MY troop and I are planning to Backpack from Onion Valley to Whitney Portal on a 7 day trip in late july. I am an avid ultra light backpacker and am look to know how a 5x7 tarp will do on the trail as a shelter. Also I would like to know the avg. temps for this time of year too. I will be using an 45 li pack. will this be enough?
Just asking these questions implies that you need to gain more experience with ultralight backpacking before taking it up to the High Sierra for 7 days. Think about developing your skills and refining your gear on more forgiving terrain with less consequence, but with challenging weather, and you'll figure this out for yourself. Your 45L pack size depends on the whole of your gear, so there's no way for people on a forum to help with that. I'd guess that 45L is a bit small and you probably can't fit a bear canister.
Stakes will be difficult in many places and you'll need to rely on tying off to rocks. Good chance you'll still get wet from wind-blown rain under a 5x7 tarp so you should have a second level of protection like a light bivy. By the time you add that up, you might consider a tarptent with a real floor and netting to keep the bugs out.
Bottom line, it seems that you might be pushing things a bit too far. There's a good chance you'll squeak through in late July, but you need to be prepared for some nasty weather such as monsoonal rains. You cant' rely on forecasts over a 7-day trip into the high country.
