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AGPN40 KWNM 200826

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
126 AM PDT Fri Apr 20 2018

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

The main forecast concern over the next few days is the potential
for marginal gales spreading from the northern California
coastal waters into the adjacent inner offshore waters Sat night
into Sun. The 00Z guidance continues to indicate the best chance
for gales Sat night into Sun over the northern California
coastal waters into the far southern Oregon coastal waters.
There is still a threat for these gales spilling into the far
eastern portion of the northern California inner zone of PZZ820
Sat night into Sun as the 00Z guidance continues to indicate
that this time frame will have the strongest pressure gradient
between high pressure W and N of these waters and a coastal low
pressure trough. The 00Z GFS and 00Z ECMWF are in very good
agreement over the region for the next several days, with the
higher first-sigma layer winds from the GFS producing widespread
gales off the northern California N to off the southern Oregon
coast Sat night into Sun which matches up well with the 1000 mb
winds from the ECMWF guidance. To maintain continuity from the
past few OPC forecast we will use a 50/50 blend of the higher
first sigma layer and lower 10 meter winds off the northern
California coast Sat night into Sun, with no major change to the
previous hazards for the early morning package. This blend in
the past has shown to be a fairly accurate predictor in these
type of situations.

Otherwise, as very good model agreement is noted over the region
for the next several days we will populate the remainder of the
forecast with the 00Z 10 meter GFS winds, which will also result
in little change from the past few OPC forecasts. A cold front
will approach Washington and Oregon waters from the W today as a
high pressure ridge weakens over the region. Widespread high-
end gales are likely N of the offshore waters, near Vancouver
Island, by later today into tonight, with some chance of storm
force winds as the cold front and associated low press up
against the coast. There is only a slight chance for gales
spreading S into Washington waters with the cold front by
tonight. We will maintain sub-gale force winds over these waters
in the early morning package for today into tonight. The cold
front will cross Washington, Oregon and move into northern
California waters tonight as it weakens. High pressure will then
build to the W of the waters Sat as a California coastal low
pressure trough strengthens. The high will continue to build off
the Pacific NW coast late Sat and Sat night as the trough
continues to strengthen and lift N. The trough and high will
both slowly weaken Sun into Sun night. Both the trough and high
will then continue to weaken Mon as a cold front weakens to the
W of the region. The front will dissipate W of the waters Mon
night into Tue as high pressure once again builds to the W of
the waters, and the coastal low pressure trough begins to slowly
strengthen. Little change in this pattern is likely Tue night
over the region.

Overall, confidence levels in the forecast are above average
over the next 5 days as there is little variance in the 00Z
model suite. Confidence levels for gales spreading into the far
eastern part of the northern California waters Sat night into
Sun remains somewhat below average, but high enough to keep in
the forecast for now.

.SEAS...Both the 00Z ENP Wavewatch and 00Z ECMWF WAM appear to
have initialized fairly well over the offshore waters early this
morning. Sea heights range from near 10 feet off the California
coast and over the NW Washington offshore waters, to around 5
feet over the inner NW Oregon and inner Washington waters per
the latest observations and altimeter data. As model guidance is
in very good agreement over the region, we will use a 50/50
blend of the ENP and WAM for the early morning package through
the weekend into next week.



.PZ5 Washington/Oregon Waters...

.PZ6 California Waters...
.PZZ820...Inner Waters from Point St. George to Point Arena...
     Gale Saturday night.
     Gale Possible Sunday.


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