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The Tempest Domain gives the ability Channel Divinity: Destructive Wrath, which says:
When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling.
Can I wait until after the target rolls their saving throw (against my spell that deals lightning or thunder damage) to decide whether or not to use Destructive Wrath?
According to every "save for half" spell I've read so far, the written order of operations is each creature makes a saving throw, then damage is rolled.
For example, thunderwave says (PH282–3)
Each creature [in the area] must make a Constitution saving throw. On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you. On a successful save, the creature takes half as much damage and isn't pushed.
And lightning bolt (PH255)
Each creature in the line must make a Dexterity saving throw. A creature takes 8d6 lightning damage on a failed save, or half as much damage on a successful one.
I could go on, but I'm sure you get the idea. The written order of operations is roll saves, then roll damage.
In general, only the player rolling the dice for the victim (and the DM) knows at the time whether or not he made his Saving Throw. The victim (in-game) only knows the effects that he perceives, and the caster (in-game) only knows what he perceives in turn. This is true for rolls in general, though there are a number of specific monster and character features which bypass this for mechanical reasons. These exceptions are always called out in the text.
It's easy to forget this basic rule in the rapid-fire action of the game. If the DM took the time to describe everything in terms of the characters' perception the game would grind to a halt. So we take a lot of short cuts and allow the players to interpret the numbers and imagine how their characters perceive each event. This makes for a faster game, but sometimes we forget that the characters have no idea that their fate is tied to whims of an icosahedral icon.
So, given this basic truth, it doesn't matter whether the Saving Throw comes before the damage roll, as the caster can't perceive the result. If the RAI was otherwise, it would have been specifically spelled out, as it is in many other spells and features.
The class feature says:
When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling.
My added emphasis shows clearly that you make this decision at the time you roll the damage. Each spell that requires a saving throw tells you when the saving throw is made and how much damage applies based on the outcome. In most cases I believe the saving throw comes logically before the damage roll, so in most cases you will be able to see the outcome of the saving throw before you decide to use Destructive Wrath.
While most answers suggest that damage is rolled after the save, based on wording or "common sense", I will offer the opposing perspective: there is no sequential temporal relation between the two dice rolls.
Spells that require a saving throw are either condition appliers (buffs/debuffs), or damage dealers. Out of those that deal damage, a vast majority are area of effect spells, with a few exceptions having a single target.
Area of effect spells offer a save-or-take-half-damage approach, imposing a saving throw on all the creatures in the spell's area. This damage is caused by the spell in an area, regardless of the number of creatures hit, and is the same for all creatures in the area. A creature succeeding on its saving throw takes half damage, but the damage that is halved has to already exist for the creature to be able to avoid the brunt of it. The fact that none, one, or all creatures succeed their save is inconsequential to the spell's damage. If anything, the saving throw is caused by the damage dealt, and not the other way around. This is further enforced by the fact that halving damage is done by taking half of the dice roll result (rounded down), and not by rolling half the dice.
For single target spells, conditioning the damage roll on the save success seems like a natural choice, but only because most (I have not been able to find any example contradicting this) single target spells that require a saving throw offer a save-or-take-no-damage approach. However, this is just a method of reducing the number of rolls by taking advantage of the fact that no damage is received on a successful save.
Typically, when a creature can save for half damage, they are doing so at the same time you are dealing the damage. The damage rolled is applied to all creatures regardless of if they save, and any modifiers applied after (like saving for half).
What you're essentially talking about boils down to this:
"Haha! I see that none of you have dodged my lightning attack! Now I will deal maximum damage!
Let's forget for the moment that maybe you can't even tell if they made their save. Even if you could, why would you be able to apply damage after the spell hit everyone?
I would treat it the same way as the Great Weapon Master or Sharpshooter: you are consciously deciding to amplify your damage, but you don't know if the attack will be as effective as it can be.
From a balance perspective, this makes sense. Even if all of your targets succeed in their saving throw, they're still taking half of maximum and that's nothing to sneeze at. The probability of rolling max damage is already pretty low, so deciding to automatically get that is strong, so you needn't feel like you're being cheated just because they save for half.
