Black oaks in Yosemite Valley begin to leaf out in April, the dogwoods bloom in early to mid-May and western azalea bushes bloom in late May and early June. Summer is warm and dry with less than 5 percent of the year's precipitation. Daytime temperatures in the valley sometimes climb above 100 degrees but nights are generally cool. Even in midsummer, temperatures in Tuolumne Meadows, at an elevation of 8,600 feet, are usually in the 70s during the day and 30s at night. Sudden thunderstorms sometimes bring dramatically changing clouds, intense but brief downpours, lightning, hail, and gusty winds.
Lightning is a danger for hikers, especially on mountaintops.
Anyone planning a hike far from shelter should check before leaving to see if thunderstorms are expected. Since melting snow feeds the waterfalls, some are mere trickles or even completely dry in August. During the fall, nights turn cooler, but days remain warm through October. September and October typically bring clear, brilliant days as the trees take on their fall colors. Autumn color peaks in late October and early November.