Hello,
First off, seeing as your data is in a JavaScript array, this really isn't a PHP-related question; the DHTML forum would have probably been more appropriate...
Anyways, what I believe you want is known as dynamic selection boxes. A quick google search brought up
this page, and there are others if you'd prefer a different solution. For instance,
this one uses list boxes instead of drop-down select boxes.
Basically the concept is dynamically changing the contents of a <select> element using JavaScript, which is fired by the selection of one of the "higher-up" select boxes. I suggest you look around the web and find some code samples you like (i.e. the links I found) and work your existing arrays into the code. It shouldn't be too complicated to integrate your existing data once you have the actual DHTML part of the code in place, but of course once you get that far you can post here again if you need help :)
hth
-snib