When you're in a NIÑA with a feeble subtropical jet you can create these setups wherein a trough comes out of the southwestern US if your northern stream wave enters the US at the right spot and divebombs far enough to the south and west to generate a decent moisture fetch off the Gulf. The early December event was the closest to a classic SE US setup with a s/w that dove south along the west coast and came into California along the spine of the Cascades and the Sierra Nevada instead of the Pacific. The longwave pattern that favors SE US overrunning events is also least favorable for s/ws to smack into California or the Baja Peninsula. This feature is often a missing key element in NIÑA winter overrunning threats, while the SE Canada vortex is usually absent during El Niños even though we tend to have more southern stream systems to work with. In a nutshell this means in a relative sense against climatology, moisture is often lacking during La Ninas relative to normal, while cold air is the missing relatively more often in NINOs even if the seasonal surface temperatures are colder than La Niñas. As discussed several weeks ago, while the observational record may not necessarily flesh this out due to sampling issues, if you assume the pattern will be cold in a La Niña and likewise in an El Niño and/or could reproduce a wide array of NINO and NINA winters hundreds of thousands of times and select the subset of years in both background states, you'd probably find that arctic outbreaks in the coldest NINA winters are actually more intense than those in the coldest El Niños thanks to the tendency for a blocking high epicenter near Alaska, the Bering Sea and NE Eurasia in La Ninas that's not favored as frequently in El Niños and allows Siberian air to be dumped into North America. The seasonal ENSO mean patterns from modern observations don't tell the entire story.