There is precedent in 5e, I believe. Hags, in a coven, are effectively a higher caster level.
Shared Spellcasting (Coven Only): While all three members of a hag coven are within 30 feet of one another, they can each cast the following Spells from the wizard's spell list but must share the Spell Slots among themselves:
• 1st level (4 slots): Identify, Ray of Sickness
• 2nd level (3 slots): Hold Person, Locate Object
• 3rd level (3 slots): Bestow Curse, Counterspell, Lightning Bolt
• 4th level (3 slots): Phantasmal Killer, Polymorph
• 5th level (2 slots): Contact Other Plane, Scrying
• 6th level (1 slot): eye bite
For casting these Spells, each hag is a 12th-level spellcaster that uses Intelligence as her Spellcasting ability. The spell save DC is 12+the hag's Intelligence modifier, and the spell Attack bonus is 4+the hag's Intelligence modifier.
So, your spellcasters could be in a coven. They could have access to a few extra spells (Control Weather plus any that are thematically appropriate for their coven and your adventure) that they don't normally have and they can cast them as 15th level casters. The advantage of doing it this way is it allows you to customize which spells they can actually cast (maybe they can only cast 2!). Also, casting any of these more powerful spells will drain their spell slots collectively, making them less powerful as a group when the PCs encounter them.
Additionally, you could say that any concentration spells cast this way must take up each casters concentration simultaneously. So, to break the effects of the spell, they only need ruin the concentration of one spellcaster.