Mammoth's track record is that they will operate as long as they think there is enough snow to attract us L.A. weekenders. That usually means skiable to the Main Lodge. I have seen them operate just the upper mountain with gondola for access a few times. The decision is not made far in advance, as the base depth could hold up much better (2003) or worse (2001) than normal. Mammoth has been running a week or two into June with a max season snowpack of 9 feet or so. The current base is 14-16 feet, so I have to believe that July 4 is a better than even chance this year, even though the last time they were open that late was 1998. You should probably call the area, maybe marketing director Joani Lynch. If they are still sitting on 15 feet at the end of March, they may be quite certain of July 4 at that point.
Despite skiing being restricted to 3-4 hours in the mornings on about 1,000 acres, there should be enough going on to keep you busy for a week. You can fly into Reno (3+ hours drive) and visit Tahoe (Squaw will have some skiing but much more limited than Mammoth) or fly into Las Vegas (5 hours drive through Death Valley, which will be about 45C that time of year).
While at Mammoth you have nice lakes and hiking available nearby in the afternoon. Tuolumne Meadows/Tioga Pass are one hour from Mammoth and Yosemite Valley (with waterfalls at the max) is 2 hours away. Also with the Sierra snowpack we're going to have an excellent whitewater rafting season this year.
As I said before any family members you bring along will have a great time whether they ski or not.
