What forebodings does Gandalf have as the company sets out from Rivendell?

"Even the good plans of wise wizards like Gandalf and of good friends like Elrond go astray sometimes when you are off on dangerous adventures over the Edge of the Wild; and Gandalf was a wise enough wizard to know it.

He knew that something unexpected might happen, and he hardly dared to hope that they would pass without fearful adventure over those great tall mountains with lonely peaks and valleys where no king ruled. They did not. All was well, until one day they met a thunderstorm - more than a thunderstorm, a thunder-battle."