Call lightning ordinarily creates a cylindrical storm cloud with a 60-foot radius, and this cloud then defines the effective range within which the caster can call down lightning bolts:
…choose a point you can see under the cloud. A bolt of lightning flashes down from the cloud to that point.
However, in stormy conditions, you can gain control over a much larger storm:
If you are outdoors in stormy conditions when you cast this spell, the spell gives you control over the existing storm instead of creating a new one.
Since the targeting requires you to choose a point "under the cloud", does taking control of a much larger existing storm cloud allow you to call down lightning anywhere under that cloud, rather than just within the 60-foot radius circle you would normally get? Or is it implied that you are only taking control of a 60-foot radius circular section of the existing cloud?
Best Answer
The rules suggest "yes" (if the recent errata is taken literally)
As explained in detail here, call lightning controls where the lightning bolts hit that are called down by the caster. That question and its answers exposed some of the ambiguities that you ask about. Salient features of the spell:
Call Lightning / Range: 120 feet / Duration: Concentration, Up to 10 minutes
How does the recent errata influence the answer?
It favors option 1a (see below) (Old text in brackets, now text in bold).
Option 1: Move the caster, move the source
One way to answer your question is to consider that the character moves; in this case, the frame of reference is the caster. The druid (or tempest cleric) is thus able to call down lightning from any part of the natural storm within range. (100' over her head). Since the caster did not have to create the ersatz storm, there isn't a problem of the basic version's summoned mini-storm's location being the limit on how far away an enemy can be. Using this logic, wherever there is natural storm, there is a place to call down lightning from. Caster moves, and from a point 100' over her head, reaches out and touches a creature 120' away, or less, with a lightning bolt.
Option 1a: Anywhere under the storm
The November 2018 errata takes this a step further. If the enemy is under the cloud (natural) it can be struck, due to the "under the cloud" update shown in the bold.
Option 2: Pick a spot and shoot from there
If the above option seems to be too powerful for a given DM / table, then pick a spot in the existing storm, concentrate on that, and shoot from there. In this case, the frame of reference is the place within the storm that the caster begins to concentrate on when casting the spell initially. The benefit in this case is restricted to the added 1d10 damage benefit from using natural lightning. Using this logic, it is the act of concentration on that spot in that part of the natural storm, to call down the lightning from the storm, that limits where the bolts can strike enemy creatures.
Which one do you think is more fun, or a better fit for our table?
Pick that one. Discuss with your DM if you are a player in this case.
One of the nice feature of D&D 5e is how much is not specified, such as cases like this. This is consistent with the general 5e theme that rulings > rules, and that the rules serve the game, and the people at the table.
Make a ruling and play on.