According to CWB, the Mei Yu officially started on May 2, but it looks like the first 'real' front is going to form over Taiwan on Wednesday. The rains will be welcome. It should then overstay its welcome by hanging around all week - as any self-respecting mei yu front should do. Expect some big rains in the north west especially and lots of thunder and lightning action. The south and the south east might have to wait a couple more days.

Also, a 'strong' El Nino event has been declared for 2015. Tried researching the effect of that on the mei yu and apparently the link is unclear. In theory it should weaken the mei yu but there have been several exceptions.

As for typhoons, the last strong El Nino was in 1997, and that saw a record number of typhoons form, though only one made landfall in Taiwan - most seemed to head north.