Disclaimer. It's gonna be high, at least I think so. May or may not be an outlier. Anyway...here it is. It's gonna be entirely based on Darkanine's calc.
I'll be doing only Gen IV to save time, and I'll use Palkia's and Electivire's height for convenience, as those are the ones in the video. They'll be a stupidly high and a bit high end respectively. I'll also use Pikachu as a low end.
Palkia is 13.75 feet tall, making the horizon distance 4.5 miles (9 miles diameter), or 7.242048 km. This serves as a radius, I think. Assuming a circle, it's area is 164.77 km^2.
According to Darkanine, storm clouds vary from 12 to 14 kilometers tall. I'll be using the middle (which is obviously 13).
Which is 2.14201e+12 m^3
Don't know why I have to multiply by 1.003, but eh. 2.148436e+12 kg.
Storm starts forming at 1 second, and is fully formed at 1.38 seconds. Which means that its moved 9 miles in .38 seconds, going at 38116.04211 m/s.
.5*2.148436e+12*38116.04211^2=3.73e+11 tons of TNT. Low End Large Island as a high end. You could also make a low end of the high end and say that the thunderstom started to be form as soon as the names of the Pokemon disappeared.
Electivire is 1.8034 meters. Makes a horizon value of 4.8 km. 9.6 km diameter
This literally becomes the calc I've been referencing the entire time here. Ends up at 871,165,680,000 kg. Timeframe is different though. Still .38 seconds.
Speed is 25263.15789 m/s.
Kinetic energy is 278000793070264880000 joules. 6.64437842e10 tons of tnt. Middling of Island level
I'll do a low end, using Pikachu's height. Horizon is 2.3 km. Diameter is 4.6 km.
16.62 km^2 is area.
216.06 km^3 is the area*height.
Translates to 2.1662e+11 m^3
12105.26316 m/s is speed using that timeframe of .38 seconds.
15919079775620174000 joules is the result of the KE. 3.80475138e9 tons of tnt. Large Mountain level+
On the assumption that not only is this not an outlier, but also legitimate, clearly only the high tiers. So basically, the tier 7s. Also, I think that we should upgrade the Fossil Pokemon to that level, as frankly, I made a mistake, and none of them are actually...weaker than most final forms.
EDIT: People are getting bitchy about using the heights of Pokemon that can use that move. Fine. Use Lucas' height, the trainer. It's static, and doesn't change with the Pokemon. 4 foot 8. 1.4224 meters. 4.3 kilometer horizon. Which is a 8.6 diameter. Area is 58.09 km^2. Multiply by 13. 755.17 km^3. 7.5517e+11 m^3. Multiply by 1.003 to get 757435510000 kg. 22631.57895 m/s. 4.63610966e10 tons of tnt. Again, middling Island level.