Xmas surprise?

A few days ago a glanced at the long range forecast and saw nothing of note. Today I was shocked and excited to see that there is the potential for major Sierra snow storm on Xmas eve!  What gives?

Its a classic case of "downstream development" in which a distant storm over the western pacific triggers a series of downstream amplifying waves in the jet stream. The below figure makes the case:

An shortwave trough (A) enters from the west and perturbs the initially straight jet. The developing ridge (red line) causes a new trough (B) to form to the east (e.g., downstream). This trough builds the next ridge, which builds the next trough, and so on. The net result is an amplifying wave train that causes a major trough (trough C) to dig into the west coast on Xmas eve. 

Here it is in motion:

As of now the trough looks to be cold (~-8C at 700 hPa), which should amount to some great Xmas day skiing for anyone lucky enough to be spending the holiday in the mountains.

Lets hope this forecast holds together!