AccuWeather's Geoff Cornish explains how upslope and downslope winds with the Rockies and Cascades affect weather patterns in the West.

Idaho

Mountain ranges in the western United States, such as the Cascades and Rockies, play a crucial role in shaping local weather patterns through a process known as orographic lift. Moist air from the Pacific is forced upward by west-facing slopes, cooling and producing heavy rain and snow, particularly in the Pacific Northwest.

On the east side of these mountains, descending air warms and dries, resulting in significantly lower precipitation. This leads to stark contrasts in annual rainfall, with some areas receiving over 100 inches and others as little as 12-18 inches. The current flood threat is concentrated along and west of the Cascades due to these dynamics.

How does topography influence weather in the western U.S.?

Let's take a look at orographic lift. What are we talking about here? How moisture runs into the west-facing slopes of the mountains, at least in this example, in the western U.S., and this is why we're seeing so much very heavy rain and heavy snow in the Pacific Northwest right now. We have this ribbon of moisture, we're calling it the atmospheric river, and as the air is shoved against the side of the mountains and it ascends, colder air can contain less water vapor, so the air goes upstairs, it's colder upstairs, it reaches saturation, and we begin to see a lot more cloud production and heavy precipitation result on the west-facing slopes of the mountains. And on the inverse, downsloping does the exact opposite. Sinking air is a process that warms the imaginary bubbles of air coming down the mountain, and warmer air can contain more water vapor. If you don't add more water vapor to this bubble of air, you're gonna see the relative humidity drop. That's a drying process on the east side of the Cascades, in this example here, or the Rockies. So the upslope side is where we see a lot more moisture, we see rain, we see the snow, this is what's happening right now in parts of the west-facing sections of the Cascades and the coastal range as well, as the snow levels eventually get a little bit lower. On the east side of that, places like, well, in many areas like the Columbia River Valley, this is the east side of the Cascades, so you got the upslope side and the rain shadow, and you may go from 40, 50, 60, in some spots, over 100 inches of precipitation a year, to maybe 12, 14, 15, 18 inches of precipitation a year. We see a similar pattern across the northern Rockies as well, and this is why the flood threat this week has primarily been along and west of the spine of the Cascades.

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