Weather in Mallorca and the Balearics
The best weather sources are:
Weatheronline: good summary for the Balearic islands, Mallorca and Menorca in English
www.aemet.es: La Agencia Estatal de Meteologìa provides an extremely detailed forecast for Mallorca, Menorca and all the other islands in the Balearics, in Spanish. You can find a glossary of wave heights here.
When to go
The sailing season in the Mediterranean is much longer than in the UK, and with the weather to match. I have enjoyed trips with glorious sunshine in both March and in late October, so there will always be an element of luck.
For many, the optimum months are June and September. The water is warm for swimming after about mid-May, while the oppressive heat and sailing crowds of July and August are less in evidence. If you are tied to the summer months by school holidays, aiming to take two weeks and getting away from Palma are both ways of finding the more deserted and enjoyable destinations.
In March-May and October-November, the weather is simply more variable. With luck, there will be bright sun, consistent winds and calm seas. The main concern is the Mistral which blows onto the steep and forbidding north coast from the Golf de Lyons to the north west of Mallorca. The mistral can be strong, and will certainly leave a legacy of a lumpy sea (marejilla (moderate) or gruesa (rough) in Spanish) which can take a day or two to disperse.
If you are stormbound for a few days, there are some options to consider. There are sheltered waters where you still may be able to sail (particularly the Bays of Alcudia and Pollensa, and potentially in the harbour of Mahon), particularly where the mountains provide protection from the north. There is also much to do across the island, and in my experience a storm that prevents you from going out to sea will usually be no more than a minor irritant on the beach or from the clifftops.