Second half of August... Are you planning a Whitney hike in that period this year? Here's what I would do: Watch the weather forecasts in the two weeks before the trip. It will be either dry and perfect weather, ....or thunderstorms.
August, in my mind, is the most prevalent time for t-storms. If there is any sort of system blowing in from the south (they come in off the Pacific Ocean over Mexico, and then head northward. In that situation, there are most often thunderstorms with even hail and cold on the summit. These are the heaviest storms in August.
The second situation is without the storms out of the south. If there is a hot and more humid pattern west of the Sierra, huge thunderheads can build over the Sierra every afternoon. Those pretty much always dump in the afternoon in the mountains. They aren't uniform -- some areas of the mountains can be missed.
In either of the above weather patterns, you should plan on carrying rain gear when hiking Whitney. Otherwise, only if the weather is steady and dry could you skip the foul weather cover. (but I would at least carry a big trash bag to use in an emergency).
