This is a really good site to use when trying to see if there is any precipitation and where it is. We know that radiation comes down with the rain and much of that can be radon washout. If you see the Japan radiation sites going in to RADCON levels, use this link to make sure the sites going in to RADCON levels do not have any rain/precipitation near them.
If you see the sites/sites going RADCON level in Japan and yet this radar map does not show any rain/precipitation near that area, this could be a sign that a real radiation event has happened.
http://www.jma.go.jp/en/radnowc/