A large intrusion of cold air into the central and eastern portion of the United States this weekend ushered in the coldest air of the season across the Northern Plains and Great Lakes region. It was slightly unusual in that there was no significant surface low/intense shortwave trough and associated sharp cold front. What starts as a mid-upper level weakly baroclinic stationary trough centered over the Hudson Bay eventually develops into a case of lower to mid-level frontogenesis, incredible cold air advection, and synoptic subsidence incited by a low amplitude upper level wave disturbance.
The analysis fields of the GFS on the 30th September, 12Z depict the trough over the Hudson Bay:
The upper tropospheric wave disturbance (12z) is seen here over northern Alberta:
By 0Z the 31st, the analysis fields of the NAM/GFS nicely capture the intensifying wave disturbance as it begins to interact with the larger trough over the Hudson Bay:
The disturbance is quite evident in the WV loop:
This is also captured nicely in the RUC Analysis field at 0Z. Note the lack of a low-level wave as no low level mass response has developed at this point in time.
Since there has yet to be any low level mass response, it makes sense there is a total lack of cold air advection in the low levels (850 hpa) along the southern Canada border into northern MN:
By 18Z the 1st October, the disturbance had progressed over the Great Lakes with the weak phasing essentially complete. Note the significant subsidence behind the wave disturbance.
Here is how the 12Z GFS has it analyzed (at 500 hpa) 6 hours later (same time as the WV image):