I've heard reports of the first buds appearing on the dogwoods in Yosemite Valley. This puts them on track for their typical blooming period, which usually begins near the end of April and continues through about mid-May. I prefer to photograph them when they first bloom, before the leaves get large enough to obscure the blossoms. You can't photograph them from behind, as I did in the photograph above, after they're leafed out. If you miss the show in Yosemite Valley, they bloom about two weeks later at higher elevations, like the Tuolumne Grove of giant sequoias.
Of course the main pursuit for Yosemite photographers in the spring is waterfalls. This is shaping up to be a typical spring, which means the peak flow should arrive around the end of May or beginning of June. Meanwhile the water flow will fluctuate with the weather—the warmer the temperature, the higher the runoff.
Yosemite Falls, the big spring attraction, gets basically terrible light this time of year. But the other big three—Vernal, Nevada, and Bridalveil—are all positioned to receive late-afternoon sunlight. You can find rainbows on all three between about 5 to 7 p.m. depending on your viewpoint. The Mist Trail opened recently, giving access to Vernal and Nevada Falls. The trail is usually crowded, but for good reason—in the spring it has to be one of the world's most spectacular day hikes.