295
AGNT40 KWNM 261205
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
705 AM EST Sun Feb 26 2017

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

A developing low over the Wrn Gulf of St Lawrence continues to
pull a trailing cold front SE across the outermost NT2 waters at
the moment. Per buoy 44013, the associated postfrontal WNW
gradient is near, or at marginal, gale force across the NT1
waters. Max associated seas are likely in the 10-12 ft range,
highest across the far SE NT1 and far NE NT2 waters, which are
being handled slightly better by the higher 00Z ECMWF WAM versus
the 06Z Wavewatch III.

Over the short term, the latest models remain very consistent and
present no major forecast problems. The 06Z NAM/GFS continue to
forecast gale force first sigma level winds at the start of the
TODAY forecast period across the Gulf of Maine so will leave the
previously forecasted gale warning in place there. Then as the
models all forecast high pressure to build offshore into the NT2
waters later today/tonight, will continue to use the previously
populated 00Z GFS winds to note diminishing conditions. Then on
Mon, as a weak cold front is forecast to approach from the NW to
varying degrees the models all forecast a strengthening WSW
gradient to develop across the Gulf of Maine with the 06Z NAM/GFS
remaining consistent in forecasting some gale force first sigma
level winds in this gradient. But at the same time, the 06Z GFS
forecasts the lower levels to become stable which would make the
weaker 06Z GFS 10m winds more likely to verify. So its going to
be close as to whether gales develop (though like the WFO
Caribou CWF with gale force gusts) in the Nrn NT1 waters Mon, but
for now with the 00Z ECMWF also forecasting a weaker gradient,
will continue to hold off on issuing a gale warning and will
continue to use the previously populated winds.

In the long range, the most significant weather feature expected
will be a strong cold front passage on Wed night into Thu. The
biggest difference between the latest global models for this
fropa is that they differ significantly in regards to both their
tracks and strengths of the associated attendant surface low. The
06Z GEFS indicates that the 06Z GFS is too far NW with the
forecast track of its attendant surface low. Therefore since they
are also more in line with the latest WPC medium range guidance,
would favor a solution closer to the similar 00Z ECMWF/UKMET in
the long range. So will continue to the previously populated 00Z
ECMWF solution for late Tue through Thu night with minimal
changes.



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PREVIOUS DISCUSSION...

Over the short term, a strong cold front extends from central
Maine south to east of Cape Cod, and then south and southwest
over NT2 waters to near 31N 77W early this morning. A low
pressure area ahead of the front is nearing the southwest coast
of Nova Scotia moving north and northeast away from the waters.
High pressure is building east from the lower and middle
Mississippi Valley. The front will clear the offshore waters by
12Z or so this morning. The latest satellite imagery and
lightning data shows locally heavy thunderstorms near the front
over offshore zones ANZ915 and 920 at this time, or well off the
mid-Atlantic coast, with the heavier thunderstorms likely moving
further east and northeast into zones 905 and 910, or the far
northeast NT2 waters, shortly. This activity will continue to
move generally east with the front, and likely clear the
offshore waters by mid-morning. Winds may exceed gale force with
these thunderstorms, along with very rough seas. Also, any post-
frontal gales near the gulf stream are forecast to diminish by
around forecast issuance time, so we will not have any gales
over these waters behind the front for the early morning
package. We will still forecast gales for mainly this morning
over the Gulf of Maine, as has been the forecast for the past
few days, as nearly all of the model guidance still suggests
that these gales are likely. The high is forecast to move east
and reach the southeast U.S. coast by this evening, and then
move east over NT2 waters tonight and Monday. A weak cold front
will approach NT1 waters Monday, and move offshore and weaken
Monday night. There is a chance for gales over mainly the Gulf
of Maine Monday as the pressure gradient tightens between a low
pressure area passing north of the waters and the high to the
south. For now, we will keep max winds at 30KT and continue to
evaluate future model guidance, as the chance for gales appears
to be too low to add to the forecast at this time. Conditions
are still forecast to improve Monday night into Tuesday as a
weak high pressure ridge forms over NT1 waters. For the early
morning package we use the 00Z GFS, which is very similar to
the previous few cycles, and has good support from the non-GFS
models into Tuesday. We will also continue to utilize the smart
tool which places the stronger first-sigma layer winds over
unstable areas and lower 10 meter winds over stable areas into
Tuesday.

Over the long term, model differences begin to develop. A warm
front is still forecast to develop over northern NT2 waters, or
around 40N or so, Tuesday night. The front will lift north over
NT1 waters Wednesday into Wednesday night. A strong cold front
will then cross the waters later Wednesday night into Thursday,
followed by high pressure building east into mainly NT2 waters
later Thursday and Thursday night. The 00Z GFS is now faster
with the progession of the cold front Wednesday night and
Thursday verses its previous few cycles. The 00Z ECMWF has
remained very consistent over the past few days regarding the
movement of the front, and now has very good support from the
00Z UKMET, and also the preliminary WPC medium range guidance.
As a result, during Tuesday we will transition the forecast
toward the more consistent 00Z ECMWF solution, and then continue
close to the 00Z ECMWF solution for Tuesday night through
Thursday night over the offshore waters. We will also boost the
ECMWF winds at times mainly over NT2 waters in order to pretty
much maintain the previous headlines over the offshore waters
for Wednesday through Thursday night. Confidence levels are
near average for the long term part of the offshore forecast.

As for seas, the 00Z wavewatch and 00Z ECMWF WAM are very
similar initially and also close to the previous forecast. We
will, therefore, use the previous grids for the early morning
forecast package for today into Tuesday. From later Tuesday
through Thursday night we will trend the forecast closer to
the 00Z WAM to match the wind forecast described above.

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


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Vukits/Mills. Ocean Prediction Center.