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000
AGPN40 KWNM 210258
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
758 PM PDT Wed Sep 20 2017

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

Satellite imagery and lightning data indicate scattered
thunderstorms over Oregon and far northern California offshore
waters this evening associated with a strong upper level trough
crossing the region. Locally gusty winds and rough seas can
occur in or near any of the heavier thunderstorms through
tonight into Thu. As the trough moves inland later Thu into Fri,
the threat for thunderstorms diminish. Overall, no major changes
will be made to the previous grids and forecast for the evening
update. We will make some minor grid edits and forecast
adjustments to fit the OPC forecast with nearby coastal WFO and
TAFB grids and forecasts a little better. The 00Z OPC-NCEP
surface analysis indicated that a cold front was moving south
from central California waters tonight. It is still expected to
then push south of the southern California waters during Thu.
High pressure will build to the west of the waters behind the
front Thu and Fri.

As far as sea heights are concerned again only minor adjustments
will be made to the previous forecast and grids so that they fit
initial conditions and nearby WFO and TAFB grids and forecasts.
Sea heights generally range from 8 to 12 feet over the offshore
waters this evening per the latest observations and altimeter
passes.

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PREVIOUS DISCUSSION...


Grids: 12z ECMWF for the first 24 hours, then a transition to the
12z GFS with slight episodic modifications. Discussion follows.

A few isolated lightning strikes were recently detected across
the northern waters, and the threat for thunderstorms will
continue overnight into tomorrow beneath digging upper shortwave
trough and subsequent steep lapse rates. Best chance generally
confined between 37N to 47N. Will continue to manually add
mention of thunderstorms into the forecast grids, with good
support from recent SREF probabilistic guidance and 4km NAM
parameters (primarily LI), generally ending west to east after
12z tomorrow as vigorous shortwave and jet streak moves inland.

Near term into 00z Fri - ECMWF: trailing cold front still draped
over the central CA waters will continue its south march,
clearing south of the far southern CA zones late tomorrow
afternoon. In the immediate vicinity of the front winds should
remain light, generally 10 to 20 kt or less. Stronger northerly
winds will develop in the vicinity of the Channel Islands as high
pressure filters southeast behind the passing front, combining
with inland trough to generate winds up to 30 kt, strongest
within the coastal zones Thu afternoon and evening. Deference to
ECMWF in the near term is primarily for the preferred gradient
across the northern portions overnight into Thu, as weak surface
low develops near Vancouver Isl, then drifts south and dissipates
just NW of Cape Blanco. GFS remains the weakest model solution
with gradient driven winds generally 25 kt or less, while the
NAM, UKMET, and ECMWF suggest the stronger and preferred winds to
30 kt.

Remainder of forecast - GFS: models remain in pretty good
agreement thru the weekend with overall synoptic pattern. E
Pacific high pressure forecast to slowly build east, while low
pressure trough remains stationary along the northern CA coast.
Based on most recent events, models have verified on the low
side of actual observations and scatterometer data, and with
this low bias in mind, intend to blend 1:1 higher 30m GFS winds
into the forecast during diurnal maxes. These will not prove to
increase surface winds too much, but provide a more realistic
good starting point to these slightly stronger expected winds.
For now do not expect conditions above 30 kt. Models then suggest
a strong cold front approaching the far northern waters late
Monday, with the GFS remaining on the slow side compared to
non-NCEP models. But with good support from the 12z GEFS and
slower 00z EPS means, prefer the slower and more conservative
GFS for now.

Seas: both the WW3 and WAM initialized fine comparing recent sea
state analysis to actual obs and satellite altimetric data, and
both remain generally within a foot or two throughout the
forecast period. Since mainly using the GFS, see no reason to
stray far from WW3 output throughout.

Extratropical Storm Surge Guidance: n/a.


.WARNINGS...Preliminary.

.PZ5 Washington/Oregon Waters...
     None.

.PZ6 California Waters...
     None.

$$

.Forecaster Mills/Collins. Ocean Prediction Center.



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