I know the "conventional wisdom" is to wait until August. The thinking is probably that this is the month with the least amount of storms. It is the best month of the year for NW gales and fog.
I have been pinned down on anchor for 7 days and nights in Aug. when it blew gale or better NW in Shelter cove. The rigging never quit howling...The NW wind sets up as the interior land warms up, and the cold pacific air rushes in to replace the rising air. Hot air + cold water = lots of wind.
The warmer it gets, the harder it blows unless it socks in with fog
April, May, and early June is your best window to go north and not have to fight the NW winds. You have a great chance of variable or south winds in these months.
If you go in Aug. it will blow in 3 or 4 day waves typically. It will blow till it fills up the caost with cool air and the fog rolls in and things go quiet for a day or two before the fog burns off and the cycle starts again.