Any idea when is the warmest time on Whitney Portal and the mountain? First week in August?
Typically August is the warmest month.
The other comments notwithstanding - while it's prudent to be prepared for anything, the weather in the Sierra is perhaps the most stable of any major range in the US. The exception to this might be an El Nino year, when the incidence of afternoon t-storms may be similar to that found in the Rockies.
Watch for long-range forecasts if you're doing a multi-day hike and you can greatly reduce the chances of being caught up high in summer storms.
