Thu night, then the 12z gfs has a strong coastal low for Fri and
Fri night while the other models are significantly weaker and
slowers. So the gfs looks like an outlyer solution for Wed
through Fri night, and will use a boosted version of the 12z
ecmwf for this timeframe since it is much more representative.
Confidence in the gale warnings through Tue night is above
average, due to good model agreement. Confidence in the gale
warnings for Wed onwards is average at best, because of some
divergence among the models.

Seas...The 12z wna wavewatch and 12z ecmwf wam both initialized
well over the offshore waters. The ecmwf wam is trending
somewhat higher with the seas for tonight through Thu, followed
by the wna wavewatch trending higher for Fri and Fri night.
Will be favoring a representative blend of the 12z wna
wavewatch/ecmwf wam through the entire forecast period, in order
to smooth out the model differences.

Extratropical Storm Surge Guidance...Latest run of estofs and
etss shows a positive surge in the 1 to 2 ft range along the SE
coast for tonight into Tue, expanding along the mid Atlantic
coast for late Tue through Wed, then expanding up into the New
England waters for late Wed and Wed night. The surge guidance
looks reasonable given that all of the medium range models have
been showing strengthening E to SE flow over the offshore waters
for tonight into Wed night.

.WARNINGS...Preliminary.

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

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

$$

.Forecaster Kells/Scovil. Ocean Prediction Center.



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