I was there September 2nd to 16th in Santa Eulalia. The big storm with all the lighting was during the second week on either the 13th or 14th as I remember going to Amnesia and it was pouring down with rain
Was definately the 13th as i spent the night of the 14th Sept roughing it at Ibiza ferry Port waiting for the ferry departure to Barcelona the following morning. Mind you it piffed it down all night that evening aswell but i found shelter by sleeping under a parked lorry trailer. Hopefully if i return this year September then the weather won't be so bad or if the the family come along it'll be May/June time with normally good weather and also a proper roof over our heads.