Hi Andy,
For pelagic species out of Monterey I recommend you look into Monterey Seabirds here:
http://www.montereyseabirds.com/ and Monterey Bay Whale Watch (see link at bottom of page on link above). MS usually runs a May pelagic trip every year. This year's trip was on the 20th. Their 2013 schedule won't be announced until Jan/Feb of next year. The MBWW trips are run almost everyday of the year. These trips are after whales so you don't necessarily get into deep water for long periods of time, nor do they necessarily have dedicated seabirders aboard. That said, these trips are less expensive and shorter in duration, you can often get on board at the last minute, and they routinely see some of the more common pelagic species in the bay that time of year such as Black-footed Albatross, and Sooty and Pink-footed shearwaters. These trips can be a great way to get out on the bay during times when dedicated pelagic trips aren't scheduled. The whale watch gang obtained nice photos of a Laysan Albatross just the other day. I would stop short of guaranteeing a Black-footed on one of these trips, but the odds are at least pretty good you will see one.
Some additional good sources of information online include "The Birding Lists Digest" here:
http://digest.sialia.com/ and eBird at ebird.org/ .
Weather during May along California's central coast (Monterey) is often dominated by NW winds. Fog is often present but often clears by mid-day. By May rain is unlikely, but not unheard of. Storms this time of year usually don't last long. Sierra Nevada weather conditions range from sun to rain to snow this time of year depending on elevation, latitude, and cold fronts coming in from the North Pacific. However, generally by May most significant precipitation events are over for the winter/spring, so getting to the mountains is normally not difficult. However, high elevation passes are usually not open in the central and northern Sierra before the end of May. Needless to say, local conditions in May 2013 are a bit difficult to predict now.
May can be a fine time of year for migrants, both along the coast (though better in April) and in the Sierra foothills. By this time along the coast and in the Coast Range many breeding species are on territory and nesting. Breeding occurs later at higher elevations. Keeping abreast of what locals are reporting via the Digest is a good way of learning what is around and where to look for it.
Dan