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000
AGPN40 KWNM 190218
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
718 PM PDT Fri Aug 18 2017

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

Update...The 00z OPC preliminary surface analysis shows a cold
front extending NE to SW across the Washington coastal and Oregon
offshore waters. Otherwise, the analysis shows a low pres trough
over California interacting with a high pres ridge over the
offshore waters to produce a tight pressure gradient in the
southern Oregon and northern California waters. Latest available
ascat and ascat hi-res passes from this afternoon show 25 to 30
kt winds in the southern Oregon and northern California coastal
waters in northerly flow.

Models...The medium range models are in good overall agreement
across the offshore waters through the forecast period, except
the ecmwf is faster than the other models with a cold front
moving SE over the PZ5 waters on Tue into Wed. Will use the
representative gfs 30m solution for the wind grids, since it
supports gales in the southern Oregon and northern California
coastal waters where the coastal WFO`s have gale warnings in
effect.

Seas...The highest seas reported were in the 12 to 13 ft range
W of Cape Mendocino. Will use the enp wavewatch for the sea
height grids through the forecast period, because it initialized
well over the waters and has good support from the ecmwf wam.
Will continue to use a slight boost to the enp wavewatch through
Sun in order to match up better with the conditions expected by
WFO Eureka.

Extratropical Storm Surge Guidance...N/A.

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

A cold front continues to drop southeast across the PZ5 waters
this afternoon per the 18Z OPC surface analysis. Meanwhile, nearly
stationary high pressure remains anchored west of the waters near
42N139W. A 1749Z ASCAT-A pass highlights a region of 25 to 30 kt
northerly winds over our inner waters extending from roughly Cape
Blanco to Point Arena.

Winds over the northern California offshore waters will strengthen
tomorrow as the area remains entrenched between strong high
pressure and a thermal low/trough over California. The resulting
gradient will generate gale force winds over the coastal waters,
extending into PZZ820 tomorrow and persisting through Sunday
night. No surprises with the 12Z guidance as the GFS/ECMWF/UKMET
have consistently depicted this event for several days now. Will
use the latest GFS for the wind grids, which results in no changes
to the gale warning in effect.

The surface ridge weakens early next week as a cold front approaches
from the west on Tuesday. The 12Z ECMWF actually moves the front
faster through our waters, with the slower solution now favored
by the 12Z GFS/UKMET. Therefore, will stick with the GFS since it
has better ensemble mean support. As the front passes through the
northern PZ6 waters on Wednesday, northerly winds will stengthen as
high pressure once again builds in from the west.

.SEAS...The highest seas were analyzed to 13 ft west of Cape
Mendocino on the 18Z sea state product. Overall, the 12Z NWW3 ENP
was favored for the wave grids, although the ECMWF WAM was very
similar for most time steps. Continued to apply a slight boost to
the wave grids through tonight to better match the latest obs
and the adjacent coastal WFOs. Still expect seas to peak around
16 ft over the northern PZ6 waters within the gales on Sunday.

.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 Saturday into Sunday.
     Gale Possible Sunday night.

$$

.Forecaster Scovil/Reinhart. Ocean Prediction Center.


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