3 season sleeping bag for Sierras

nm

"the answer is within"

having read all posts above mine…here's my $0.02

the real key to your answer is you – r u a cold or warm sleeper?
not knowing can be a real disadvantage in this context.

in the end however, perhaps a better question 4u is this: If you do not have a down jacket, and thus cannot augment your bag…then is the weight difference b/t a 20 and 30 degree bag really something to deliberate? Would you risk your safety and/or a sleepless night over half a pound?

Be safe. Be warm.
happy trails.

"the race is long and in the end, it's only with yourself"

Having frozen my a$$ off in August on the PCT in a high velley (24 F. at 8,000+ ft on the way to Olancha Peak) I recommend an OVERSTUFFED WM Megalite bag. Warm at 25 F. and large enough to accomodate my EB down jacket (in my avatar).

Taking a light down jacket into the Sierras or Rockies in summer is wise. Nice around camp in the AM and just before bedtime and good enough to keep you warm when the temps drop 10 F. below your bag's rating.

But if you can get a 20 F.down bag WITH DWR treated down do it and you'll never regeret it.

June, July, August in the High Sierra

I just take my 1.3 lb synthetic 40* bag and shiver a bit if its not enough.
During the day when I'm hiking I never regret the decision.

I'll echo the suggestions for the 20-degree bag and make a suggestion for a killer bag at such a low price that you will be able to also buy the Costco Kirkland down sweater and a really large tri-latte delight at a yuppie coffee joint: the Kelty Cosmic Down 20.

It's reviewed on this site in the BPL reviews and in several threads. You will find it under $150 and it is excellent. Several of my Scouts and adults who've spent time in the eastern Sierra (Garnet, TI, Clark lakes,Kearsarge Pass, PCT, JMT) with me will offer their endorsements whenever asked: it's warm, it's light and it's very affordable. When the weather turns to "surprise mode", it's a great bag for 3-seasons.

The Cosmic Down 20 even comes in a shrimpy-size for midgets under 5'5" that weighs 2 lb, 3 oz… and can be found for as low as $70 on sales right about now…

"WM Ultralite (20*)"

+1 particularly if you don't carry good insulating layers. Even if you do, the Ultralite will reliably extend your season on either end by a month. If you go that route it would be well worth your while to have WM add 2 oz of overfill to the bag.