concept

Atmospheric blocking

A large, quasi-stationary high-pressure system that stalls the normal west-to-east progression of weather for days or weeks, usually by splitting the jet stream into two branches that detour around it. Forecasters name them by shape: an omega block traces the letter Ω in the flow, while a Rex block stacks a high directly above a low. The hazard is not the high itself but the fact that downstream weather stops advancing — rain that should have passed over settles in and floods, and heat or drought that should have lasted a day digs in until it becomes a lethal heatwave.