What kind of spider is this?
This is probably Araneus marmoreus var. pyramidatus.

Here is a better image for comparison:



Source: http://www.uksafari.com/marbled_orb_weavers.htm

PS: Despite A. marmoreus being found in US, this variety (pyramidatus) is  normally an European one. I don't know any observation of it in US. However, I'll leave this answer here until another ID is provided.