Calc The Witcher - Fritjof summons a storm

Fritjof, a vaedermakar (druid who can control the weather), summons a huge storm to the horizon.
Before the storm...
Some clouds, but mostly blue skies. Afterwards?
Storms cover the sky to the horizon. Including, may I add, from up on a mountain.
For reference, that last one is up on Hail Hill. The mountain is covered in snow, and given Skellige is a cross between Britain and Scandinavia, we'll go with a low end of the snowline of Svalbard and a high end of Southern Scandinavia, which is 300m and 1200m above sea level respectively. Using this calculator, we can determine that is 61.9km (61900m) and 123.7km (123700m). The average height of a cumulonimbus is 12,000m. Volumes as cylinders.

(Low end)
V = πr^2h
= π X 61900^2 X 12,000
= 1.44448294e14m^3

(High end)
V = πr^2h
= π X 123700^2 X 12,000
= 5.76860123e14m^3

Density of a cumulus cloud is around 0.5 grams per m^3.

(Low end)
M = 1.44448294e14 X 0.5g
= 7.2224147e13g

(High end)
M = 5.76860123e14 X 0.5g
= 2.88430062e14g

Condensation of water vapour to liquid is 2260 joules per gram.

(Low end)
E = 7.2224147e13 X 2260
= 1.63226572e17 joules
= 39.012086998087951883 megatons

(High end)
E = 2.88430062e14 X 2260
= 6.5185194e17 joules
= 155.79635277246654823 megatons

Final Results
Fritjof summons a storm (low end) = 39.0121 megatons
Fritjof summons a storm (high end) = 155.794 megatons

And this isn't taking kinetic energy into account. The djinn Geralt fought should much stronger than Fritjof.