Hi!
Check this thread
http://www.ageod-forum.com/showthread.php?25306-AJE-Strategic-Maps
Post 21 and 22.
There you have screenshots showing all the sea zones that can have storms. Only the ones shown there with storms can have them, even in winter. The rest of the sea zones are 100% safe.
You can download this images and used them to plan your sea movements. I've been doing it for a long time and they never failed me!
On the sea zones that can have storms, they are more frecuent on winter months and very rare on summer months.
Season is also important because storms aren't always as bad.
Winter storms can wreak havoc on a fleet an eliminate many units. A total disaster... On the other seasons storms are usually weaker, specialy in summer and if you are afected by one it will only do moderate damage to your ships.
But even on summer is better to try to plot your moves through safe sea zones as much as you can.
Regards!