April weather?

april is already a bit late for snow in Nagano. It doesn't mean you WON'T see snow, there's a chance you can.

you can take a look at a live webcam in the area for example: http://www.jigokudani-yaenkoen.co.jp/livecam/monkey/index.htm

But you can definitely see the monkeys any time of the year. If you go early April, there are changes you will still see snow