Marine Interpretation Message
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AGPN40 KWNM 180252

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
752 PM PDT Sat Mar 17 2018

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

High pressure just west of the region will move across the
southern waters and maintain a ridge cross most of the forecast
waters in the short term. Low pressure will approach the waters
from the southwest and move across the central waters in the
extended period. Satellite images show clouds with cold tops over
the northern waters and cyclonic circulation just north of the
region near Vancouver Island. Few warmer clouds are depicted over
most of the southern waters. Scatterometer pass at 18Z missed
the northern waters and is described in the previous discussion
section below. The 00Z NCEP weather map has high pressure 1022 MB
just west of the southern waters near 36N130W and extends a
ridge into most of the region and maintains a very weak pressure
gradient across the forecast waters. West of the high is low
pressure 1010 MB near 45N150W that will strengthen as it moves
southeast and will then combine with another developing low
before moving northeast and force a tight pressure gradient over
the region in the extended period.

Seas are relatively large over the central waters with a peak at
9 ft while they are less than 6 ft over the western portion of
PZ5 waters. NWW3 and ECMWFWAVE models fit well the observed seas
pattern and they have been quite consistent in the previous runs.
the only small differences are in the high seas area, near the
low pressure mentioned above, where they are underdone by up to 3
ft. Otherwise, both wave models agree well in the short term on
subsiding seas across the region to below 6 ft by Monday and
then will start to build over the western portion of the central
waters in response to the approaching low pressure. Will stay
with NWW3 for seas.

Global models, GFS/CMC/ECMWFHR/UKMETHR, have initialized well
the 00Z surface observations with just very minor differences in
the wind direction around the high pressure were winds are very
weak. Otherwise, in the short term models generally agree on
pushing high pressure into the southern waters and eventually
inland. In the extended period models have a general agreement on
low pressure moving toward the waters but they differ on the
actual track the low will take. ECMWFHR has the low move farther
south into the southern waters and weakens it while GFS has the
low strengthen as it moves into the central waters. CMC is in
linewith GFS solution and so is UKMETHR. I will favor the more
aggressive model solution and so will go with GFS.


Weak low pressure remains positioned over Vancouver Island this
afternoon with a surface trough extending southward along the
Pacific Northwest coast. A surface ridge extends northward from
1023 mb high pressure analyzed west of the waters. Elsewhere, a
weakening cold front extends across the far southern California
waters. A recent ASCAT pass across the central and southern PZ6
waters supports 10-15 kt north to northwest flow over the area.

Conditions will remain relatively quiet over our offshore waters
through the weekend into early next week before a complex low
and its associated frontal system bring deteriorating conditions
by midweek. The 12Z models agree that a surface ridge will cross
the PZ6 waters tonight through Sun night, bringing light and
variable winds as the ridge axis passes over the region.
Meanwhile, deepening low pressure will become occluded well west
of the offshore zones and push a frontal system towards the PZ6
waters Mon through Tue. The 12Z GFS is the only model that depicts
marginal gales over the outer PZ6 zones ahead of this front Mon
night, while the 12Z ECMWF/UKMET/NAM all keep winds at 20-30 kt.
Therefore, will transition our official grids to the 12Z ECMWF
beginning 00Z Tue. Even applying a boost to the ECMWF grids still
keeps winds below gale criteria once the front reaches our

Unfortunately, the 12Z guidance offered little clarity in the
medium range. As the complex low west of the area approaches the
waters by Wed, the 12Z GFS tracks the low across the region about
12 hours faster than the rest of the global guidance. Meanwhile,
the 12Z ECMWF brings the low much further south than its previous
run, while the 12Z UKMET is most similar to the solution shown
by the 00Z ECMWF. Clearly, forecast confidence for the middle of
next week is quite low at this time. Have opted to use the 00Z
ECMWF wind grids from Wed night through Thu night until some
model consensus is reached. These grids are supported by the
12Z UKMET, but produce stronger winds behind the cold front that
crosses the region Wed through Thu night as low pressure drops
southeast from the Gulf of Alaska and nears the northern PZ5
waters Thu night.

While gales appear virtually certain west of the offshore zones
as the low deepens early next week, the 12Z guidance overall was
not as bullish with the wind speeds over our forecast waters by
midweek. Therefore, will cap the 00Z ECMWF winds at 30 kt on day
5 and not carry any warning headlines with this forecast package.

.SEAS...The 12Z Wavewatch/ECMWF WAM guidance initialized well
based on the latest sea state analysis. In fact, differences
between the wave models were fairly minor until the wind
solutions diverged late in the period. Favored a blend of the
12Z Wavewatch/WAM seas for the official wave grids through Wed.
Then, transitioned to the 00Z WAM Wed night through Thu night to
match up the highest seas with our wind grids.



.PZ5 Washington/Oregon Waters...
.PZZ915...Outer Waters from Florence OR to Point St. George...
     Gale Possible Wednesday night.

.PZ6 California Waters...
.PZZ920...Outer Waters from Point St. George to Point Arena...
     Gale Possible Wednesday night.


.Forecaster Musonda/Reinhart. Ocean Prediction Center. is the U.S. government's official web portal to all federal, state and local government web resources and services.