The National Geospatial Intelligence Agency has a link to an atlas of pilot charts. Google "pilot charts" to find the link then poke around looking for the charts of the North Pacific then select the month(s) of your trip to see the chart(s). This will give you a general, long range idea of conditions of interest to the mariner in open ocean. The Straits of Juan de Fuca aren't very well covered by the pilot charts.
The wikipedia site does address the climate there, but it may not be too useful for your purposes. Perhaps a short term forecast as you approach the Straits would be more useful.
Hope this is helpful
Rich