>"What's going to be the earliest to go down there where it won't be raining the whole time, cold, and just nasty? March? April? Later?"
Well, the last 4 days were good on the Lost Coast, sunny days but cold nights.
(Note: We usually have a 'nice' spell in January).
That said, it usually doesn't stop raining on the North Coast until July through mid-September. At that point, you're often getting typical summer weather... fog on the coast and sunshine inland. Hopefully, the fog blows away on the coast during the afternoons, but that means wind.
I recommend Benchmark Maps... California (and other states). Good for route planning.