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!

Santa Ana Winds

We're seeing the ideal set up for Santa Ana winds (and Diablo Winds, and Mono Winds) today across California. Starting late last night a surge of cold air associated with a digging upper level trough slid down the east side of the Sierra Nevada and is now surging to the SW across southern California. 

Some of cold air gets block by the high Sierra, apparent as an extremely tight temperature gradient spanning the mountains, but south the the Sierra the air can expand across the Mojave, and then run down through the gaps into the LA basin. This coupled with the strong upper level winds drives so very strong gap flows, and in places mountain waves as well. 

The onset of the strong winds in and around LA has a clear signature: Rapid drying accompanied by consistently strong winds with an easterly component:

The cold air is spilling across the Sierra Crest near Tahoe, where the ridges are a bit lower, and producing some impressive ridge top winds. Maybe not the nicest day at the top of Squaw!