Marine Interpretation Message
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000
AGPN40 KWNM 210244
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
744 PM PDT Sun Aug 20 2017

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

Gale warning will be continued over the central waters through
06Z because the current tight pressure gradient will become
relaxed as high pressure to the west will weaken while slowly
retreating southwest and inland low pressure will also weaken.
Synoptic systems that will impact the waters thereafter, will
remain weak through the period keeping a relaxed pressure
gradient across the waters. Satellite images show clouds moving
in anticyclonic circulation over the north waters but are moving
in cyclonic direction over the central and southern waters.
Lightning density map does not show lightning associated with the
clouds over the region and even the SREF model has PROB 0 for
TSTMS threat but inland over California. Latest observations show
winds in gale force range over the eastern portion of the
central waters. Otherwise winds are mainly from the north to
northwest 5 to 20 kt over the north and southern waters. NCEP
weather map at 00Z has high pressure 1028 MB center near 40N140W
with its northeast ridge extending into the north waters while
inland trough extends northwest from low pressure over southern
California. Pressure gradient is tight over the central waters.
low pressure in the gulf of Alaska has a cold front that
stretches southwest passing west of the high pressure.

Seas range between 6 and 12 ft over the central waters with a
peak at 14 ft while they range between 3 and 6 ft over the north
and far south waters. NWW3 and ECMWFWAVE models fit well the
observed seas pattern and have been quite consistent in the
previous runs. Both wave models are in good agreement through
most of the forecast period and so will not deviate from the
previous model choice. Will stay close to NWW3. In the short
term, seas will remain higher over the central waters but will
subside to less than 8 ft in the extended period.

Models GFS/CMC/ECMWFHR/UKMETHR/NAM have initialized fairly well
the 00Z synoptic observations with just minor differences on the
central pressure value within 1 MB of the high pressure.
Differences are similar in the extended period as high pressure
retreats to the southwest. Otherwise, models are in fairly good
agreement on the general synoptic pattern and wind speed. Will
continue to use GFS as our main model guidance. In the short term
the high pressure will slowly move southwest as it weakens. In
the extended period, the high pressure will finally dissipate and
another high pressure over central Pacific will move east and
extend its ridge into the north waters. Inland trough will weaken
in the short term but will start to strengthen in the extended
period.

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

The 18z surface analysis indicated strong high pressure centered
near 40N138W with ridge extending northeast to Vancouver Island,
with low pressure inland over northern California. An ascat pass
from 1802z indicated gales persisting over the far southeast
Oregon offshore waters and the far northeast portion of the
northern California waters. These gales extended east into the
adjacent coastal waters.

The 12z models were all in very good agreement during the entire
forecast period. The high pressure ridge will slowly weaken and
shift southwest later tonight into Tuesday as a weak cold front
approaches the Washington and Oregon waters Tuesday. Meanwhile,
low pressure near the northern California coast will also weaken
through Tuesday night. This will allow winds to decrease below
gale by early Monday over the northern California and far
southern Oregon waters. For the evening forecast issuance, plan
on maintaining gales for PZZ815 and PZZ820 through about 06z with
winds dropping below gale later tonight over the offshore waters
while continuing over the coastal waters through late tonight.
The weak cold front mentioned above will move across the
Washington and Oregon waters Tuesday night through Wednesday
before dissipating over the central California waters Thursday.
The models also indicate the coastal trough slowly strengthening
again Wednesday night through Friday night. This will result in
gales again over the northern California/southern Oregon coastal
waters late Thursday night into Friday night. At this time, it
appears winds will remain just below gale over the offshore
waters. Will populate wind grids using the 12z GFS 10m winds
through the period.

.SEAS...The 18z sea state analysis indicated seas were mostly
within a foot of the latest WaveWatch III and ECMWF WAM, although
based on a recent altimeter pass the seas over the far east
portion of the northern California offshore waters were about 2
to 3 feet too low. Went with a 16 ft max for the analysis. For
the wave grids, will use the WaveWatch III, however will add a
few feet tonight over the northern California waters.

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



.WARNINGS...Preliminary.

.PZ5 Washington/Oregon Waters...
     None.

.PZ6 California Waters...
.PZZ820...Inner Waters from Point St. George to Point Arena...
     Gale tonight.

$$

.Forecaster Musonda/Kosier. Ocean Prediction Center.


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