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000
AGPN40 KWNM 291600
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
900 AM PDT Sat Apr 29 2017

.FORECAST DISCUSSION...Major features/winds/seas/significant
.weather for the North Pacific N of 30N and E of 150W.

Gale warnings in the far north zones today will diminish over
the forecast waters by tonight and winds over the region will
then remain below gale force threshold through the rest of the
forecast period as high pressure will dominate. The satellite
images show anticyclonic air flow over most of the region except
the far north waters where clouds with cold tops are moving east.
The latest observations show higher winds over the north waters
in the gale force range from the south and southwest. Winds are
mainly 15 to 25 kt and they are form the north and northwest over
most of the region. The NCEP map at 1200Z has low pressure 997
MB about 360 NM northwest of the region with its warm front
stretching southeast into the western edge of the washington
waters. A trough lies along the coast of California into the
southern parts of Oregon coast. High pressure 1029 MB is
centered west of the southern waters near 36N134W. The pressure
gradient is tight over the northwest portion of washington
waters. Elsewhere, low pressure 1011Mb in the gulf of Alaska
which is part of the low pressure described above. Farther west
is an area that is dominated by low pressure with several
centers. One low pressure that demands close monitoring is 1008
MB east of Japan near 31N154E as this low will end up in the gulf
of AK and force winds to be elevated to storm force threshold
and will also bring large period west swells to the forecast
waters.

The seas have peaks to 14 ft over the southern waters while seas
range between 6 and 12 ft over the southern waters mainly due to
large period swell but range only between 6 and 9 ft over the
northern half of the waters. The wave models NWW3 and ECMFWAVE
fit very well the observed seas pattern and they have been quite
consistent in the previous runs. Will continue with NWW3 for the
seas.

In the upper levels, the models GFS/CMC/ECMWFHR/UKMETHR suggest
energy embedded in an upper level trough stretching southeast
from the Gulf of AK that is moving over the for north waters. An
upper level cut off high pressure has a ridge with little to no
energy just west of the region. Meanwhile, there is a cut off
upper level low with significant energy centered just north of
Hawaii. Another area of significant energy is a cut off low just
west of the Aleutian Islands which extends a trough that has
energy that is maintaining low pressure east of Japan mentioned
above. In the short term, the models agree on pushing the energy
over the north waters to the northeast while building and
maintaining high pressure to the west of the region. The energy
over north Hawaii will drift south while most of the energy
associated with low to the west of the Aleutian Islands will
consolidate and move toward the Gulf of Alaska.

The models GFS/CMC/ECMWFHR/UKMETHR/NOGAPS/NAM have initialized
well the 12Z surface observations.with just minor differences
within 1 MB on the central pressure values of the low pressure to
the northwest and high pressure west of the region. As for the
wind speed, ECMWFHR is quite weak by 10 kt over GFS and so has no
gale force winds over the north region. UKMETHR is equally
weaker than GFS with winds to only 30 KT. CMC and GFS are evenly
matched on winds over the north waters. So, will stay with GFS
for this forecast. In the short term, low pressure will move
north of the region and will pull a warm front northeast across
the north waters while high pressure will continue to build west
of the region. The pressure gradient will remain relaxed through
most of the forecast period.


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

The 00Z global models are in pretty good agreement over the
region into the middle of the upcoming week, with the models
diverging by later Wednesday and beyond. For today into
Wednesday we will stay close to the 00Z GFS, and then transition
toward the 12Z ECMWF from yesterday, or previous grids, from
later Wednesday through the end of the forecast.

Over the short term, we will use a blend of the stronger first-
sigma layer 00Z GFS and somewhat lower 10 meter 00Z GFS winds
for the forecast for today over PZ5 waters, and the GFS 10 meter
winds elsewhere. This blend of winds appears to match up well
with the latest ASCAT overpass which shows gale force winds
occurring roughly 100 NM west-northwest of the northwest
Washington offshore waters at 0626Z. The low pressure system
producing these stronger winds was centered near 50N 140W per
the preliminary 06Z NCEP-OPC surface map, with high pressure
extending from eastern PZ5 waters southwestward to west of the
central PZ6 waters. A coastal trough was located from southwest
Oregon to southern California. A cold front associated with the
low pressure system will move east and southeast, and cross PZ5
waters today into tonight as it weakens. Before it weakens, we
are now forecasting a period of marginal gales over northwest
Washington waters, or zone PZZ900, today, with more widespread
gales to the northwest and then north of the offshore waters
today into tonight, with storm force winds likely west and
northwest of Vancouver Island by this afternoon. Improving
conditions are likely tonight as the front weakens as it moves
southeast toward northern California waters. High pressure will
then dominate most of the region Sunday into Monday, with a weak
low pressure area developing west of the PZ5 waters Sunday
night, dissipating just west of these waters later Monday. A low
pressure trough will persist over coastal California for the
next few days producing moderately strong north and northwest
winds over many portions of the California offshore waters.
Confidence levels are near to above average over the region
through Monday night.

Over the long term, the 00Z global models are in decent agreement
Tuesday into early Wednesday. We will rely on the 00Z GFS 10
meter winds as a result for this time period. This will result
in little change from the previous few OPC forecasts, with the
next cold front approaching PZ5 waters from the west, high
pressure dominating areas from coastal Washington and Oregon
south and southwestward over northwest California waters, and a
low pressure trough persisting along the California coast. No
headlines appear likely over the offshore waters Tuesday into
Wednesday, with a chance for gales over California coastal
waters, and also north and northwest of PZ5 waters. For later
Wednesday and Wednesday night we will transition the forecast
toward the previous grids which used the 12Z ECMWF. This appears
to be the best option for now, as models diverge late in the
forecast, especially regarding the development of low pressure
along the cold front as it moves into PZ5 waters Wednesday
night. The 00Z GFS has a weak low moving northeast along the
front late Wednesday, with the 00Z ECMWF now showing a stronger
low moving northeast along the front and passing just west of
the PZ5 waters Wednesday night. The 00Z UKMET also shows this
low, but it is weaker, or more similar to the 12Z ECMWF. The 00Z
Canadian GEM is also closer to the 00Z UKMET or 12Z ECMWF
guidance. We will have no headlines in the offshore forecast for
later Wednesday and Wednesday night, as confidence levels are
below average at this time.

.SEAS...The 00Z ENP Wavewatch III model guidance appears to be
running within a foot or two of the observations and altimeter
data over the offshore waters early this morning. The 00Z ECMWF
WAM guidance is also similar. We will rely on the 00Z ENP
Wavewatch III guidance today into Wednesday, with a few manual
edits mainly near the adjacent coastal waters and TAFB waters.
For later Wednesday and Wednesday night we will transition the
forecast toward the previous grids or something closer to the
12Z WAM guidance from yesterday to match the wind forecast as
described above.

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



.WARNINGS...Preliminary.

.PZ5 Washington/Oregon Waters...
.PZZ800...Inner Waters from Cape Flattery to Cape Shoalwater...
     Gale today.
.PZZ900...Outer Waters from Cape Flattery to Cape Shoalwater...
     Gale today.

.PZ6 California Waters...
     None.

$$

.Forecaster Musonda/Musonda/Mills. Ocean Prediction Center.


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