Something that prevents a really good event from being successful
or
an alternative, really crap, event that people choose in preference to what you've organised.
(derived from the strange success of the 'b' movie Sharknado).
My party was hit by a sharknado as only a few people showed up.
I don't understand why no one is here, there must be a sharknado going on.
1. A tornado that dooms large metropolitan areas with dumping thousands of ravenous sharks ready to eat people's appendages off. No one is safe from the sharknado's destructive path of multiple vortexes and the only hope for survival is Ian Ziering (from the original 90210) and a washed up Tara Reid, oh and a bad ass bikini barista who also seems to be an expert marksman.
2. When many events happen all at the same time, making someone's life or a situation overwhelming. This comes from the SyFy original movie "Sharknado" where a storm of multiple tornadoes hit LA full of ravenous sharks.
Example 1: If you are trapped in a school bus flooded with water and sharks waiting to make you Purina Shark Chow, just know a sharknado is on it's way. Don't worry, Tara Reid will save the day.
Example 2: Jan: Fuck dude, I've had a totally shitty day today. First my car broke down which made me late for my meeting and which made my boss rip me a new one and I got mud on my new Prada shoes.
Anything that is big, confusing and potentially dangerous that probably shouldn't exist but you still have to deal with anyway. See also, Sharknado, the movie