Blue Lightning (Green Ray?)

I was sitting on the couch tonight, chatting with my parents about earthquakes and strange behaviors of cows (more on that later) when I was startled by a small and then larger, bright flash of blue-green light coming from west of the Golden Gate Bridge. My first impression was that it was lightning, except there was no thunder, the color was wrong, and it was too low in the sky and without the usual bolt of light seen with lightning. Hmm, well perhaps it was an explosion? But I heard no explosion, and haven't heard about any explosions on the news here. Okay, maybe fireworks? But what is the occasion? And it was 2 quick, isolated flashes, not resembling the normal appearance of fireworks. I posted a listing on the local news section of craigslist asking if anyone else saw it, and have had 2 responses so far. One guy suggested that it could be a green flash, which is a meteorological phenomenon typically occurring right after sunset. It seemed like a possibility because of the foggy, cloudy conditions tonight, but it was 2 hours after sunset, and green flashes typically occur right as the sun sets, or soon after.

I emailed an astronomy professor in London who wrote an article on green flashes. I hope he might have some insight. Perhaps it was a green ray?

Here's the basic info.
I saw it around 7:10 PM on November 5, 2007.
It appeared near or to the west of the Golden Gate Bridge, north and slightly west of where I viewed it in the approximate center of San Francisco. I would estimate the distance at about 5 miles.
It appeared to come from the same elevation as the Golden Gate Bridge, or a little lower. It appeared to originate from below and flash up, brightening the sky. I believe there was a smaller flash before the larger, brighter flash. The bigger flash was only a few seconds long at most.

I suspect some transformer could have exploded, or perhaps it originated from a ship on the Bay. But I really don't know. I find Roswell and the X-Files amusing but don't really buy into that stuff, so I'm looking for a more plausible explanation.

Did you see it?

Follow-up
So thanks to my friend Sarang, the mystery has been solved. According to the San Francisco Chronicle, a plastic balloon flew into some power lines on 2nd and Clement, causing them to spark and resulting in a power outage. Who knew a balloon could cause something like that? Here's a great youtube video of a branch falling on some power lines. These guys were lucky:


Thanks also to the two astronomy professors who humored my requests and wrote me back explaining that it could not be a green flash so long after sunset. Dr. Andy Young from SDSU was closest to solving the puzzle. He said,

"It certainly sounds to me like electrical sparks between copper conductors. Are there any trolley lines in that direction?. . . Arcs between copper wires can also be caused by falling tree limbs, large birds, etc. -- though of course wires rubbing against something they normally wouldn't can also happen in high-wind conditions."

And I particularly enjoyed the email from Dr. Mike Dworetsky, Director of the London Observatory in London, UK, who enlightened me on green flashes, cloud-to-cloud lightning, and sprites. In the end he offered my favorite explanation:

"Hmmm. Nov 5th: It's Guy Fawkes night in England, traditionally celebrated with fireworks, so maybe a long-shot guess is that a British ship was letting off fireworks at sea...or maybe a British family on the Marin peninsula or at Drake's Bay?"

I'd like to see if there was a Union Jack on that balloon!