Could a sharknado theoretically occur? Well, if there were a substantial number of sharks swimming in shallow waters right where a powerful waterspout touched down, then it would be physically possible to have a sharknado - briefly. Unfortunately, it would very likely kill all the sharks from the extremely low air pressure, not to mention depriving them of the liquid environment they need to breathe, so if by some magic the sharknado kept all the sharks in the air until it hit land, all that would happen is that you might be lethally pelted with shark corpses. If it didn't reach land, the sharks would just be thrown into the air (it happens with 18-wheel trucks, so it can happen to a great white very easily) and belly-flop back on to the ocean some distance away. If they weren't already dead, the impact would probably kill them.
Final verdict: The most plausible sharknado scenario that could kill a human being would be a waterspout right off the coast that tosses a shark on to land that ends up landing on top of someone. Not gonna happen, obviously, but it's conceivable. But here's a movie that needs to be made: Snakes on a Sharknado. I really want to see some motherfucking snakes on a motherfucking sharknado. And I would especially appreciate it if the CGI for it is created on a Windows 95 machine, and if they get that guy from Thinner to be the lead - you know, that guy who was totally snubbed at the Oscars. And is Judge Reinhold doing anything? The world needs him back in the game.
I actually see a bit of a franchise in store for Sharknado, and possibly some reboots: Badgerricane, Spiderquake, Wolfalanche, Snakeunami. Then, of course, you need to re-release each of them in 3D. Then the unrated versions on Blu Ray - not because there's anything salacious in them, but just because people are dumb and will pay for the word "unrated" on the cover. Add like 1.5 seconds of cut footage and call it "The version the studio fatcats didn't want you to see!" BTW, when are they gonna make a remake of Freddy Got Fingered? Still waiting on that, Spielberg! Got enough time for Lincoln but not Tom Green, huh?