I just watched Sharknado, and it is definitely going in my random encounters list.
A tornado at sea passes over 300-500 sharks of various species swimming as a pack. The sharks are lifted into the sky where they fly around in the tornado, which then makes landfall. As sharks fall from the sky, they use their fins to swoop from the sky and swallow people whole. When the players encounter a Sharknado, roll 1d20.
A critical success allows them to get to high ground and avoid the sharks, which rip through the region and leave a lot of loot for them to pick up.
A normal roll means there are 1d6 sharks flying towards them, with an attack chance of 15 (150% in Runequest, attack as 10th level in AD&D). The sharks do standard shark damage (according to your system) unless they roll a critical hit, which means they swallow the defendant whole as they fall. The defendant has only 1 minute to cut his/her way free (Chainsaws give +20%) unless they have a Breath Shark Blood spell or some other way to stay alive.
A critical fumble means the Sharknado is dying down, and 200 sharks are landing in their vicinity. They face immediate attack from 2d6 swooping sharks, plus another 3d6 swimming in the water dropped by the tornado. If the party can survive 15 minutes, the water will drain away and the sharks will have only a quarter of their normal chance to hit. Be sure to roll Acrobatics for every character to maintain their footing in the flood. Mages with magic swimming speed have the best chance of survival.