Dan,
Get yourself 2 different jackets. One should be a very large, very well made Gore-Tex jacket with hood. This will handle any 'bad weather' situation (incessant rain, snow, sleet, etc.). The other should be a rather large, thin urethane-coated nylon. This washable jacket can be used (and stored right in your bag) for any spur-of-the-moment bad weather situation and won't mind the sweating you'll do in it - as it can be washed a lot.
Karl
Ps: Also - very important - make sure that the arms are cut on the bias so that they're either "articulated" or at least there is NO difficulty in lifting your arm over your head (the rest of the jacket doesn't "come with your arm"...and obscure your view, etc.) just in case you have to throw an overhand
