system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The difference between a Polar Vortex (responsible for last winter's extreme temperatures) and an Omega Block (responsible for the current outbreak)
gpt
The polar vortex is a persistent vortex which is in the upper atmosphere (there are two of them, one over each pole) . Think of them like a permanent hurricane of cold from "The Day After Tomorrow", except instead of breaking the laws of physics it is just normal cold weather. Last year, it dipped to the south over North America, and brought the cold with it. 

Right now, the culprit is an Omega Block in the Pacific, basically a high pressure front with low pressure fronts to the west and east of it. The jet stream is being routed far north, up to Alaska, and when it comes back south over North America it's still cold.

The polar vortex is still up there, but so far this year it's staying put in the Arctic. 

Not a meteorologist, so this is probably oversimplified.