Marine Weather for HS 102


marine weather discussion for N Pacific Ocean NWS ocean prediction center Washington DC 827 PM PDT Sat Apr 29 2017

.Forecast discussion...Major features/winds/seas/significant .Weather for the North Pacific N of 30n and E of 150w.

Per the latest observations and satellite data, gales remain north of the offshore waters this evening, associated with low pressure moving east over the Queen Charlotte islands off the coast of western Canada. A trailing cold front extends south and southwest over pz5 waters this evening, with a large high pressure area remaining centered west of the northern and central California offshore waters. For the evening update, we will make only minor adjustments to the previous forecast and grids so that the opc forecast and grids fit current conditions and nearby coastal WFO and TAFB grids and forecasts.

For seas, again little change will be made to the previous forecast for the evening update. As was done for winds, we will tweak the previous forecast and grids to fit current conditions and nearby coastal WFO and TAFB forecasts a little better. Sea heights currently range from near 20 feet to the west and northwest of Vancouver Island, associated with the low pressure area mentioned previously, to around 4 or 5 feet over Oregon and northern California waters.

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

Gale warnings in the far north zones have diminished over the forecast waters and winds over the region will remain below gale force threshold through the rest of the forecast period as high pressure will dominate. The satellite images still show anticyclonic air flow over most of the region except the far north waters where clouds with cold tops are still moving east. The latest observations including scatterometer pass at 1746z show higher winds over the north waters in just below gale force threshold from the south and southwest. Winds are mainly 15 to 25 kt and they are form the north and northwest over most of the region. The NCEP map at 1800z has low pressure 993 mb now about 300 nm north-northwest of the region with its warm front stretching southeast into the eastern portion of the Washington waters while its associated cold front is now entering the Washington waters. A trough still stretches along the coast of California into the southern parts of Oregon coast. High pressure now strengthened by 2 mb to 1031 mb in the last 6 hours is still centered west of the southern waters near 36n134w. The pressure gradient is just tight over the north portion of Washington waters and quite relaxed over the rest of the region. Elsewhere, low pressure 1010 mb in the Gulf of Alaska is part of the low pressure described above. Farther west is still a broad area that is dominated by low pressure with several centers. One low pressure that still demands close monitoring is now 1006 mb east of Japan near 33n156e as this low is still slated to end up in the Gulf of AK and force winds to be elevated to storm force threshold and will also bring large period west swells to the forecast waters.

In the upper levels, the models GFS/CMC/ecmwfhr/ukmethr suggest energy embedded in an upper level trough stretching southeast from the Gulf of AK that is moving over the for north waters. An upper level cut off high pressure has a ridge with little to no energy just west of the region. Meanwhile, there is a cut off upper level low with significant energy centered just north of Hawaii. Another area of significant energy is a cut off low just west of the Aleutian Islands which extends a trough that has energy that is maintaining low pressure east of Japan mentioned above. In the short term, the models agree on pushing the energy over the north waters to the northeast while building and maintaining high pressure to the west of the region. The energy over north Hawaii will drift south while most of the energy associated with low to the west of the Aleutian Islands will consolidate and move toward the Gulf of Alaska.

The models GFS/CMC/ecmwfhr/ukmethr/NOGAPS/NAM have still initialized well the 18z surface observations with still just minor differences within 1 mb on the central pressure values of the low pressure to the northwest and high pressure west of the region. As for the wind speed, ecmwfhr is still quite weak by 10 kt over GFS and ukmethr is equally weaker than GFS with winds to only 25 kt. CMC and GFS are evenly matched on winds over the north waters and match the 30 kt winds over the north. So, will stay with GFS for this forecast. In the short term, low pressure will move north of the region and will pull a warm front northeast of the north waters as a cold front moves east. High pressure will continue to build west of the region and will maintain a relaxed pressure gradient especially over the central and southern waters through most of the forecast period.

.Seas...The seas now have peaks to 12 ft over the southern waters. The seas range between 6 and 11 ft over the southern waters mainly due to large period swell and they range only between 6 and 9 ft over the northern half of the waters. The wave models nww3 and ecmfwave still fit very well the observed 18z seas pattern and they have been quite consistent in the previous runs. Will continue with nww3 for the seas. Seas will continue to subside over the southern waters to below 8 ft through the forecast period but will build to 10 ft over the central waters. Briefly before subsiding to 8 ft in the extended period. The seas will start to build over the northwest waters in the extended period mainly due to the large period swell from the west.

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


.Pz5 Washington/Oregon waters... None.

.Pz6 California waters... None.


.Forecaster Mills/musonda. Ocean prediction center.

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