I've seen it online as the sign for many churches. The churches themselves look buried in snow, and it seems the sign person had enough: "WHOEVER IS PRAYING FOR SNOW, PLEASE STOP." It's a funny sign, and it makes me chuckle every time. But there's also a belief behind the message: prayer works. Someone (or multiple someones) has been praying for snow, and that prayer was answered -- in spades. How do you stop the snow, then? Stop praying for it!
Jesus tells us in Matthew 21:22, "If you believe, you will receive whatever you ask for in prayer." Earlier in Matthew, he says this: "Ask and it will be given to you; seek and you will find; knock and the door will be opened to you" (Matt. 7:7). 1 John 5:14-15 puts it this way: "This is the confidence we have in approaching God: that if we ask anything according to His will, He hears us. And if we know that He hears us—whatever we ask—we know that we have what we asked of Him."
Those are powerful verses. If we pray in faith, and if our prayer is according to the will of God, He hears it and gives us what we requested. That should tell us how we should pray: in faith, according to His will. If we pray for our own will, our prayers are answered with "No." And if we all got everything we wanted, regardless of how good or bad it would be for us, imagine how evil the world would be. Just as in the Lord's Prayer, our prayer should always be, "Thy will be done." If Jesus himself can pray, "Not my will, but Thine" (Mark 14:36), so can we.
Even if that means we end up praying for snow.