Why did the temperatures stay so warm last night?

For many locations in western Washington, temperature stayed quite high overnight. 

Normally, when there are clear or nearly clear skies aloft, temperatures cool rapidly in the evening as the earth radiates heat to space.

But much less so last night.  

Consider the temperatures at Sea Tac Airport over the last week (below).  Red lines are the observed temperatures and the purple/cyan lines are the average highs and lows, respectively.  The highs on the last two days were way above normal, but look at the lows!  The low temperatures were roughly the same as the typical high temperatures.

But there is more interesting aspects of the low temperatures.  Below are the minimum temperatures last night (click on image to expand).  In places like Cle Elum temperatures dropped into the 30s, and in much of eastern Washington 40s were prevalent.    But western Washington was way warmer, ranging from the LOW SEVENTIES to the mid-50s.   It was particularly along the Cascade foothills in places such as Enumclaw.  Why?  And SeaTacwas warmer that Boeing Field and North Seattle. Why?

The main explanation:  strong easterly winds above us.

Easterly winds (wind from the east) result in air sinking down the western slopes of the Cascades, warming by compression as it declines from roughly 5000 ft to sea level.   Such compression causes adiabatic warming, by roughly 10C per kilometer (roughly 27F in 5000 ft).

During the last two nights the easterly winds were quite strong aloft, something shown by the winds above Sea Tac over the past 24 h (see below).  Time is along the X axis, height in the Y axis (700 is around 10,000 ft, red is temperature, winds in blue). Easterly and southeasterly winds of 15-25 knots were right above us).

During the day, surface heating causes mixing that brings down the warm air to the surface...that is why we had record breaking warmth on Saturday (and later today).

But at night, there is a battle going on.  Without solar heating, there is cooling as the surface radiates heat to space.  Without any mixing of warm air down from aloft, our temperatures would cool substantially.

But the strong winds aloft cause enough turbulence on their own to mix warm air down, keeping the temperatures up at night.  Places with the strongest easterlies (from Sea Tac Airport to Encumclaw) downstream of the Stampede Gap in the Cascades had the highest temperatures.  And the greatest warming tended to be in the foothills of the Cascades--places exposed to the uber-warm downslope air.  The temperature prediction by the UW high-resolution model at 8 AM this morning shows this effect.   Warm temperatures were also found downstream of the Olympic Mountains.

And in places like the University of Washington, you could see the "battle" taking place (see below).  The plot below shows conditions at the top of the UW Atmospheric Sciences building during the past day, with time increasing to the right.  The top panel is wind speed, the second is wind direction, the third shows temperature (black) and dew point (red) and the fourth is relative humidity.  Temperatures only dropped into mid-60s. 

But look carefully.  The temperatures, dew point and relative humidities were jumping up and down overnight (indicated by red arrow).   Now look at the wind direction....jumping between northwesterly flow (cool, low level air) and northeasterly air (warm downslope air).  Turbulence as occasionally mixing the warm air down....and when it did the relative humidities would drop.

As soon as the sun came up the battle was over....the warm, downslope air was being mixed to the surface.  And we are going to get very warm because of it.