Perhaps these excerpts from my outlook last fall will prove somewhat illuminating....
Lee et al (2019) identified the transition from this Pacific trough regime to the Alaskan ridge regime as most conducive to the type of wave propagation needed to trigger a reflection event, which renders the Pacific trough regime the precursor pattern. This is a remarkable extended forecasting tool given the expectation for a mid January onset of a reflection event, as this implies that the first half of January is likely to be mild and characterized by Pacific trough regime with a rapidly consolidating PV, thus the arctic low regime is also worthy of consideration here during this mid-winter-thaw period.
This period will also feature a considerable Pacific jet extension as mild Pacific air infiltrates the vast majority of the CONUS and flooding becomes a concern for areas that amassed a snowpack during the early season arctic high regime. Any seasonably cold air masses are likely to be dislodged from the east coast by a primary storm track across the interior. The expected midseason progression is illustrated in the schematic below that details the typical evolution of reflection event. Note that the pattern begins to reverse in earnest approximately 5 days after the event onset, during which time the anonymously strong PV reverts to normal strength, and begins to stretch due to the building Alaskan and Aleutian ridging that retrogrades from North America.
This is why it very mild at the onset of these events, during the antecedent Pacific trough pattern, but by ten days post reflection the pattern has reversed and is characteristic of an Alaskan ridge regime (bottom right above). Once the process terminates and run its course by early February, a more canonical La Nina pattern will likely ensue in the absence of the expected SSW. However, should one materialize as anticipated, another arctic high interval is possible for potential grand finale to the season.