been wanting to do this myself for like forever. it looks like they are using high level summary data from the NWS ... they also have this amazing thing called MADIS which focuses on micro climate research (and more) ... so in theory you could get a high resolution version of this for san francisco or nyc, and that'd be really great and possibly rather useful too. I think the Weather Underground is a subscriber of MADIS, and their "rapid update" feature has some of this, and also I should mention some of this is collected by ham radio people in their spare time too.