A river of rain lashed parts of Northern California Friday, flooding roads, leaving thousands of PG&E customers without power and forcing area airports to cancel hundreds of flights. To which the Bay Area soggily sighed, “Welcome! May we have more, please?” Friday afternoon, an enormous rainbow appeared briefly in the skies over Mineta San Jose International Airport, as if to open the storm door to a jet stream that has proved stubbornly resistant to delivering rain to California.