I use a Giant brand handlebar bag for my long rides with unknown conditions. holds a spare tire, full fingered gloves, lunch, spare co2, arm warmers, leg warmers, and a wind breaker. Plus it has a clear top where you can insert a route sheet or other notes you may have. Used it on every self supported century I've done, and while it sometimes goes unused, the times I've needed it have made me very glad I had it.

http://wheelworld.com/product/giant-...ag-44673-1.htm