We've been to PP in September a number of times, generally for two weeks.
Sometimes thunderstorms seem to start rolling across the Med this time of the year and we've had a couple of mornings or days most years which were non-beach days. Either it was to chilly to go down to the beach the morning after the thunderstorm or the whole day would be breezy and overcast.
But we've been lucky too and had two great weeks full of sunshine.
Shorts and t-shirts are generally still enough, but a light waterproof and a jumper/cardigan (as mentioned above) as backup can come in useful.
You might have to plan for a few non-beach things to do - either for the morning or the whole day.
It's a great time to explore the island with it's huge variety.
Pollensa and Alcudia still have some hidden corners to explore and it's easier to get around if it's not roasting hot. Checking out the market days around the island, venturing up the Tramuntana mountains for a short visit to Lluc (the virgin, choir, gardens) or further down to Deia and Valdemossa. And naturally Soller is a great excursion. Nothing wrong with a shopping round trip to Palma either. Also plenty of bodegas around Mallorca to visit/sample/stock up.
Top on the agenda for any non-beach day this year for us is the aquarium in Palma.
If you don't have access to TV or Internet the Sail & Surf people stick up a daily weather forecast at the Torre Playa every day (around 10ish if I remember correctly).
A lot of the possible excursion places have very informative websites - e.g. Lluc (see links below)
Santuari de Lluc - Mallorca (Illes Balears)
Santuari de Lluc - Mallorca (Illes Balears) (time table)