More frigid temperatures always prevail on hills, and certainly toward the Cascade foothills and above. Exposures also vary from the shelter of woods to a fully exposed bare hillside. Plants don't experience the wind chill factor that affects warm-blooded creatures, but drying winds can cause water loss from plants. Some may be damaged by both freezing and dessication. Cloud cover helps to moderate the effect of freezing, because plants don't dry out quite as fast as they do in a long period of sunny days with frozen soil.
Snow also moderates plant damage, acting as an insulating layer.
In addition to the variables presented by the weather and the site of the plant, plants react according to their own genetic hardiness and their place of origin. Plants such as Scotch pine and Mugo pine originated in climates generally colder than western Washington. Many gardeners have been able to grow this plant and many like it through the last five winters but may find in the spring that they are totally dead. If the plants haven't gone dormant, they will experience more freeze damage. I expect freeze damage on all the flowers and shoots of hardy fuchsias frozen entirely.