We have a nice example of a Rex Block setting up over the western US this week. A Rex Block features a high over low configuration at upper levels, with easterly flow between. They are not unusual over the western United States.
The GFS dynamic tropopause forecast below provides a bit of a "modern" view of the Rex Block, which is often described in terms of 500 mb height. Note the pronounced anticyclonic PV anomaly over the northwestern United States and cyclonic PV anomaly over the southwest, with easterly flow between.
These two anomalies are closely related as the wave breaking event responsible for the anticyclonic PV anomaly also provides the necessary deformation to compact and fracture the cyclonic PV anomaly. This is readily apparent in the 06 UTC 16 Oct GFS forecast available here and suggests that Rex Block development may be intrinsically related to anticyclonic wave breaking in the presence of a downstream neck of cyclonic PV.
With "severe clear" conditions in place last night, Pony Express Marker (PEM) had a 52F diurnal cycle. Not bad, but not as great as seen in September.
This may change tomorrow as the NAM remains fairly optimistic about the development of clouds and precipitation as the Rex Block develops and the cyclonic PV anomaly approaches and fractures off the Pacific Coast.