A Winter Weather Advisory has been posted for both the mountains and the valleys in western Nevada and eastern California starting late Saturday night and continuing to the early afternoon Sunday. A split storm system has slowed the front so that any rain in Reno will likely hold off until after the sun goes down, but cold air moving in behind the initial front will drop snow levels to the valley floors before the sun comes up Sunday. Total snow accumulations in the lower valleys will probably be up to a couple of inches, with significantly more expected over the mountains. Strong winds expected ahead of the storm have also kicked off a Wind Advisory Saturday from 4 am to 4 pm.

So what causes a rain shadow? In order to see what stops the rain, you have to understand what makes rain (and snow) in the first place. Water vapor in the air has to first condense in order to make a cloud, and to condense the air has to cool. If you take moist air and force it upwards, the decrease in pressure makes it colder (it’s called adiabatic cooling). So in oversimplified terms, if you take moist air and make it go up, you get clouds and rain.

There are a lot of ways that can occur in nature…a cold front or converging airmasses in the lower atmosphere are just two examples. But wind sweeping up the west side of the mountains is another way. The lift provided by the Sierra from the dominant westerly winds is the reason that you get more precipitation at the top of the mountains than you get in San Francisco right next to the ocean.

While lift on the windward side of the mountains enhances precipitation, the opposite happens on our side of the hills. As I mentioned before, rising air cools, condenses and precipitates. Falling air warms, evaporates (clouds), and clears. If you are on the lee side of mountains like we are here in western Nevada, it greatly reduces the amount of precipitation you receive. That’s why we live in a desert.