First, forget the waves (well, the swell anyway: we have "waves" with westerlies on Leman (or Lake Geneva, one good spot is Preverenges) or with Bise (N-NE) on Lake Neuchatel (Grandson, Yvonnand). Once, I even saw a guy surfing (no sail, I mean) !).
Also, forget Silvaplana (S.Moritz) if you're really into shortboarding, it's too high (1850 m): most people go with al least 7.0...
The best wind can be caught with a low pressure and bad wheather south or north of the Alps. The wind ("foehn") blows from the rainy part to the other side of the Alps and can be very strong there (7-10 bft !).
Best spots:
If you happen to be in Valais (spring,summer) try Martigny. You won't see any lakes on the map, but there's a small pond right of the highway (exit at the gas station/restaurant). The thermal usually picks up in the afternoon from the NW. It's a little like indoor sailing... (in the summer you can go snowbording in the morning (Verbier is 20 minutes away) and sail in your shortie in the afternoon)
Another spot is in Cham on Zug Lake, but I never sailed there.
The best thermal wind can be found on Como Lake in Italy, 1 hr. from Lugano (It's so popular among swiss sailors that in some spots you will see less italians than swiss or germans (Domaso)).
Good luck and please let me know if you find any other spots !
I was in CH (stands for Le Canton Helvitique BTW) in 1992. I saw lots of shortboards zipping around Lac Leman from the train as I was coming into Montreux. That's the NE corner of the lake. They were going at amazing speeds, as the wind was a steady 25 to 30 and the waves were not big. I was told later that windsurfing is quite big in CH and has been for years, for the simple reason that the mountains cause a reliable wind to form each day.
I live in the norh of Switzerland, so my favorite spots are maybe different than yours. There is Bielersee (best with north-west wind (Bise)), Urnersee and Alpnmachersee, good with west wind, excellent with wind from the south (Foehn). There is a phone number which gives you *some* hints about the wind for the next day, but several times it's abolutly wrong. (Tel: 157 12 62 25). I'd go if wind is more than 3-4, all below such as 2-4 is shit.
From: Mac
Here some infos about the EST half of the lake of Biel (Bielersee):
good spots for the different wind directions
Bielersee Windsurf Clubs: Windsurf Club Ispach, Windsurf Club Piranha
The best spots are in the northern part of the Lake around Pino and Tronzano, where the lake is narrower than elsewhere. Although usually the wind isn't as good as on Garda Lake or Como Lake, the wind patterns are similar. There are mainly three kinds of wind:
Ascona (CH) 1300 - 1600 2-3 Bft Porto Ronco(CH) 1300 - 1600 3 Bft Brissago (CH) 1300 - 1600 3-4 Bft Canobbio (I) 1300 - 1600 3-4 Bft Punta di Pino Tronzano (I) 1300 - 1600 3-5 Bft best spot
Ascona (CH) 1700-1900 2-4 Bft Minusio-Tenero 1700-1900 2-3 Bft Carmine (I) 0700-1000 3-5 Bft Maccagno (I) 0700-1000 3-5 Bft
I would like to suggest another spot on Lake Maggiore; it is at "Ispra Beach" (the name is obviously a joke..).
You can reach it driving to Ispra and following the signal to the pier; on the left of it there is a beach with a windsurf and sailboat clubs with a parking and the access to the water is easy.
The best wind is the Tramontana which is present usually from April to October early in the morning (let'say from 6.30 to 10:00 depending on daybreak). If there is wind I see people planning with 6.5/7.0 sails, in case of Favonio people use 4.5/5.0 sails.
Unfortunately due to the location of the beach there is no Inverna in the afternoon. Just for further information a windusfers' shop (EMMECI) has been just moved from Angera to Ispra a few hundreds metters far away from the beach.