East Wind, Rain Shadow

Below is the 700 mb pressure surface (about as high as the mountains). We have a cut-off low pressure centered near near the Utah-Arizona boarder. The green indicates the relative humidity is pretty high, above 80%. This can be associated with clouds and rain. Notice the winds in Utah bringing the moisture are blowing from the east. The mountains will cause some topographic lifting, cause some rain on the east side of the mountains, and the west side--Salt Lake and Utah counties--will be stuck in a rain shadow. The west side is expecting to get less rain than the east side, all because the wind is blowing from the east. Let me explain...

When air flows up a mountain-side it cools. Clouds are created. When air flows down the backside of the mountain the air compresses and warms, dissipating the clouds. Kind of a cool concept. Below is a picture I took today of what I thought illustrated the rain shadow effect. This is looking south towards Mt. Loafer. The winds were blowing from the east, from the left side of the picture to the right (as the model suggested). Air moving up the mountain cooled, allowing clouds to form up to the crest. On the west side of the mountain air moved down the mountain. The air compressed and was warmed. There where no clouds on the west side of the mountain.

(c) bkb

This same principle is the same reason why Seattle is generally wet and Spokane is generally dry. Weather patterns tend to blow from the west to the east in the Northwest. The rain shadow in this case is on the east side of the mountain while the wet side (the windward side) is on the west side of the Cascade range.