September is still hot vinylgroover, it's more usual to rain, but much more the 2nd half than the first. In this webpage you'll find it in great detail:
http://boreal.inm.es/wwc/html/dclimat/IBIZA_.html
If you don't understand Spanish, these are the inscriptions in English (it would be useful if the webmaster of Ibiza-spotlight added it to the website). It belongs to the spanish National Institute of Meteorology and it's measured at the airport of Ibiza:
T - monthly/yearly average temperature (°C)
TM - monthly/yearly average for maximum daytime temperatures (°C)
Tm - m/y average for minimum temperatures (°C)
R - m/y average precipitation (mm)
H - average relative humidity (%)
DR - m/y average number of days with precipitation equal or higher than 1 mm.
DN m/y average number of snowy days
DT m/y average number of stormy days
DF m/y average number of misty days
DH m/y average number of days with frost
DD m/y average number of clear days
I m/y average number of sunny hours