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000
AGPN40 KWNM 240804
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
104 AM PDT Mon Jul 24 2017

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

A very timely Ascat overpass from 0550 UTC this morning went
directly over the coastal and inner offshore waters, and
indicated a large area of gales to the W of Point Saint George
in the strong pres gradient between strong high pres to the W of
the offshore waters and the inland thermal trough. Ascat
indicated a large area with 40 kt in the coastal waters and 35 kt
in the inner nrn California offshore waters. 00Z GFS 10m winds
are well initialized when compared with the Ascat data, and
indicates the gales will continue into tonight before the high
starts receding to the W Tue and Tue night while slowly
weakening. In comparison, the 00Z ECMWF/UKMET surface wind are
initialized a little low, while the 00Z NAM/GEM are initialized
too strong with widespread gales up to 45 kt. However, the models
all agree well on the strength of the pres gradient, and also on
the timing and slow weakening of the ridge to the W. Confidence
in the 00Z GFS`s timing and intensity is high as it has been
fairly consistent on the timing and intensity. As a result,
planning on continuing gale warning in zone PZZ820 into tonight
with high confidence.

Otherwise, the 00Z models agree fairly well over the forecast
period, and generally maintain a ridge to the W of the offshore
waters as the thermal trough persists near the coast. The 00Z
GFS/ECMWF also indicate a very weak cold front will pass through
the PZ5 waters Tue night into Wed, and the weak cold surge is
expected to move down the coast by Thu as high pres continues to
the W of the area near 140W. The GFS is hinting at the
possibility of gales Thu night, though the rest of guidance is
weaker. However, the GFS is generating the higher winds over the
coastal waters in the gradient between the high and the inland
thermal trough on Thu night despite the high slowly weakening. As
a result, confidence is somewhat low in the gales right now. The
GFS only indicates up to 25 kt in the offshore waters, and this
seems representative further offshore. Also, the 00Z models also
indicate a weak low pres system will approach the PZ5 waters on
Fri night. All but the 00Z ECMWF keep the associated frontal
boundary W of the area by late Fri night, as the ECMWF appears to
be a slightly more progressive outlier. The 00Z GFS appears to
be representative of the consensus of the 00Z UKMET/GEM, so
planning on using the 00Z GFS for the remainder of the forecast.

.SEAS...A CryoSat altimeter overpass from 0220 UTC across the
offshore waters indicated a large area of 12 ft seas in the
California offshore waters with up to 14 ft roughly 150 nm
offshore. When compared with the 00Z Wavewatch and ECMWF WAM
guidance, both models are initialized low once again in the
vicinity of the strong gradient winds off nrn California. This
has been the case for the past few runs, so will need to adjust
wave heights to account for the low initialization. Otherwise,
the models are in somewhat decent agreement over the forecast
period, so planning on using the 00Z Wavewatch to match the
preferred GFS winds.

.EXTRATROPICAL STORM SURGE GUIDANCE...N/A.

.WARNINGS...Preliminary.

.PZ5 Washington/Oregon Waters...
     None.

.PZ6 California Waters...
.PZZ820...Inner Waters from Point St. George to Point Arena...
     Gale today into tonight.

$$

.Forecaster Kells. Ocean Prediction Center.



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