Lowland Snow - Giant Flakes - How do they form?

We saw flakes coming down today in the Shoreline convergence zone that were up to 3 inches in diameter. They looked like birds floating down, or like whole snowballs gently floating out of the sky.

Anybody know what conditions caused them?

I'm no scientist and I've never played one on TV, but if you're asking me... Ramming Speeeeed. But it may be more complex than that

It was beautiful! The ones falling on Phinney Ridge this morning were as big as cotton balls.

A meteorologist explains

Physics Fact Book

Department of Energy answer

The huge flakes yesterday in Seattle around 8am were probably the largest I've ever seen.  UW prof Cliff Mass mentioned them in his blog ( cliffmass.blogspot.com/ ), with his explanation that "The snow is full of large aggregates (many crystals stuck together)...that is why they are so large (the surfaces of the snowflakes are melting and wet...and this acts as a sort of glue to allow them to stick to each other). This happens in marginal temp situations, like we have now."

Here's a nice article from the NY Times:
Snowflakes as Big as Frisbees?
www.nytimes.com/2007/03/20/science/20snow.html

UW prof Cliff Mass mentioned them in his blog ( cliffmass.blogspot.com/...http://www.nyt.../science/20snow.html

I'm reading Mass' book on NW weather right now; it is interesting and well-written and a link to it is on his blog.