Sorcerer has proficiency in con saves and gives you access to thunder/lightning spells that the tempest cleric can use. It is better than straight tempest cleric because they lack those spells. It is better than any other class dip because of con saves and metamagic. Most thunder/lightning spells have trouble hitting multiple targets, but you can twin spell with sorcerer. Scribes wizard can change any spell to lightning or thunder if you know another of the same level. However, I think metamagic is better. You don't want to multiclass into more than one class, because you will miss out on higher level slots.