00:00Let'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.
00:10And this is why we're seeing so much very heavy rain and heavy snow in the Pacific Northwest right now.
00:16We 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.
00:26So the air goes upstairs, it's colder upstairs, reaches saturation, and we begin to see a lot more cloud production and heavy precipitation result on the west-facing slopes of the mountains.
00:38And on the inverse, down-sloping does the exact opposite.
00:43Sinking air is a process that warms the imaginary bubbles of air coming down the mountain, and warmer air can contain more water vapor.
00:51If you don't add more water vapor to this bubble of air, you're going to see the relative humidity drop.
00:56It's a drying process on the east side of the Cascades, in this example here, or the Rockies.
01:03So the upslope side is where we see a lot more moisture, we see rain, we see the snow.
01:08This 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.
01:16On the east side of that, places like, well, in many areas like the Columbia River Valley, this is the east side of the Cascades.
01:24So you've got the upslope side and the rain shadow.
01:27And you may go from 40, 50, 60, some spots over 100 inches of precipitation a year, to maybe 12, 14, 15, 18 inches of precipitation a year.
01:36We see a similar pattern across the northern Rockies as well.
01:39And this is why the flood threat this week has primarily been along and west of the spine of the Cascades.
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