The parched Bay Area is grateful, heading into America’s day of Thanksgiving, for any rain we can get. It was coming; the weather folks said so, the doppler radar said so. But for a couple of hours after the storm was expected to start on Tuesday morning, it was all expectation and no precipitation. Birds sensed it, fall leaves tossed in sudden wind, clouds gathered, but … nothing. It finally did rain, for a bit, and temperatures dropped, but now we’re facing a weekend of crisp winter sun. For a moment before the clouds opened, though, the skies played with a dramatic duality: the dark stormy look that tradition defines as “threatening,” and a shard of rainbow to remind us that the threat, in a period of drought, is also a promise.