There are a number of weather stations throughout the park.
Depending on what exactly you are looking for... cdec is the way to go for backcountry info imo.
You can get a decent idea of how much snow is out there and the river flows without tooo
much trouble. What stations record varies... so you have to poke around.
And many snow surveys are now done by either Satellite imagery or by foot.
Anywho... I swear I did some analysis of stream flow from a number of years in a thread ..
but can't find it... you can get all kinds of historical info from cdec.
For example:
Maybe go here and play around:
http://cdec.water.ca.gov/On the left... maybe just click on station locator to find the station you want... and then go back and click on historical data.
This is about the only one I use for historical data...
Couple this with trip data from previous years and I find it not tooooo difficult to get a general idea of conditions out there.
YMMV
Have fun