blocking

A meteorological condition, when a pressure system remains stationary for, perhaps, weeks. It is linked with a blocking pattern in Rossby waves. Blocking anticyclones develop when the Rossby pattern changes from zonal to strongly meridional, often forming one or two high-level, closed anticyclonic circulations, so that the jet stream splits around the high pressure system(s). The upper air flow then guides depressions around the edge of the anticyclone(s). Unusually long periods of fine, dry weather result in summer; winter conditions are very cold, but rather dry.