The High Desert, a (mostly) dry place

As moist air travels from the west and hits the Cascades, the air cools, clouds form and it rains. As it sinks over the east side of the mountains, the air warms up and dries out, Mittelstadt said. Oregon State University researchers once labeled the stretch from Santiam Pass to Sisters as the most drastic rain shadow in the world, Mittelstadt said — until they found a more dramatic drop in China.

RAIN SHADOW

As the prevailing westerly winds bring moist air from the coast across Oregon and approach the Cascades, the air rises and cools. This causes most of the water vapor to condense and precipitate on the windward side of the mountains.

Source: WeatherPages.com

Some cities ride the rain gradients

Smaller changes in topography influence the weather as well — everything from the gradual slope down from La Pine to Maupin to the buttes around Bend can change where rain falls, where sun shines and where the cool or warm air flows. Areas closer to bodies of water like the Deschutes River can also see more fog than neighbors farther away, Mittelstadt said. Bend is in a transition zone, he said, where weather systems coming from the west over the mountains or from the southwest and La Pine can hit the area.

Those systems could bring either rain or snow or could peter out before they reach the city. Across Central Oregon at night, dense, cool air masses sink and flow downhill, he said, frequently settling in places like Sunriver. "On a calm night, that cold air will flow downhill like water, and then it'll pool in the lowest spot," Mittelstadt said. The chart at right illustrates the extreme rain gradient in the western part of the region.

Source: Oregon Climate Service