Marine Weather for HS 102

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marine weather discussion for N Pacific Ocean NWS ocean prediction center Washington DC 755 am PDT sun Apr 30 2017

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

No warnings through the forecast period over the forecast waters but seas will be building over the north waters in the extended period as strong low pressure slated to reach the Gulf of Alaska passes to the northwest. The satellite images show anticyclonic flow over most of the forecast waters except the far north waters which is dominated by clouds with cold tops. The lightning density map shows no lightning strikes across the region and even the sref model has prob 0 for tstms threat. The latest observations indicate higher winds only to 25 kt in a few areas over the far north and south waters. At 1200z, the NCEP map has a cold front across the northern California waters that passes through high pressure to the west resulting in two centers 1030 mb about 120 nm west of Oregon waters and the second center 1030 mb about 300 nm west of central California waters. Low pressure 1010 mb along 130w about 300 nm north of the region with gale force winds in its south sector but north of the region. Inland trough stretches near the coast from south California to 40n. The central and western Pacific is till dominated by low pressure with several centers. One complex low pressure is in the Bering Sea with another low 1006 mb east of Japan near 170e and this low is on track to reach the Gulf of Alaska and bring storm force winds there but will also bring large period swell to the north forecast waters.

The seas range between 6 and 10 ft over the north and south waters but they range between 4 and 6 ft over the central region. Peak seas to 12 ft are over the far north waters. The wave models nww3 and ecmwfwave have initialized the observed seas pattern very well and have been very consistent in the previous runs and so no need to deviate from the previous model choice. Will stay with enp for seas.

The models GFS/CMC/ecmwfhr/ukmethr in the upper levels indicate a cut off upper level high pressure along 30n and extends its ridge north through the western portion of the forecast region. There is still a cut off low with some energy over the Hawaiian islands and also over the Aleutian Islands. The models agree well on maintaining upper level high pressure ridge over the region that will block energy from the west to reach the waters but will instead Glide northeast and end up in the Gulf of Alaska.

The models GFS/CMC/ecmwfhr/ukmethr/NAM/NOGAPS have generally initialized well the 12z surface observations and the small differences are mainly within 2 mb on the central pressure value for the high pressure to the west of Oregon. The ecmwfhr is the weakest with wind speed. Otherwise the models still have these minor differences through the short term but do not alter the general synoptic pattern and so will continue with GFS. In the short term, the high pressure Center West of Oregon waters will pass east across the region and dissipate inland while the high pressure Center West of the southern waters will remain in the same vicinity and maintain a ridge across the region with a relaxed pressure gradient. There will be no warnings through the forecast period.

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

Upper-level high pressure is centered near 33n 132w with upper level low pressure systems located near Hawaii, and over the south-central U.S. Early this morning, per the latest GOES water vapor satellite imagery. A nearly zonal flow is located further north, roughly along 45-50n, over most of the eastern North Pacific Ocean into the Pacific northwest. This pattern will be slow to change over the next few days, with the high moving east toward the western states by later Wednesday into Thursday night, as a low pressure trough deepens just west of the offshore waters. At the surface, a cold front is nearing northern California waters early this morning, with surface high pressure persisting west of the offshore waters. The front will move south today, and then dissipate by tonight. Weak low pressure will develop west of the pz5 waters later today, then drift east and northeast toward the waters tonight, before dissipating Monday. Another weak low will pass just west and northwest of the pz5 waters Wednesday into Wednesday night with an associated cold front moving east into the waters Thursday and Thursday night, followed by a large high pressure area building well west of the waters. Further south, the interaction of a coastal low pressure trough and high pressure west of the California waters will remain the main weather features over the next few days. At this time, chances for gales over the offshore waters are too low to add to the offshore forecast. The best chance for gales will be over the California coastal waters tonight into Tuesday night when the pressure gradient between the coastal low pressure trough and high pressure area to the west of the waters becomes rather strong.

The 00z models are in decent agreement over the offshore waters for the next few days. Model differences increase later in the forecast period. The 00z ECMWF appears to be an outlier at this time, as it brings a deepening low east and then southeast toward the waters later Thursday and Thursday night, while most of the remaining guidance lifts this system more toward the northeast toward western Canada. For now, we will stay with the majority of the model guidance, which resembles the 00z GFS for the most part. Also, the GFS has remained quite consistent from the 12z run from yesterday, with confidence levels in the early morning forecast package near to above average.

.Seas...Both the 00z enp wavewatch and 00z ECMWF wam models appear to have initialized quite well over the offshore waters per the latest altimeter data and observations. Maximum sea heights near 20 feet are occurring north of the offshore waters, to the northwest of Vancouver Island, with seas heights ranging from 5 feet over far northwestern California waters to near 15 feet over far northwestern Washington waters at this time. For the early morning package, as we are staying close to the 00z GFS for winds, we will also remain close to the 00z enp wavewatch for sea heights over the region during the next few days.

.Extratropical storm surge guidance...N/A

.Warnings...Preliminary.

.Pz5 Washington/Oregon waters... None.

.Pz6 California waters... None.

$$

.Forecaster musonda/Mills. Ocean prediction center.

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