What are Foehn Winds?

Foehn Winds (or Föhn winds) are locally-strong examples of cross-mountain winds (in mountainous areas) that cause warming and drying of descending air on the lee (leeward)  side of the mountain.  In some areas, there are local names, such as "Chinook Winds", for these winds that are a component of the Rain Shadow Effect.

In this particular post, there is a video on the types of rainfall and within the video, the Rain Shadow Effect is explained at approximately 5:19 in the video.   

In the linear mountain ranges of the North American West Coast (including Canada and Alaska), when the prevailing winds bring Pacific moist air inland, the Orographic Effect occurs as the air moves upslope and chills along the western mountain slopes, triggering precipitation.

Once the winds cross the ridgelines and start to descend the eastern, leeward slopes, the winds become warmer, drier, and faster (due to gravity), producing the Rain Shadow Effect and when particularly strong, the Foehn (Chinook) Winds.

There are times when temporary conditions "reverse" these prevailing directions.  When the sub-tropical Northern Jet Stream is in a "non-wavy" shape, inland High-Pressure Systems "stalled" in the warm Western Interior produce winds that blow westward towards the Pacific Ocean.

As these winds blow to the west and southwest, when they descend the western slopes of the Sierra Nevada and the Pacific Coast Ranges, they "reverse" the "normal" West-to-East Rain Shadow Effect.  When these winds are funneled through mountain passes, their speeds further increase due to the "Venturi Effect".  These are the yearly "Santa Ana winds" or "El Diablo winds" that dry the seasonal vegetation and help spread wildfires.