Oof, I didn't know you /already/ had a rain jacket with you. I suppose
the over-the-head thing might have worked with breathing holes, but it's
a bummer when those things slip and cover your eyes on a 40mph descent. :)
But I had read too quickly and "heard" Mt. Hood when you "said" Mt.
Bachelor, so I assumed this was a shorts-and-jersey ride in a warmer clime.
Frank has already noted the newspaper-up-the-jersey-front trick (which
does work; I have used century route sheets at times in desperation,
though crumpled newspaper works better, though I wouldn't try it in a
downpour.)
I've also had good results with a plastic grocery bag between two thin
jerseys in a pinch, during dry-but-cold rides; keeps drafts off the
chest, and is easy to (responsibly) dispose of at your first stop when
it warms up. For those not-cold-enough-for-a-jacket rides.
Fortunately, summer has arrived/returned to western Oregon for now. :) :)
Mark J.