Dan, 63ChevyII pointed out in this post that the forecasts have been predicting the low about 10 degrees higher than actual. He was watching the actual temperatures reported at the Cottonwood Lakes remote station just a few miles south of the Whitney Trail.
(Link is on the Whitney Weather page: Cottonwood Lakes, elev. 10150' 7-day Temperatures

Based on that, I would plan on 20 degrees instead of the 30. Unfortunately, that means all your water will freeze overnight, so plan accordingly. Fill up in the morning, and anything you want to keep overnight could be heated and then put inside your sleeping bag, or at least insulated somehow.