San Diego is pretty dry, with an average of 10.3 inches of rain a year; an average of about 3.6 days per month with some precipitation - ranging from one day in June, July, August and September to seven days in February and March. That small amount of rain tends to leave the roads pretty slippery for a 2 wheeler since it isn't enough to wash away the oil from cars. On average, there are about 263 sunny days per year in San Diego so there's plenty of days to ride without having to venture out in the rain.
My experience riding in Columbus Montana was somewhat similar with about 14 inches of precipitation per year - 69 days with any measurable precipitation, and an average of 202 sunny days, per year. The difference in Montana is that some of the precipitation is in the form of snow and some of those sunny days can be below freezing.
Everett, Washington is a different sort of place with an average annual rainfall of 37 inches which is spread out over an average of 148 days with measurable precipitation.
If I stuck with the "no riding in the rain" rule I'd end up not riding - 43 days in San Diego, 69 days in Montana and 148 days in Everett. The comparison with Montana isn't fair though since there's probably 148 days where there's either snow on the ground or it's so cold that you wouldn't want to ride a 2 wheeler.
The nice thing about Everett is that even though it gets some rain, it generally isn't that cold. Snow is fairly rare here and the average monthly low temperature is never below freezing so most days are okay for riding. I wouldn't want to spend much time on the busy Interstate Highways around here in the rain, particularly at night - but for my daily sixteen mile 35 mph commute - riding in the rain is fun.