This is a wind model NILU

North America's continent will be covered from this model. There are models for three different isotopes: I 131; Cs 137 and Xe 133. The model changes with the winds and is modified every 6 hours. And it covers Asia, Pacific, Japan, North America and the Northern hemisphere Check it out. (I found the link from http://news.sciencemag.org/ and thus trust the source.