Perhaps late blight. If so, it's the end of the road. Remove absolutely all debris and put in trash.

Where in CA are you? I ask because in some parts, late blight is very uncommon.

That said, the disease can be facilitated by overhead watering -- via rain or sprinkler or hose.

Many excellent photos of Late Blight here, to help you with identification; also links to info:
http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm

Nothing's quite as swift as Late Blight can be. You have my wholehearted commiseration! (Particularly since I may have found a bit on a leaflet this morning.)

I'm San Francisco. I feel so bad for my tomato plants. So they are pretty much goners then? Looks like a bomb hit the center of them..

If it's Late Blight, those which are badly infected are definitely goners (sometimes completely destroyed within 48 hours).

Those which aren't badly infected can be kept alive for a while if you remove the damaged parts immediately and dispose of them properly. Some of those plants may last for weeks; in 2009, some of the varieties I grew seemed resistant, but they all succumbed eventually -- but in the meantime, I got some edible fruit from some plants. One grape's foliage remained immune, but eventually all its fruit rotted.