Is calling down lightning from an existing storm (stormy weather) restricted to a small location?

Question

Based on this question, and a tweet from Mike Mearls in this answer: how flexible is the second form of call lightning for the spell caster?

Call Lightning has two forms. In the first there is no natural storm to get lightning from, so the spell caster conjures one with modest dimensions: a 10' tall cylinder with a radius of 60' that begins 100' above the head of the spell caster. You can call down lightning once per turn on any target within range(120').

The tweet is a terse dev ruling that the conjured storm cloud stays in the spot where it was created (the rules don't specify that) and does not move with the spell caster. The rules don't constrain the spell caster to standing in place while concentrating on the spell while she calls down lighting.

The tactical problem to address

{form 1}A storm cloud appears in the shape of a cylinder that is 10 feet tall with a 60-foot radius, centered on a point you can see 100 feet directly above you.
{snip}
{form 2} 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. Under such conditions, the spell’s damage increases by 1d10.

There's a storm cloud already there because we have stormy weather. The spell is cast and down comes the first stroke of lightning. As each turn passes, the caster is not constrained by location and could do this:
Move, call down a strike. Move, call down a strike. Move, call down a strike. (And so on for ten minutes or until spell concentration ends).

Natural storms range in size from a small cell to a very large storm that stretches across many miles. The "above the spell caster's head" criteria suggests that the spell caster can move up to 3000' (ten minutes at ten turns per minute, base movement speed of 30) and call down lightning from anywhere in range each turn as long as the spell caster remains under the natural storm cloud. (Be wary of angry Druids! You can run, but you'll only die tired!)

Question:

Is there a restriction where the spell caster can call lightning down from, when the lightning is called down from a natural storm in stormy weather?

If any of you encountered this in Adventure League play during the Elemental Evil season, rulings on that point during AL are of interest and would be useful in answers to this question.

Answer 1

First of all, there is nothing in the spell description that suggests that "control over the existing storm instead of creating a new one" does anything other than subvert rather than create the same "storm cloud" described in the first paragraph. If there is already a cloud 100 feet above your head you get that one (and the extra damage); if not, you create one. So the answer to your question is the same in both cases.

The spell's description says:

When you cast the spell, choose a point you can see within range [120 feet]. A bolt of lightning flashes down from the cloud to that point. ... On each of your turns until the spell ends, you can use your action to call down lightning in this way again, targeting the same point or a different one.

There are two things to consider:

Range

The target of a spell must be within the spell’s range.

Once a spell is cast, its effects aren’t limited by its range, unless the spell’s description says otherwise.

"in this way again" means that range is still a factor. And this tweet (apart from the fact that the range is misquoted) indicates that the range is measured from you - not your storm cloud:

@mikemearls As a follow-up, as long as you can see a target within that 60 ft. radius, you can move as far away from the storm as you want?

@TheShieldComics Yes

Target

To target something, you must have a clear path to it, so it can’t be behind total cover.

I think that in the case of this spell it is the cloud, rather than you, that must have the clear path.

Is there a restriction on the location of where the spell caster can call lightning down from when the lightning is called down from a natural storm in stormy weather?

Yes, the target must be within 120 feet of you and have a clear path to the cloud

So yes you can do exactly what you want - move up to 3000 feet and keep calling lighting from your pet cloud that is 3000 feet away. Given that a lighting leader travels at 61 km/s and the return stroke at 27,800 km/s; the lighting will take about 15 milliseconds to make the journey - well within the 6 second round. You can do this irrespective of if there was a pre-existing storm or not.