A long loop of GFS dynamic tropopause (jet-stream level) analyses and forecasts covering the period from 0000 UTC 5 May – 0600 UTC 10 May shows this trough has little inclination to move eastward, but instead is going to run as quickly as possible to Mexican border, making to the Gulf of California by Wednesday (blue areas have low dynamic tropopause pressures and are upper-level troughs).
This is a case of what meteorologists call trough fracture. Note how the trough appears to fracture away from the parent upper-leel trough and then dig southward and amplify. This is a common occurrence when a strong ridge builds upstream, as is the case in this event. In this instance, the ridge building occurs in response to the development of a surface cyclone over the Gulf of Alaska (not shown).
Also of interest is how the upper-level trough interacts with a weaker trough over Arizona. If you have good eyes, you might notice that they appear to rotate around each other for a few frames. This is known as the Fujiwhara effect in honor of Japanese meteorologist Sakuhei Fujiwhara.
So, this trough is not only heading south to the border, but it has picked up a partner to tango with when it gets there.