that might work, but on the other hand, thiswould be available to wizards at 17th level, and sorcerors at 18th level. 24 dice of damage all in one go (over an area, no less) seems a bit overpowered when compared to other direct damage attack spells of that level. generally they deal 1 die per level. I'll have to think about it.
well, compare it to meteor swarm, it does the same damage, but with a 40 foot radius, so lets do the math, (pi)40^2, or about 5000 ft^2, or 960 ft by 5 ft, or about 4800 ft^2, so slightly less area for more versatility, as well as the fact that if you choose to roll to hit with the lightning, you can do 26d6, while meteor swarm can do 32d6. so in the end, its (slightly) less area, and less damage, for more versatility in how it's spread out, so consider that.