Is it worth visiting Yosemite National park in the beginning of May? Are the hiking trails already open?

This answer was originally written on Quora

Yes, Yosemite is worth visiting in the beginning of May. Most trails are open and accessible year round with the exception of a few that are very dangerous in winter conditions (ex. Mist trail, 4 mile trail). You’ll want to check current conditions to get the best idea of what it is like. 
Yosemite is worth visiting any time of the year in my opinion. While most would say that a limiting factor would be the weather, I say that you base your activities on the weather/conditions and have a great time regardless. BUT since you are talking about hiking you will want to make a fully informed decision based off of the current condition. There are many resources available that I have outlined below. These sources take a little getting used to and it helps if you have seen the spot before IRL but I’ll try and explain them with details. These will roughly be in order of priority/importance/reliability to me when Imake the trek over there but will change from person to person.
Weather report:
Check out whatever weather resource that you like to use. I personally like National and Local Weather Forecast, Hurricane, Radar and Report  for a quick reference. However, if you want something a little more technical you can take a visit to your friends at the National Weather Service and see weather warnings and all that good stuff.
Webcams:
The Yosemite Conservacy has four webcams of the area. I like to give them a look every now and then to imagine being there. I wish they had more locations but I bet the cost of running them is not worth it to them. These will give you a general idea of the snow levels and current conditions. If there is snow on the Valley floor you can bet and there is going to be snow all around.
The cams include (With a description from YC itself):
Happy Isles: This is an extremely popular trailhead that leads to Half Dome, Clouds Rest, the John Muir Trail, and many other very popular sites. It is nice because you can see the river level which alludes to the snow pack conditions as well as how wet some of the trails will be. Further down I will speak to the river level piece. 
El Capitan: The granite formation to the left of Half Dome is El Capitan. This vertical rock formation extends about 3,000 feet (900 m) from base to summit along its tallest face.
Half Dome: This view of Half Dome is just east of Yosemite Village from Ahwahnee Meadow
High Sierra: This webcam shares a stunning view of Yosemite’s high country, including Tenaya Peak and Clouds Rest.
Yosemite Falls: Yosemite Falls is one of the tallest waterfalls in North America with a total drop of 2,425 feet. The Lost Arrow spire is located immediately adjacent to Upper Yosemite Fall and is a popular spot for climbers.
Yosemite News Information
Next site that I use is Yosemite News. There is a surprisingly active community of Yosemite experts, amateurs and everything in between. Other than providing great information and detailed reports the “regulars” on the site often go on side tangents about Yosemite which leads way to things you would never know to ask about. There are people on there who go to Yosemite 2-3 weekends a month and recount their trip reports. Often there will be current pictures of the area that you can check out. If you have any questions Yosemite related I guarantee that you’ll be able to ask and find an answer on YosemiteNews. Here is an article about park expansion as an example.
River Level
At the bottom of the Happy Isles site you can see a graph showing the river level. You can see the river flow is the greatest a little after mid-day and then comes by down at night. I just did a quick look at the gage read 8ft at its highest point in the last couple of years. This won’t have a huge effect on your hike but its good information to know.
Other resources
If you really wanted to jump into more information you could always check the snow pack levels of Badger Pass Ski Resort, check which roads are open, whenTioga Road has opened in the past couple of years, or get information from the best source possible, Yosemite Park Staff.