Blocks in meteorology are large scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g.- precipitation for some areas, clear skies for others). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic oceans.
Rex blocks consist of a high situated to the north of a low. Very often both the high and the low are closed, meaning that the isobars (or constant geopotential height lines) defining the high/low close to form a circle. Rex blocks are not so-named because they are considered the "king of blocks" (see Rex), but rather they were named after the meteorologist who first identified them.
When an upper level high or low pressure system becomes stuck in place due to a lack of steering currents, it is known as being "cut off". The usual pattern which leads to this is the jet stream retreats to the north, leaving the then cut off system behind. Whether or not the system is of high or low pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southwestern United States in late spring and early summer of 2007, when a cut off low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to Texas and Oklahoma (see June 2007 Texas flooding).
If the block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed; and rainy, cooler weather if the block is a low.