Typhoon season in Japan can kick off as early as mid April as it did in 2005. Usually however the early typhoons, April, May June usually stay at much lower latitudes because of the warmer sea temps. Some will send swell and occasionally its good quality.
For typhoon surf in Japan we are watching for typhoons with the following attributes:
a. High velocity winds
b. that this wind that blows over a long distance in the same direction (Fetch) pointing towards Japan
c. that the wind blows for long enough to generate long period swells
The following graph records the typhoons of 2009 that ended up sending swell to Japan, including some that sent Epic swell to Japan.
And this is a screen shot of Choi-wan - the whole country lit up with great swell.