Right now, temperature in Japan is around 20 degrees. I suggest you take a look at this link:
About.com: http://www.weather.com/outlook/travel/businesstraveler/wxclimatology/monthly/graph/JAXX0085
It shows the average temperature values of each month. Knowing that you are freezing with 30C, March and April may be still too cold for you (around 12-18C).
Keep in mind though that 30C may feel alot warmer (steamy) then you are used to, because of the humid climate. I was in Japan this summer for a month and even though it was unusually warm for that time of the year (close to 40C), you don't feel alot of difference between 30C and 40C, it's both steaming hot.