Crimson calced this for me, so using this I'll calc a storm feat.
We have a radius of 2089.2 m. We can also assume that it's 8000 or 10000 meters tall since it's a cumulonimbus, which gets us a volume of 109677297235.57 meters3 or 137096621544.46 meters3 using the cylinder volume calculator.
1.003 KG/M, so 110,006,329,127 kg and 137,507,911,409 kg
I'm pretty sure using strong instability is fine, since it can destroy a city and creates tornadoes strong enough to rip out trees as well (Tough they are not as big as a normal tornado). So 2.5 kilojoules per kg.
Low end: 275,015,822,818 kilojoules (2.75015823 × 1014 joules)
High end: 343,769,778,523 kilojoules (3.43769779 × 1014 joules)
DMUA's edit: The storm has higher instability, so he remade it with values he finds more reasonable.
π×8000×2089.2^2 is 109698299159.45
Times 1.003 is 110027394056.92
Times 4,000 is 440109576227714 Joules, 105.188713247541 Kilotons, Large Town level
Times 5,900 is 649161624935878 Joules, 155.153352040123 Kilotons, Large Town level
Times 8,000 is 880219152455428 Joules, 210.377426495083 Kilotons, Large Town level