School Closings in Michigan Today?

Why do some School Closings in Michigan today, why? I heard it in the radio, there's going to be lots of closing in Michigan today but for what reason?
Could anyone explain me?

I live in Michigan and found out this morning that my school is closing because of the weather and the snow....
Some school in other states closed for the same. There were lots of school closing a few months ago in Michigan because of the swine flu, now it's the weather.