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AGPN40 KWNM 250320

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
820 PM PDT Fri Mar 24 2017

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

At 00z a trough of low pressure was located over the PZ5 waters
while a cold front was situated across the central PZ6 waters.
There was continued good agreement when comparing the 18z
GFS/NAM to the 12z model package...hence see no reason to make
adjustments to the current grids in place considering this is an
intermediate update. Current hazards remain as gales anticipated
ahead of the cold front forecast to move through the waters Sat
night through Sun. Will continue to monitor for the possibility
of gales needing to be introduced in the forecast Mon night into
perhaps early Tue over parts of the southern PZ6 waters in the
increased n-lies as a result of a tightening pressure gradient
between inland troughing to the east and a high pressure ridge to
the west.


Grids: will use the 12z GFS into late Sun night, transition to
the 12z ECMWF Mon until Tue afternoon, then transition back to
the 12z GFS Tue night onwards. Discussion follows.

As of 18z, strong frontal boundary that was responsible for
widespread gale force winds and seas in excess of 20 feet
overnight has finally bowed inland and cleared even the coastal
zones n of about 39n. Farther south, the cold front emerges
offshore near Point Reyes and extends southwest across the
central CA offshore waters. ASCAT MetOp-B overpass just before
18z caught a good swath over the aforementioned area, and similar
to 12z GFS initialization, returned mostly 15-25 kt wind
retrievals immediately south of the boundary. Some stronger 25 kt
winds (with embedded 30 kt flags) were present in the coastal
zones from Point St George north to about Cape Lookout, likely
associated with low pressure trough crossing the WA/OR offshore

Sat night through Sun night: echoing its run from 06z, the 12z
GFS again compares favorably to global consensus with respect to
both timing and intensity of a passing frontal boundary. For the
past few days we had a pretty good notion that gales would
likely be present ahead of this boundary, but with erratic runs
of the GFS and UKMET developing potent triple point lows, could
not pinpoint warning timing or location with any degree of
confidence and had been playing it rather conservative. Now with
all 12z guidance sharing very similar solutions, and supported
by the 12z NAM, will introduce widespread gale force winds across
the WA/OR waters Sat night into Sun. Warning confidence is above
average. Also worth noting -- similar to this most previous
fropa and confirmed by ASCAT overpasses -- expect relative wind
maxima to develop in the vinicity of Cape Blanco as low level jet
increases as the front drawing nearer to the coast. The other
maxima will be located just south of Vancouver Island, but
similar to the southern max, expect the strongest of the surface
winds to be confined to areas just outside of OPC waters.

Sun night through Tue aftn: strong pressure gradient still
progged between intensifying low pressure trough along the CA
coast and building EPAC high pressure. Most model guidance has
been consistent building strong prevailing n-nw flow into the
25-30 kt range. For the past couple of days, the GFS has remained
the most robust solution, signaling a gradient strong enough to
support gale force winds. Recent GFS cycles have backed off
slightly, and shifted core axis of possible gales a little
farther south in basically a 120 nm west semicircle from Point
Conception. No major changes in forecast thinking at this time;
will continue to rely on the much more consistent ECMWF
solution(s), and continue to monitor latest guidance. At some
point, would not be surprised to see gale headlines added into
this portion of the forecast, whether that be overnight tonight,
or more of a nowcast as the event draws nearer.

Farther north, deference to ECMWF offerrs a middle of the road
compromise as low pressure passes NW of the forecast area while
dragging another cold front into the northern waters. Low track
is spread about 500 nm between global models, with little support
offered from ensemble means.

Tue aftn onwards: models signal yet another shortwave trough
ripping across northern offshores enroute the Pacific NW. Models
split into two different low tracks; the ECMWF and UKMET are
farther south along 43n, while the GFS/GEM along a more northern
trajectory across 49n and towards Vancouver Isl. Recent model
tendencies into verified weather have been favoring systems along
a more northern track, so with this in mind, will cautiously lean
more towards the GFS camp (which resembles the older 00z ECMWF
run, and is supported by the 12z EPS mean).

Seas: despite the official wind grids vacillating between the GFS
and ECMWF, wave models remain in fairly good agreement. Will use
a 1:1 blend of the 12z ECMWF WAM and WW3/ENP this afternoon to
iron out small differences among the guidance. Will however be
making significant edits/increases to the seas in the pre-front
gales Sun and Sun night, and this adjustment is based on the
recent boundary that pushed through overnight where observed seas
were anywhere from 3-6 feet above the much too low guidance.
While these most egregious differences between obs and wave
height models were confined to the coastal zones, sensible
upcoming seas edits will yield forecast values 3-4 feet over raw
guidance across inner OPC zones. This may still prove too low.



.PZ5 Washington/Oregon Waters...
.PZZ800...Inner Waters from Cape Flattery to Cape Shoalwater...
     Gale Sunday.
.PZZ900...Outer Waters from Cape Flattery to Cape Shoalwater...
     Gale Sunday.
.PZZ805...Inner Waters from Cape Shoalwater to Cape Lookout...
     Gale Sunday.
.PZZ810...Inner Waters from Cape Lookout to Florence OR...
     Gale Sunday.
.PZZ815...Inner Waters from Florence OR to Point St. George...
     Gale Sunday.

.PZ6 California Waters...


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