Tempest Clerics can use their channel divinity to guarantee max lightning damage.
With spells that only use dice (like lightning bolt, or later chain lightning) that means you double!!! lightning damage.
If enemies are wet, that doubles lightning and cold damage. So 4x in total.
A Storm Sorcerer 10/ Tempest Cleric 2 can first cast create water with quicken spell, and then in the same turn do 160 AoE lightning damage. Guaranteed, without any variance.
You also have the normal sorcerer stuff, like twinned spell, subtle spell making you immune to being counterspelled or heightened spell to hit, and, most importantly, con proficiency, so you will not loose your concentration easily.
Thanks to Cleric, you have heavy armor proficiency and a very high armor class, especially for a caster, without any major investment.
And with their strange rules for multiclassing, it seems that you get the spell lists of BOTH the cleric and sorcerer by multiclassing, which is otherwise a very powerful Divine Soul Sorcerer subclass feature. That means all the fun cleric spells, but with metamagic!
For leveling, go 5 levels in sorc, then 2 cleric, then the rest sorc.
That way, you are strong at every single level, and can use Lightning Bolt as the go to lightning spell before you get chain lightning. Still a respectable 96 guaranteed damage
You would loose access to chain lighting with this build though as you need atleast 11 sorc levels
