Blocks in meteorology are large scale patterns in atmospheric pressure fields (high and low pressure), which are nearly stationary, and as a result there is little or no movement of weather systems. These “blocks” can remain in place for several days (or longer) producing the repeating weather pattern.
On such block is known as, the Rex Block (named after the meteorologist that first identified them). This blocking pattern occurs most often in the spring. A Rex Block has two adjacent highs and lows. The most impressive Rex Blocks will have a strong low pressure next to a strong high pressure. The high pressure will be located in a generally north direction from the low pressure. The low associated with the Rex Block is not completely cut-off from the upper level flow, but does have little east-west movement. Strong ridging north of the low causes the airflow to move from high latitudes to low latitudes with little comparable west-east movement of the air. The airflow pattern follows a backward “S” trajectory.