One of the things I’m watching closely this year is the Sierra snowpack. For the past several years with the drought, it was perfectly possible to tackle the JMT in June. Snow on the JMT means that you’ll spend more time hiking up and down passes, as they’ll be slick and the trail may not be visible. It also means you’ll be dealing with swift, high stream crossings, which can be very dangerous.
The last year with a considerably above normal snowpack was 2011. That year, there was snow on passes into August. If you want to check up on how the snowpack is, I like the following two links:
This site gives regional information about the percentage of the normal amount of snow the Sierra has gotten compared to this date in an average year and as compared to April 1st.
A great visualization where you can compare past years to this year. Check out 1997-1998 for a similar El Nino year – looks like we’re on track for a snowy year!