METEOROLOGIST JEFF HABY
The Rex Block is a
blocking pattern that in relation to the United States occurs most often in the spring
and most often occurs along or near the West Coast.
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. In the example
image, air flowing into the West Coast of Canada then flows south into the Pacific off the Mexican West Coast.
Once the flow leaves the Rex Block, it can finally make some significant progress to the east.
An example diagram of the Rex Block is shown below: