Rex Block

Idealized 500mb map of a Rex Block.  The dominant high height center is found poleward of a equally dominant low height center.  The primary deformation zone associated with this block is shown in the diagram (west of the high wrapping around the western side of the low).  The isoheights drawn on this diagram show a backward "S" pattern wrapping poleward of the high then between the high and the low, then equatorward of the low.


The Rex Block is a variation of either the blocking high or cut-off low in which the opposing upper-level circulation becomes as large and as intense as the original height center. This pattern appears as a strong east to west jet on water vapour imagery with a diffluent plume at its western end which reinforces the primary meridional deformation zone west of the height centers. The block will remain nearly stationary until one of the height centers changes intensity, unbalancing the high-over-low pattern, or until the meridional deformation zone dissipates.

Associated Weather

With a Rex Block, conditions under each height center are typical of that height center. The abnormality of the Rex Block is the strong easterly winds at and below 500mb between the high and low. This easterly flow when centered over the mountains of the West Coast of the United States often produces Santa Ana type winds and can exacerbate conditions which may already be ripe for wildfires. Upstream from the block, west of the deformation zone, conditions are typical of zonal flow. After the recombination of the split flow on the east side of the block, expect to see warming and drying conditions more typical of a progressive ridge.


Below is an example of a Rex Block over the Pacific Coast of the United States from January 26-29, 2007. To show/hide the 500mb height centers, deformation zone, and associated warm/dry and cool/wet weather area, use the checkboxes at the bottom of the animation.


Compared to other blocks, what is abnormal about the 500mb wind field associated with a Rex Block?
Choose the best answer.

Select the correct answer.

Although a trough is present on the equatorward periphery of the block, a ridge is present on the poleward periphery, so the pattern is neither predominantly trough-like or ridge-like.
All blocks should contain a meridional deformation zone.
That's right. The Rex Block is abnormal because it develops a strong easterly jet, which can significantly alter the weather underneath that region of the Rex Block.
Blocks should not have a disproportionate amount of either absolute vorticity sign.