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000
AGPN40 KWNM 170345
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
845 PM PDT Wed Aug 16 2017

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

Gale warnings will be retained in the forecast tonight in the
PZZ815 zone and will also retain for those in the extended period
because a low pressure trough along the California coast will
force a tight pressure gradient as high pressure builds in from
the west in the short term and the trough will also strengthen
in the extended period to re-enforce tight pressure gradient over
the central waters. Satellite images show clouds with brighter
tops over the central waters and indicate clearing over the other
areas. latest observations show highest winds in the gale force
range over the far eastern portion of the central waters. At 00Z
the NCEP weather map has high pressure 1031 mb west of the region
near 40N143W with its northeast ridge into the north waters and
southeast ridge passing the western portion of the southern
waters. Inland low pressure 1010 mb over Arizona/california
border has trough extending northwest into the central waters
with pressure gradient just tight over the central waters that
has resulted in gale force winds. Models are in good agreement on
keeping the low pressure inland but weaken the high pressure to
the west and that will relax the pressure gradient and so will
keep warnings over the central waters. High pressure will weaken
slightly though the inland trough will persist, the gradient
will briefly relax then strengthen in the extended period. Will
continue with GFS.

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PREVIOUS DISCUSSION...

The 18Z NCEP/OPC surface analysis depicts a weak cold front
across the northern PZ5 waters this afternoon. Elsewhere, 1029 mb
high pressure is located near 41N143W, and 1012 mb low pressure
is analyzed over northern California with a trough extending to
the southern Oregon coast. Between these features, a 1745Z ASCAT-B
pass revealed 20 to 30 kt northerly winds over the coastal and
inner offshore waters from Florence, OR to Point Arena, CA. Still
expect a brief period of gales to develop early tonight over the
northeast portion of PZZ820 as high pressure moves eastward and
the thermal trough sharpens along the coast.

The 12Z models remain consistent with the synoptic pattern through
Monday night. High pressure west of the area will continue moving
east through Saturday, then slowly drift south through Sunday
night before weakening on Monday. This means periods of gales
remain likely off the northern California and southern Oregon
coast, especially over the coastal waters as diurnal heating
strengthens the thermal tough and tightens the pressure gradient.
For the offshore zones, there is low to moderate confidence for
gales Friday night in PZZ820. The 12Z GFS remains the most
bullish model for gales, with some support from the 12Z UKMET as
well. The 12Z ECMWF winds are marginal, but at this point it
seems prudent to maintain the warning by sticking with the GFS.

There is still above average confidence for an extended gale
event over PZZ820 and the adjacent coastal waters from Saturday
night into Sunday night. The surface high makes its closest
approach to the waters during this time, which further enhances
the gradient off the coast. The GFS/ECMWF/UKMET winds all support
gales over our inner waters from Point Saint George to Point
Arena. Therefore, will use the 12Z GFS to reflect gales in the
grids beginning at 00Z Sunday and diminishing over our waters by
06Z Monday. Winds will continue to relax on Monday as the surface
ridge weakens and moves southwest away from the waters.


.SEAS...The 12Z NWW3 ENP and ECMWF WAM appear to have initialized
fairly well based on the latest sea state analysis, although still
perhaps a bit low off the northern California coast. Other than
some minor enhancements in this region through tonight, used the
12Z ENP for the wave grids to match the GFS winds. It is worth
noting that the ENP/WAM are in very good agreement for the gale
event this weekend with max seas to 16 ft off the northern
California coast.


.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.
     Gale Possible Friday night.
     Gale Possible Saturday night into Sunday night.

$$

.Forecaster Musonda/Reinhart. Ocean Prediction Center.


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