Marine Weather for HS 102

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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.

----------------------------------------------------------------- Previous discussion...

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 waters.

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.

.Extratropical storm surge guidance...N/A.

.Warnings...Preliminary.

.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... None.

$$

.Forecaster Holley/Collins. Ocean prediction center.

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