What is an ‘omega block’?

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Dear Tom,

On rare occasions I hear you refer to a weather system known as an “omega block.” What is it?

— Mill Fransisco, Chicago

Dear Mill,

An “omega block” occurs when the jet stream undergoes extreme buckling and becomes wavy, instead of flowing west to east across the country. Such a weather pattern interferes with the normal progression of weather systems, thus the term “block.”

The designation “omega block” is specifically applied to a flow pattern that takes the shape of the Greek alphabet letter omega at least several hundred miles across. Such a block can have global weather effects. Blocks are notorious for their persistence and can go on for days, sometimes even weeks. Blocks play a huge role in some of the Earth’s most notable weather extremes like droughts, heat waves, arctic outbreaks and repetitive rains that set up widespread flooding.