marine weather discussion for N Pacific Ocean NWS ocean prediction center Washington DC 823 am PDT Tue may 21 2019
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Pacific N of 30n and E of 150w.
The latest NCEP surface analysis shows a gale force low pressure system in the eastern Pacific this morning, with the associated cold front moving through the central and srn pz6 offshore waters. In addition, the analysis shows high pressure to the W of the pz6 waters, and the GOES water vapor imagery indicates an amplifying ridge near 150w moving slowly to the E. Overall, the 00z/06z global and regional models agree fairly well over the short range, and indicate that the low will move inland today while weakening as the cold front continues S along the coast. In addition, the GFS/ECMWF show the upper pattern amplifying as the ridge W of the area extends into the Gulf of Alaska late today and tonight. The models show the high at the surface shifting E toward the offshore waters while strengthening over the next 36 hours. At the same time, the cold surge along the coast is expected to destabilize the lowest layers of the atmosphere and allow stronger winds aloft to mix down. Simultaneously, the GFS indicates a low pressure trough will strengthen along the California coast, and interact with the building high to the W of the area. The tightening pressure gradient is expected to increase the winds across the offshore waters, and the models have a strong signal for maintaining the current gales across the area. As a result, confidence remains above average on the previous gale headlines since they are well supported by the latest model guidance, so planning on making minimal edits to the previous grids in the next forecast.
Late Thu into Fri, the models indicate the ridge will be bolstered by a Second Ridge that moves in from the W into the Gulf of Alaska and merges with the current one already in place. This will build a second surface high to the W of Vancouver Island as a coastal trough develops along the coast. This will tighten the pressure gradient and subsequently increase the winds offshore, and the GFS/ECMWF agree on gales developing to the N and over the far NE portion of the Washington offshore waters. Confidence remains about average on the previous gale headlines in this region on Thu night into Fri, so again planning on making minor edits to the previous wind and hazard grids with the models remaining fairly consistent and in good agreement on this scenario. Otherwise, will not be making major changes from the previous forecast through the remainder of the forecast period since the guidance has been consistent and remain in decent agreement on the overall features.
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The GOES infrared satellite imagery shows strong low pressure over the central Oregon offshore waters, with an associated cold front over the central pz6 waters. Ascat wind retrievals from 0450z to 0550z passes indicated gales mainly to the SW of the low center and a 06z ship report shows W 37 kt at southern edge of pzz915. The water vapor imagery shows the short wave trough supporting the low showing signs of dropping more se in response to upstream building of the upper ridge to the W, with the shortwave heading toward a long wave trough with the axis inland over the U.S. Great Basin. The 00z GFS/ECMWF winds are initialized fairly well when compared with the current data, and the models agree fairly well on the surface low moving inland later Tue while pulling the cold front se across the remainder of the waters. The models agree well on the system increasing winds in the offshore waters tonight and Tue, mainly in the SW quadrant of the low. Since the ascat passes indicated an area of W to NW gales already present in the SW quadrant, confidence is well above average on the gales initially in the northern pz6 and SW Oregon waters shifting se over the pz6 northern and central offshore waters Tue. The previous forecast had headlines up consistent with the GFS. Planning on continuing with the previous headlines to start the forecast since there is good support from the 00z guidance for them.
The 00z models then indicate that the cold advection behind the front will continue moving S down the coast Tue and Wed, which will act to destabilize the surface boundary layer. Model consensus of 00z GFS/ECMWF, especially the latter, is to allow for a deeper mixed layer near the surface with negative static stability, with low level winds at 925 mb up to 45 to 50 kt Tue into Wed. Therefore the likelihood of at least minimal gales mixing down to the surface seems to be above average especially with the ridge amplifying to the W of the offshore waters and a deep trough to the E allowing for a strong surface pressure gradient to develop. The 00z GFS indicates gales will persist into Thu along the California coast, and the tightening pressure gradient and increasing winds are supported by the rest of the 12z guidance. As a result, confidence remains above average on gales persisting into Thu in this region, so planning on staying close to previous gale headlines through Thu. Am also planning on using an even blend of the higher first sigma level winds with the 10 meter winds of the 00z GFS through Thu which appears initially close to observations.
In the medium range, the GFS indicates that the ridge will rebuild to the N into the Gulf of Alaska late Thu into Fri as the upper trough persists just inland. The 00z models show a second surface high building W of Vancouver Island by late Thu, as it interacts with a coastal trough along the coast from Washington to Vancouver Island. As one would expect, the models show the surface pressure gradient across and N of pz5 tightening in response, while the winds along the coast increase. The GFS shows another round of gales developing, but over and N of the pz5 zones Thu night into Fri night. The rest of the 00z global models generally support the synoptic features of the GFS through Fri night, but with variation in extent of gales. Confidence is about average on the gales. Since there were gale headlines present in the previous forecast, will blend the previous grids with the 00z ECMWF through Fri night. Then for Sat and Sat night the 00z models maintain a trough along the coast while the high pressure to the W shifts S while weakening. Used an even blend of the 00z GFS and ECMWF after 1for Sat and 2z Sat.
Seas...the 00z wavewatch and ECMWF wam are initialized a couple of feet low initially in the Oregon coastal waters when compared with the 06z sea state analysis. However, the models agree well during the forecast period, so planning on using an even blend of the two models to account for minor differences while making adjustments for the initial area of poor initialization.
.Extratropical storm surge guidance...The 00z surge models show no significant positive surge events over the next few days. However, there is a weak signal in the guidance for a positive surge of 0.5 ft or higher along the S Oregon and N California coasts early Tue with the strong westerly flow S of the approaching strong low center moving into the region. Please consult products issued by your local National Weather Service forecast office for additional information.
.Pz5 Washington/Oregon waters... .pzz800...inner waters from Cape Flattery to Cape Shoalwater... gale tonight. Gale possible Friday. .Pzz910...Outer waters from Cape Lookout to Florence or... gale today. .Pzz915...Outer waters from Florence or to point St. George... gale today into tonight.
.Pz6 California waters... .pzz820...inner waters from point St. George to Point Arena... gale today into tonight. Gale Wednesday night. Gale possible Thursday. .Pzz920...Outer waters from point St. George to Point Arena... gale today into tonight. .Pzz825...Inner waters from Point Arena to Pigeon Point... gale tonight into Wednesday night. Gale possible Thursday. .Pzz925...Outer waters from Point Arena to Pigeon Point... gale tonight. .Pzz830...Inner waters from Pigeon Point to Point Piedras Blancas... gale Wednesday into Wednesday night. Gale possible Thursday. .Pzz930...Outer waters from Pigeon Point to Point Piedras Blancas... gale Wednesday into Wednesday night. Gale possible Thursday. .Pzz835...Inner waters from Point Piedras Blancas to Santa Cruz Island, CA... gale Wednesday. .Pzz935...Outer waters from Point Piedras Blancas to Santa Cruz Island, CA... gale Wednesday. .Pzz840...Inner waters from Santa Cruz Island, CA to San Clemente Island, CA... gale tonight into Wednesday.
.Forecaster Kells/Bancroft. Ocean prediction center.