Marine Interpretation Message
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000
AGPN40 KWNM 170245
MIMPAC

Marine Weather Discussion for N Pacific Ocean
NWS Ocean Prediction Center Washington DC
745 PM PDT Mon Oct 16 2017

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

The latest satellite imagery and lightning data shows scattered
thunderstorms occurring west of the southern California offshore
waters near 30N 126W. This activity appears to be associated
with a weak surface low pressure trough and an upper level
disturbance lifting north, to the west of the southern
California offshore waters this evening. The 18Z GFS and latest
RAP models bring some of the thunderstorm activity north and
then northeast into portions of the southwestern and central
California offshore waters later tonight through Tue, with the
activity weakening and diminishing Tue night. We will adjust the
weather grids and forecast accordingly for the evening update.
Locally gusty winds and rough seas can occur in or near any of
the thunderstorms through the rest of tonight and Tue.

Otherwise, little change will be made to the previous headlines,
winds and seas, with minor adjustments to fit the grids to
current conditions and nearby coastal WFO and TAFB grids and
forecasts. Gales will spread east over Washington offshore waters
tonight as the first in a series of cold fronts moves into the
region from the west and northwest. The 00Z OPC-NCEP analysis
shows the cold front just west of the NW Washington waters with a
high pressure ridge extending westward from southern Oregon and
northern California waters. A coastal trough extends from far
southern Oregon south and southeast along the California coast.

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

A 1908Z ASCAT-A overpass indicated winds running generally 20 to
25 kt in the SW flow ahead of a cold front approaching the NW
waters, covering most of NW half of PZ5 waters. These winds are
close to 12Z GFS 10m winds. Winds are light in the SW-NE ridge
over the SE PZ5 waters while ASCAT indicated northerly winds to
20 kt in the gradient between a coastal trough and the weak ridge
to the NW. The 1730Z pass of ASCAT showed light winds elsewhere
to the S with the trough. These features are weakening as the
front approaches. Low pressure was centered near 53N 130W per
the 18Z NCEP preliminary surface analysis with an associated
cold front extending southwest from the low center. Although no
gales are apparent from ASCAT, another low will form just north
of the Washington waters tonight in response to some digging of a
shortwave trough coming from the W with an associated cold front
pushing southeast over the Washington and Oregon offshore waters
tonight into Tue, with the front dissipating over Oregon waters
later Tue night. Weak high pressure will cross the waters north
of the front late Tue and early Tue night. Most of the 12Z model
guidance including the GFS and ECMWF bring a period of gales to
the NW Washington offshore waters near the front tonight, and we
will also add gales to NW part of inner zone 800 for the
afternoon package, for tonight. Visible GOES-W satellite omagery
and GFS stability guidance suggest there could be negative static
stability at times behind frontal passages. With good agreement
between GFS and other model guidance in the near term will
populate grids with the 12z GFS modified with the smart tool that
places stronger first sigma level winds in unstable areas and 10m
winds in stable areas. A stronger cold front will then push E
and move over Washington, Oregon and northern California offshore
waters later Tue night and Wed. This front will bring a good
chance for gales over all of the Washington and the NW Oregon
offshore waters late Tue night into Wed. The front will weaken as
it moves into northern and central California waters Wed night
into Thu with winds forecast to diminish below gale force, except
over NW Washington waters where gales may re-develop as a strong
upper level disturbance moves east into the area with associated
negative static stability. For now plan to keep gales out of the
two southern PZ5 zones. Any gales will then end over the
offshore waters during Thu as the disturbance slowly lifts north
of the waters, and the front continues to slowly weaken as it
moves further to the south. A weak high pressure ridge will then
build into the region Fri ahead of the next cold front.

Starting Fri night the models start to diverge with the timing of
the next frontal system to affect to waters Fri night with the
GFS looking like a slow outlier while the 12z UKMET/ECMWF are
faster and 12Z GEM agood compromise. Since grids arev not
available for the GEM plan to the use the more representative old
00Z ECMWF for winds starting 18Z Fri and for the reminder of the
forecast, which is close to WPC guidance.

.SEAS...Both the 12Z ENP Wavewatch and 12Z ECMWF WAM appear to
have initialized quite well over the offshore water per the
latest observations and earlier altimeter data. Sea heights range
from 5 feet over southern California waters to around 10 to 12 ft
in the far NW. For the new package we will use a 50/50 blend of
the Wavewatch and WAM for tonight into Fri and after that with
old ECMWF used for winds, will use the 00Z ECMWF WAM.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a.


.WARNINGS...Preliminary.

.PZ5 Washington/Oregon Waters...
.PZZ800...Inner Waters from Cape Flattery to Cape Shoalwater...
     Gale tonight.
     Gale Wednesday.
     Gale Possible Saturday into Saturday night.
.PZZ900...Outer Waters from Cape Flattery to Cape Shoalwater...
     Gale tonight.
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night into Thursday.
     Gale Possible Friday night into Saturday.
.PZZ805...Inner Waters from Cape Shoalwater to Cape Lookout...
     Gale Wednesday.
     Gale Possible Wednesday night.
     Gale Possible Saturday into Saturday night.
.PZZ905...Outer Waters from Cape Shoalwater to Cape Lookout...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.
     Gale Possible Friday night into Saturday night.
.PZZ810...Inner Waters from Cape Lookout to Florence OR...
     Gale Possible Wednesday night.
     Gale Possible Saturday into Saturday night.
.PZZ910...Outer Waters from Cape Lookout to Florence OR...
     Gale Wednesday.
     Gale Possible Wednesday night.
     Gale Possible Friday night into Saturday night.
.PZZ915...Outer Waters from Florence OR to Point St. George...
     Gale Wednesday.
     Gale Possible Wednesday night into Thursday.
     Gale Possible Saturday into Saturday night.

.PZ6 California Waters...
     None.

$$

.Forecaster Mills/Bancroft. Ocean Prediction Center.


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