I was going to suggest a normal SLA for the chain lightning and a unique Sp for the forked. Something like
Forked Lightning (Sp): Twice per day, a herd cloud can release a forked lightning bolt consisting of two X ft long lines of electricty, each doing 6d6 damage. A DC X Reflex save halves the damage, and the two lines cannot overlap. This is the equivalent of a 5th? level spell.
The only problem with the Sp is that the DC will be a bit weak unless we boost the Cha. On the other hand, DC 21 is probably too high at the likely CR.