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000
AGNT40 KWNM 271202
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
702 AM EST Mon Feb 27 2017

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

As previously expected, as high pressure builds offshore into
the NT2 waters, generally weak gradients with light winds persist
throughout much of the coastal/offshore waters, except over the
Nrn Gulf of Maine where a WSW gradient has begun to strengthen
(generally up to 20-25 kt) as the next cold front approaches from
the NW. Max seas throughout are in the 8-10 ft range, highest
across the far SE NT1 and NE most NT2 waters, which are being
handled fine by the 06Z Wavewatch III and 00Z ECMWF WAM at the
moment.

Over the short term, as the models all forecast the cold front to
continue to approach from the NW today, then move offshore and
weaken tonight, the 06Z NAM (and the 00Z GEM) continues to
forecast the WSW gradient to strengthen today with some gale
force boundary layer (BL) winds developing across the Gulf of
Maine. For now would favor the similar 06Z/00Z GFS solutions for
this gradient as they look like a reasonable compromise between
the stronger NAM/GEM and weaker 00Z UKMET/ECMWF solutions. So for
now will stay the course with the previously populated 00Z GFS
winds and will not forecast any associated gale warnings (but
agree with WPO Caribou that some gale force gusts in their
coastal waters certainly look possible).

Then with some generally small timing differences, the latest
models forecast a warm front to develop off the Nrn mid Atlantic
coast and then lift NNE across the NT1 waters Tue night with a
strengthening SSW gradient developing throughout much of the
waters. So with the 06Z GFS again remaining consistent in
regards to the forecast strength of this gradient, will again
continue to use the previously populated 00Z GFS winds Tue/Tue
night, with just some minor additional edits in deference to the
00Z ECMWF.

In the long range, the 06Z GFS continues the trends of its
previous respective runs in being slightly faster than the 00Z
ECMWF in regards to a strong cold front sweeping offshore Wed
night into Thu. Though the 06Z GFS differs somewhat in regards
to the forecast track of its associated attendant surface low, it
forecasts similarly strong associated gradients with this fropa
as the previously populated 00Z GFS solution. So again will just
make some minor additional edits to these 00Z GFS winds in
deference to the 00Z ECMWF.

Then further out in the long range, the 06Z GFS continues its
previous model trends of forecasting a vigorous clipper-type
surface low to race across the Srn NT1 waters Fri with widespread
associated gale force winds. But with the 06Z GEFS Mean
indicating that the 06Z GFS is likely both too fast and too
strong with this system, for now will continue to favor the 00Z
ECMWF solution on Thu night into Fri night, though will beef up
the 00Z ECMWF winds somewhat Fri night in the cold air advection
it forecasts to develop in the wake of its weaker surface low.


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

The current GOES infrared satellite imagery indicates moderately
strong cold air advection across the nrn offshore waters in the
wake of a cold front well east of the area. Ascat wind retrievals
from late last night indicated winds up to 30 kt in the ern Gulf
of Maine. However, current surface observations only indicate up
to 20 kt in the NT1 zones with a few higher wind reports to the
east. In addition, the observations and Ascat indicate variable
winds 5 to 10 kt in NT2 where high pres is building over the
area. The 00Z GFS winds are well initialized when compared with
the data, and the model indicates that the winds will remain
light over NT2 today as high pres passes east across the waters.
However, the GFS indicates that another cold front will pass
through the nrn waters tonight, and increase the winds over NT1
in the SW flow ahead of the front. The GFS indicates 30 kt in the
first sigma winds over the Gulf of Maine with a marginally
unstable environment and a strong low level jet with winds to 40
kt at 925 mb. The 00Z NAM/GEM agree with the GFS on the
intensity, though the NAM is slightly stronger with a few gales.
However, the 00Z ECMWF/UKMET both only show 25 kt, so the NAM
sees a bit overdone. The previous forecast went up to 30 kt, and
this still seems reasonable given the good model agreement on the
timing. Also, taking into account the low wind biases of the
ECMWF and UKMET, the GFS seems to be the best compromise. As a
result, planning on starting out with the 00Z GFS in the short
range, but will use the first sigma winds over areas with
negative static stability to account for the stronger mixed layer
winds.

Otherwise, the 00Z models all indicate an active pattern through
the remainder of the forecast period. The 00Z GFS indicates the
next system will start impacting the offshore waters on Tue
night as it approaches from the west while extending a warm front
offshore. The model then indicates the cold front will approach
from the west Wed and pass through the area Wed night. The
models have all been indicating gales in the SW prefrontal flow,
and this seems reasonable with a strong low level jet setting up
along the front in the unstable environment over the Gulf of
Stream. However, there is a slight difference on the timing as
the GFS is slightly faster than the rest of the guidance. The GFS
has been consistent with this faster solution, but the
ECMWF/UKMET/GEM have been consistently slower. Preferring the
slower solutions as support is greater, so will time shift the
GFS slightly slower starting Wed night as a compromise. Confidence
is above average with the prefrontal gales as most solutions are
showing a strong pres gradient and strong winds. However,
confidence is slightly lower with the postfrontal gales in the
cold advection as the ECMWF/UKMET are all weaker than the GFS.
However, the ECMWF/GEM both indicate a few small areas with 35
kt winds, so planning on maintaining previous gale headlines with
this system.

On Fri, the 00Z models all generally indicate that another low
pres system will pass through the offshore waters. The GFS has
been the strongest solution by far, and is also a bit north with
the track from the rest of the 00Z models. The 00Z GEFS mean and
individual members are mostly supportive of the GFS solution, but
the 00Z ECMWF/UKMET/GEM are all weaker with the low and
associated winds. With all of the uncertainty raised by the
model differences, confidence in the strong gales forecast by the
GFS is low. However, with the ECMWF/UKMET/GEM showing marginal
gales, will continue previous warning headlines on Fri with this
system. Will also switch to the 00Z ECMWF for the grids which
better reflect the preferred track and intensity for this
system.


.Seas...The 00Z Wavewatch and ECMWF WAM have both initialized
the current seas well. The models are in good agreement into Thu
night, so will populate with a 50/50 blend of the two models
with the Wavewatch time shifted 3 hours slower Wed through Thu
night to account for the fast timing of the associated GFS winds.
Will then switch to the 00Z ECMWF exclusively from Fri onward as
the models disagree starting from that point and am preferring
the ECMWF for the wind grids.

.EXTRATROPICAL STORM SURGE GUIDANCE...N/A.


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Vukits/Kells. Ocean Prediction Center.


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