As someone who LIVES in Mammoth Lakes, ca...please come and visit !
The summer months are nice but we do have a lot of visitors.
Over the last 13 years I have lived here, by far my favorite months are may and mid September through late October. This is due to the low visitor turnout and the fact the generally the weather is still fairly stable.
It is a pain in the ass to access though. Hwy 395 off the 14 out of the L.a basin is the only road north, or driving hwy 395 south out of Reno is truly beautiful as well.
From Yosemite valley to mammoth lakes is about 2.5-3.0 hours depending on how many blue haired winnebago driving grannies you get stuck behind, lol.
I have expirienced snow in All months of the year, although from may until mid oct it is usually measured in inches. Oct 16th 2004 we did receive 5 feet in 48 hours.
This is very unusual though.
The mountain and road biking is wonderful, as is the Climbing, Hiking and mountaineering ! Truly a place for 4 season adventures.
Hope this helps a bit ?
If you are going to be in California for 6 months or more then take your time.
It is a very loong state..800 Miles (1400kilometer) and wide 200 miles(320 kilometer) so be prepared for that.
LosAngeles to Mammoth lakes is about 3.5 hours...
LosAngeles into Yosemite Valley is about 5 hours...
LosAngeles along the coast to/from SanFrancisco is about 10 hours.