I posted my theory on the Vegetable Forum. I have read that a few CA growers have had their onions bolt this year.....my theory is the really cold winter weather we had in Cali this year....but I don't have any proof of course...

My red and white onions from Dixondale bolted.....my browns didn't, but also didn't bulb either.....go figure....just one of those years I guess.

Here is a link that might be useful: Onion Bolting Theory

Mine are grown here from seed. Yellow seed I bought. Stockton Red grown from seed grown here. Neither bolted at all. We had one day down to 20 F. Otherwise a very mild winter.

Ooops! I posted my comment on the the onion bolting theory thread

Among other great things about the pecularities of growing onions in the bay area, here's what "Golden Gate Gardening" says
"Gardeners want the biggest bulbs possible, so they plant as early as they can. or even start with partly grown onion sets. But if the seedlings have made plants with lower stems larger than a pencil in diameter when the grip of winter sets in in December, the plants are likely to bloom before they can bulb up."