Summer is definitely the safest time to make the trip. Getting on to
winter a series of lows march across the
North Pacific on a weekly basis. When I say 'low', read storm. The chances are very good you'll get hit by a significant storm at least three times if you leave other than late May through early September.
The typical
route for a sailboat is to
head north from
Hawaii and go around the top of the high at anywhere from 40-60 degrees north depending on its location. Typically it's a close reach out of
Hawaii that will ease to a beam reach as you go around the High. Even in summer, it will be a cool trip as sea
water temps will not be above 60 degrees once you leave the environs of Hawaii. Even in Summer, you can get some nasty
weather.
Boats returning from Hawaii after the
Single Handed
Race in Augustna couple of years back got a succession of lows that resulted in one being abandoned. One
skipper, even though very experienced and familiar with his
boat, abandoned it largely due to exhaustion with the prospect of many more days of foul
weather that had driven him way south.
Another strategy to use if you have plenty of
fuel and want to go to points further south on the mainland involves powering through the high. You'd sail North staying as hard on the
wind as you can comfortably live with till you enter the high. In the high, there is little
wind so you plot a rhumb line course direct to your
destination and
motor till you come out of the high on the other side. Once you get wind which will typically be NW, you then sail on a reach to SF or LA for the final part of the
passage. Might even
work for Seattle depending on the location of the high. This takes lots of
fuel as you will have to
power for a week or more to get through the high. Know of one guy who made the trip several times lashing a 40 gallon drum in the
cockpit to augment the
boats fuel tankage. His last trip had to be abandoned. The wave conditions were pitching the boat about so much the lashed barrel threatened to break loose before he reached the high. He turned around, sailed back to Kona and
sold the boat.
Last and not usually possible is to sail under the high. That will usually result in a beat the whole way. A very uncomfortable and wet trip. Shortest distance though probably not the quickest way to get tbere. A sailor did it this past
winter to SF without
self steering. He hove to at night and was on a very
small boat so he spent several months making the
passage, IIRC.
You should be able to find many accounts of the trip in 'Lattitude 38' magazine if you search their
archives.