Hypersmurf
Moderatarrrrh...
So, the Meteor Swarm explodes upon impact with the first creature struck, not when reaching the destination, as per the spell's description?
I think the key word in the spell description is "if".
Any creature in the straight-line path of these spheres is struck by each one and takes 24d6 points of fire damage (no save).
If the spheres reach their destination, each bursts like a fireball in a spread. (6d6 damage each.)
The implication is that if a sphere does not reach its destination, it doesn't burst.
There is no reason explicitly stated in the spell description for why a sphere might not reach its destination to justify the inclusion of the conditional modifier... but the paragraph does immediately follow the description of what happens when the spheres are intercepted by a creature.
The 24d6 seems to be the result of absorbing 4 unburst 6d6 fireballs... by my reading, they neither pass through the character who takes that damage, nor burst.
Interpretation, though, so YMMV.
-Hyp.