Downslope Windstorm and Extreme Precipitation Contrasts Across the Olympics

The Olympic Mountains are a meteorological laboratory par excellence.   As shown below, the Olympics are nearly ideal as a meteorological research zone:  an isolated, near-circular barrier approached by uninterrupted flow from off the Pacific.  Plus, lots of observations around and within the barrier.  

It is a topographic island of extremes, with the wettest locations in the lower-48 states only miles away areas in which aridity and cacti flourish.

Consider this week.  Strong moist flow approached the barrier from the south, producing a stunning contrast in precipitation.  Consider the total precipitation from Sunday to today (Thursday) at 6 PM (below).

Extraordinary.  10-12.5 inches over the southeast slopes of the barrier near Lake Cushman, but less than an inch from Sequim to Port Townsend.  Only 0.28 inches on northern Whidbey Island.

The Skokomish River is found on the wet southeast side of the Olympics and according to the National Weather Service, it reached flood stage.

The Skokomish River cam confirms the wet situation!

The origin of this precipitation contrast is no mystery:  there was strong upslope flow on the windward (southern) side of the barrier, which produces heavy precipitation, while strong downslope flow on the lee (northern) side resulted in drying and lack of rain (see schematic below)

But there is more!   Strong downslope winds produced powerful warming and drying on the northeast side of the barrier that was captured by several weather stations on the lee slopes of the Olympics.

Consider the Indian Creek/Lake Crescent site shown by the red arrows.

On Tuesday evening, the winds surged across the Olympic Crest and headed down to the lower slopes where this station occurred....and things went wild.  The wind went from nearly calm to almost 60 mph in a few minutes!

As the winds descended on the NE side of the barrier, the temperature (cyan color) surged upward from about 45F to around 53F and then jumped up and down wildly.  Relative humidity dropped (green colors) as did dewpoint. (blue line).

What happened?   Air descending terrain warms rapidly since the air is being compressed as it goes from low to higher pressure during descent.  Warms like your bicycle pump as you compress the air inside it.  Warm air can hold more water vapor than colder air, so relative humidity drops at the same time.. 

Pretty dramatic changes.   There are more impacts of the Olympics that will have to wait until another blog, from the Puget Sound Convergence Zone to the intense low that can form on its NE side.

Another reason that Northwest weather is so interesting and fun to study.