Your pretty much good to go until mid-October in the Sierra’s. In '05 I finished a flip-flop PCT thru-hike on Whitney on October 9th. I talked to many locals about the weather and they all said the same thing - normally only minor (up to 6") snow until October 15th.
In '05 I had excellent weather through late September and early October - daytime highs in the 60’s and nights around freezing. There was one small ‘storm’ in late September that dropped about a half a foot on the highest passes and Whitney. This didn’t pose a problem at all, except on the climb down from Whitney.
Whitney can be very tricky when the never ending switchbacks on the Portal side are icy. This was the one area that made me slow down and be really careful - as I wasn’t carrying an ice-axe or crampons at this point.
Happy Trails!
freebird