The Sierra Crest is noted for having the capacity to produce four seasons of weather during a single Summer Day.
Rain, snow, hail, fog, lightening, and a bit of sunshine can all happen in quick sequence.
The High Sierras are noted for rapid changes in weather and temperature, and the sudden appearance of harsh weather trends. You may experience anything from intense heat to bitterly cold Summer snow storms. Or you may not.
In any case, weather must be considered and proper gear preparations made to deal with the range of weather BEFORE entering the High Sierras.
Long Distance Backpackers who were on the High Sierra Crest in early August of 2009 experienced just such a series of Summer Snow Storms.
Temperatures dropped to the high teens and low twenties, snow and hail fell which partially obscured the trails, and all of those who entertained the misconception that the Sierras only have fine weather during the Summertime were bitterly disappointed. Luckily for the many backcountry travelers who were not properly prepared, those storms blew apart, rather than consolidated and deepened.
Weather considerations should guide your gear selection.
Weather Preparations for Backcountry Travel
Always check the National Weather Service Satellites, Maps, and Forecasts
The NWS Satellites will give you an orbital view of potiential storm activity far off the coast of California. Combined with the NWS Maps and Forecasts, you can anticipate weather. Note: especially check the 28km infrared and water vapor animations.
Check the National Weather Service Maps
NWS Maps Note: Especially Check the 24 to 48 Hour fronts and Precipitation Maps.
Consult the National Weather Service Home Page
The National Weather Service Home Page displays large and small scale weather warnings which may pertain to your High Sierra backpacking plans.
All Sierra Web Cams: Sierra Visions