Atmospheric river’s downside (Part Three)

Just to remind my readers, I wanted to know how atmospheric rivers were affecting our winter snowpack in the mountains of Park City as well as our flood risk at high altitudes. Atmospheric rivers behave like a high-pressure fire-hose of moisture. 

While they bring heavy rain to the coast, their impact changes dramatically once they hit the "vertical wall" of a mountain range, with a complete list of risks and benefits. First, in coastal areas, the primary threat is volume of water and its intensity. Because coastal temperatures are usually well above freezing, 100% of the atmospheric river’s moisture falls as rain. 

There’s the risk of flash flooding as the ground quickly becomes saturated, leading to immediate urban flooding and mudslides. This situation doesn’t last too long and often ends shortly after the "river" passes, as the water drains quickly into the ocean. 

The mountains present a different set of circumstances that specialists call "Orographic Lifting". As the atmospheric river hits the mountains, the air is forced upward. This is called orographic lifting. As the air rises, it cools rapidly, causing it to "wring out" even more moisture than it did at the coast. This means that mountainous regions often receive double or triple the precipitation seen at sea level.

There’s also a "Double-Edged Sword" effect for snow-pack at high altitudes where the atmospheric river becomes a game of temperature. These storms are typically warm, which creates two very different scenarios. First there is the good news also known as the "Big Gulp". 

If the storm is cold enough, it can drop 5–10 feet of snow in a single weekend. Some of these "drought-buster" events have been known to provide the bulk of the Western US water supply for a full year! Then there is the bad news, which seems to be happening more and more as we advance into climate change. It’s known as the “Rain-on-Snow”event. 

This is a big flood risk for mountain communities. Because ARs are warm, the "snow line" (the elevation where rain turns to snow) can climb much higher than usual, to 9,000 feet compared to Park City’s 7,100 feet. The Result is that warm rain falls on top of an existing deep snow-pack. The rain doesn't just run off; it melts the snow beneath it, releasing weeks' worth of stored water in just a few hours. 

This creates catastrophic "riverine" flooding downstream. This is what happened a lot until now and why I’ve been skiing so little this season!