If a spell has an area of effect with a radius of miles, like Control Weather, or Mirage Arcane, is there anything stopping a level 5 mage, hiding somewhere within the area, from dispelling the terrible storm cast by the archmage?
It feels like these massive spells are extremely vulnerable since, at most, they’d need to beat a 19 on an ability check. Surely it’d make for a very anticlimactic fight if the mega-storm conjured by the BBEG was dispelled by a couple of apprentice mages.
Control Weather causes a magical effect with a 5 mile radius, centered on self. A successful cast of Dispel Magic from within 120 feet of that area ends the spell, as you’d expect.
Some notes based on your question description however:
If you wanted to dispel it, you’d have to know the weather is magical, you’d need to have Dispel Magic known/prepared (depending on your class) and a 3rd level spell slot available, and then pass DC 18/19 so check to dispel it (pure spellcasting ability check, so a caster with a 20 in their spellcasting ability would still need 2-3 tries on average, even if they’re a bard unless it’s a relatively high level bard). Now all of these individually aren’t extremely high barriers, but they’re still there.
Even if you do end the spell, this doesn’t immediately stop the weather conditions caused by Control Weather, rather the spell description specifies:
When the spell ends, the weather gradually returns to normal.
So if you did dispel the spell, the caster would be very annoyed but the immediate threat caused by any existing storm or the like wouldn’t disappear instantly. (And if the weather wasn’t bad enough to be a problem already, how’d you even guess it’s a magical effect or bother with dispelling it?). A sufficiently powerful caster could simply resume casting the spell (if they have more castings/spell slots available). And of course, considering how long this spell needs to take effect no competent caster would bother with it if they weren’t relatively sure they’d not get interrupted.
