Visiting in May
Yosemite's waterfalls (mostly in Yosemite Valley) usually peak in mid- to late-May, so this is the best month to see them. While it may seem like summer, snow is usually present in much of the park, and the Tioga and Glacier Point Roads may be closed.
The Tioga Road (Highway 120 through the park) and Glacier Point Road are closed, typically until late May or early June. While it's impossible to predict when these roads will open because April and May weather significantly affect plowing, you view a list of opening dates since 1980 to get a feel for when they can open.
Areas to Visit
Hetch Hetchy, a valley similar to Yosemite Valley that was dammed, is generally snow-free and Wapama and Tueeulala Falls are full.
The Mariposa Grove Road and the Mariposa Grove are closed for a restoration project until approximately spring 2017. Get additional information, updates, and information about nearby sequoia groves.
Fire & Smoke: Hazy or smoky conditions may occur even during spring. In some areas, the park may have prescribed fires to mimic natural fires to promote forest health and reduce the risk of larger fires; conditions in spring are often good for such fires. Less common, but possible, are fires in the region or in the park. Lightning-caused fire occurs naturally in the Sierra Nevada. Additionally, accidental and arson fires can also occur, but firefighters typically attempt to suppress these fires immediately.Hiking and Backpacking
Hiking and backpacking options are still limited, even in May. Expect snow above 7,000 feet (this varies from year to year). Day hikes from Yosemite Valley are usually snow-free, but trails along the northern and southern rims (e.g., El Capitan-Eagle Peak-North Dome and Pohono Trail) tend to be snowy and/or flooded. The Four Mile Trail to Glacier Point is closed, usually until late May, but may be open to Union Point, about halfway up.
Outside of Yosemite Valley, trails in Wawona, Mariposa Grove, and Hetch Hetchy are usually snow-free, but may be wet.
Few multi-night backpacking options are available in May, depending on conditions. The trails in best condition will be those leaving from Yosemite Valley, Wawona, and Hetch Hetchy, but it's unlikely you'll be able to find a hike more than two nights long that doesn't involve significant snow. Wilderness permits are required and may be difficult to get.
The Upper, Lower, and North Pines (Yosemite Valley), Wawona, and Hodgdon Meadow Campgrounds are on the reservation system with no sites available the same day.
Horse/mule rides: The Valley Stable typically opens in late April, conditions permitting.
Biking: Bicycle paths in Yosemite Valley are snow-free in May and an excellent way to get around eastern Yosemite Valley. Bikes are available for rent at Yosemite Lodge and Curry Village.
Rafting: The Merced River is usually closed to rafting and kayaking in May, except following relatively dry winters.