Do creatures attempt a saving throw immediately when a Whirlwind is moved onto them on a turn subsequent to the initial casting?

Question

The Sage Advice Compendium (pp. 19-20) contains a ruling about the spell moonbeam and other similar spells. The ruling provides a list of spells that are covered by it:

• blade barrier
• cloudkill
• cloud of daggers
• Evard’s black tentacles
• forbiddance
• moonbeam
• sleet storm
• spirit guardians

Whirlwind is not listed. But it does not say that it is a complete list; it merely says that these are "some spells with the same timing as moonbeam for their areas of effect."

The ruling states that (emphasis mine):

Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell's area of effect if the area is created on the creature's space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature's space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn't count. If the creature is still in the area at the start of its turn, it is subjected to the area's effect.

The moonbeam spell description states:

When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw.

The whirlwind spell description states (emphasis mine):

A creature must make a Dexterity saving throw the first time on a turn that it enters the whirlwind or that the whirlwind enters its space, including when the whirlwind first appears.

The bolded words are the key difference here. They mean that, unlike with moonbeam, with whirlwind, a saving throw is attempted by any creature within the whirlwind's area of effect immediately when whirlwind is first cast.

But what about when the whirlwind is moved on a subsequent turn? Do creatures attempt the saving throw immediately when the whirlwind is moved onto them? Or, does the Sage Advice Compendium ruling apply, meaning they only attempt the saving throw if and when they are in the whirlwind's area of effect at the start of their next turn?

Answer 1

Whirlwind has effects beyond those covered by SAC

The Sage Advice Compendium is attempting to explain a certain kind of spell - the kind that affects a creature when that creature enters an area of effect:

Some spells and other game features create an area of effect that does something when a creature enters that area for the first time on a turn or when a creature starts its turn in that area.

For the spells it lists as examples, one could justifiably ask, 'is a creature entering the area the same as moving the area onto a creature?' For these spells, the answer is no:

Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count.

Whirlwind is subject to this ruling. Like all the other examples, entering its area is not the same as having its area move into your space.

These effects include forcing a save when it is moved

Whirlwind says (emphasis mine):

A creature must make a Dexterity saving throw the first time on a turn that it enters the whirlwind or that the whirlwind enters its space, including when the whirlwind first appears.

The Sage Advice ruling explains that entering an area of effect is not the same as the area moving into your space, and this applies to whirlwind as well. But whirlwind says explicitly that the Dexterity save is also required when the effect is moved onto a creature, something none of the spells in the Sage Advice ruling say. It follows the same ruling but goes beyond it by explicitly having an additional trigger condition. It is not the same trigger, even though it happens to have the same result.

OP also asks whether this save would be required on subsequent turns of casting, whenever either triggering effect occurred: a creature entered the area, or the area moved over the creature. By using the word 'including', whirlwind is indicating that the save occurs on both the turn of casting and subsequent turns; the initial turn is included in the same effect that subsequent turns have¹.

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¹If the save was intended to be triggered only on the initial, and not subsequent turns, Whirlwind would have said "A creature must make a Dexterity saving throw the first time on a turn that it enters the whirlwind, or the whirlwind enters its space when it first appears."

Answer 2

You attempt the save immediately

The key part in the spell description is this:

A creature must make a Dexterity saving throw the first time on a turn that it enters the whirlwind or that the whirlwind enters its space, including when the whirlwind first appears.

The ruling in Sage Compendium differentiates between a creature entering some space and a moving area of effect entering the space of a creature, and all the spells listed in it only work in case of the former.

Whirlwind is not mentioned because it's different, it explicitly says that it works in both cases, going as far as to specify that the effect takes place when it first appears.