On the 6th of October, a series of vortices developed over the northwestern portion of the United states over a region high horizontal wind shear aloft. This was preceded on the 5th by a large cutoff low over the US SW and a sharp shortwave trough along the US-CAN border. A region of deformation develops later on the 5th seen here in WV:
This deformation zone transitions into a region of strong cyclonic shear on the 6th, streamlines on WV :
The NAM also captures this region of enhanced cyclonic vorticity (250 hpa):
From there, a vortex street develops, seen here in WV:
Seems to be a case of barotropic instability, but there are other potential (or simply other mechanisms) theories on the development of these vortices including stratospheric intrusions. Unfortunately I do not have cross section data, so this simple analysis will have to suffice for this post.
Update: It is very well possible the unusually sharp shortwave (PV anomaly) trough penetrated the stratosphere resulting in the very dry air behind the system, resulting in a combination of the two effects discussed above.