The ornamental design for a weatherbonnet, as shown and described.
FIG. 1 is a perspective view of a weather bonnet showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof; and,
FIG. 6 is a top plan view thereof.
The broken-line outline of the partial human figure in FIG. 1 is environmental subject matter that assists in showing how the claimed design may appear when worn, but forms no part of the claimed design.