In meteorology, blocks are areas of high pressure that remain nearly stationary and distort the usual eastward progression of pressure systems. The blocks can remain in position for more than a week which will lead to areas under the high to have similar weather for a prolonged period of time.
There are two types of blocks; an Omega Block and a Diffluent Block, and are most common in spring. Exceptionally they can persist for months around mid-summer, like in 1976, or mid-winter, like in 1963.
Omega blocks are named due to pattern they form which resembles the uppercase Greek letter omega, Ω. An area of high pressure will be sandwiched in between two lows to the east and west, and also slightly to the south. These blocks frequently occur on the eastern edges of the Atlantic and eastern Pacific, and can lead to easterly flows to the UK.
A split in the eastwards flow can lead to a Diffluent Block. Examples with a closed high centre to the north of a closed low centre in the south are more likely to last for a prolonged period of time.