There is plenty of good water that is open year round. They can be found under the Special Regulations Section from PFBC. Many of these streams are the more common and popular streams found in the state. The biomass is typically pretty good, but there are some which are stocked and don't hold over the winter as well as others.
What you will want to do is educate yourself on what are looking for in the type of water: limestone streams, freestone streams, stocked, naturally reproducing and of course your geography. I wouldn't focus only on Fly Fishing Only all these streams offer something for a fly fisherman.
I would start here: http://fishandboat.com/specreg.htm
There is a lot of information found on this site of course. You can look into the stream reports to read about fishing that has been done over the last four months and compare that to some of what you have come up with.
After you have done a little homework with that I would suggest asking specific stream questions in the
Fly Fishing Locations Forums.
Some of the other factors for you might need to consider where you will be staying and the weather too. Generally, State College, Huntingdon and Carlisle are all towns that put you close to these streams.
