The lights bivy that I can find that comes with a waterproof bottom is:

http://www.backpackinglight.com/cgi-bin/backpackinglight/oware_drawcord_bivy_sack_dwr.html

I am looking for the lightest bivy/tarp combo that will still keep my dry and safe during those moments in the colorado mountains that it decides to snow in the middle of July.

I don't really have a problem finding the right tarp....looks like weight will be between 6-13 ounces for something that protects from rain. I want to keep the bivy around 6 +/- 3 ounces. I don't want a partial bivy because this bivy will be the only ground cover I have for my sleeping back. The drawstring bivies seem pretty cool because I lose the pole weitht and I can just hang it from the top of my tarp and still keep the mesh off my face.

I appreciate any suggestions.