Froome has looked unconquerable thus far, but as the saying goes, it ain't over until it's over. A bad day or a crash can doom anyone, and there are always wildcards in cycling. Tomorrow's wildcard could be the weather. The Alps are presently under the influence of a weak trough, and the models call for showers and thunderstorms to develop over the region during tomorrow's stage.
Addendum @ 5:00 pm:
More on this at Velonews.