Saturday, December 5, 2015

What is the Rain Shadow and What's Wind Gotta do With It?

The dreaded rain shadow - something that those of us who live east of the Sierra crest are more than familiar with and hate to hear. So what is this rain shadow and why does it even exist? Lets start with the topography just to our west...

The Sierra Nevada mountain range runs 400 miles from north-to-south and is approximately 70 miles wide. It includes the highest peak in the lower 48, Mt Whitney at 14,505 feet, and rises above 10,000 feet in many locations.
Left: The Sierra Nevada Mountains highlighted in blue. Right: A relief map showing elevation (in meters) of the Sierra.

The Sierra are a significant barrier for Pacific storms which typically move from west to east. The movement of air masses and associated weather cannot go through the mountains and is forced up and over the mountain range. As Pacific moisture is driven eastward by the wind, the air mass is lifted up and over the Sierra. Rising air will cool and once it reaches the point of saturation, condense into clouds. These clouds can then produce the rain and snow in the mountains that millions of us depend on for water. The majority of the moisture will precipitate out on the west side of the Sierra, not leaving much for the eastern slopes. To add insult to injury, once the air crosses the Sierra, it begins to sink into the Great Basin and this sinking motion acts to warm and further dry the air. This is the basis of our rain shadow and rain shadows seen across the world. The rain shadow can clearly be seen in the average annual precipitation and snowfall maps below as well.

A graphic showing the rain shadow effect of the Sierra Nevada Mountains on top, with the average annual precipitation on the bottom.

Average annual snowfall. Some great examples of the rain shadow: Average for Donner Pass: 412", Truckee: 200", Reno: 24". Span from Donner Pass to Reno: About 40 miles as a bird flies!

Now that we know in general terms what the rain shadow is, lets talk about what enhances it and what type of Pacific storms will actually bring better rain and snow to the eastern Sierra and western Nevada.

Obviously we are going to need a lot of moisture in a storm if we are going to get a decent amount of precipitation east of the crest. One thing we like to see is a moisture tap known as an Atmospheric River (AR). An AR is a relatively narrow region (250-375 miles wide) of the atmosphere that is responsible for transporting a large amount of water vapor. In a strong AR event, the amount of water vapor transported is roughly the same as 7.5 to 15 times the average flow of water at the mouth of the Mississippi River. In fact, on average we will get up to 50% of our total annual precipitation in the western coastal states from just a few AR events! Several of our flood events have been caused by warm ARs dropping a ton of rain on a large snow pack, but not all ARs are this devastating. If we are able to combine a significant moisture tap with a cold low pressure system this is a recipe for significant amounts of snow. In fact, some of these events have brought us up to 10 feet of snow in just a couple days in the high Sierra! For more information about ARs, please check out the NOAA Earth System Research Laboratory page.

An example of an Atmospheric River pushing into the west coast in the yellow and green shading.

The other significant factor in how much shadowing or spillover we will see is the wind. Many think it has to do with wind speed, and yes that can play a role, but direction is much more important. Think about crossing the Sierra--which is faster...going directly west to east or slowly moving eastward on a north to south route? As we talked about earlier, it's a much shorter distance going west to east and you guessed it...a west wind is much more favorable for getting spillover! Southerly winds will create a significant rain shadow almost every time. Just look at the event we had on Dec 3-4 -- 12 to 15 inches of snow fell along the Sierra crest and not much more than sprinkles in western Nevada. What did we have ahead of and with this storm? Strong southerly winds! If we have a strong westerly wind along with moisture it helps to bring the moisture eastward quicker and can actually lead to less shadowing on the eastern side of the Sierra. Now, what if the winds are southwesterly? Well, then it depends on a variety of factors including the degree of which the winds are turned more south or west, how much moisture is available, and what the terrain is like southwest of the area of interest. Will it completely block the flow of moisture or are there gaps in the terrain which could aid in spillover? 

While there are still other factors that can contribute to whether we'll be shadowed, or on the other side of the spectrum, see spillover, just keep in mind that the amount of moisture and wind direction are two of the biggest factors. No matter what, for storms that cross the Sierra, western Nevada will almost always see some degree of shadowing, but certain factors can at least help us to get additional precipitation. The good news is that the forecast over the next 7-10 days looks to remain active, so we have additional chances for rain and snow throughout the area!