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An atmospheric river is about to dump heavy snow and flooding rains

Snow already is covering parts of California courtesy Kamil Krzaczynski / AFP - Getty Images

By now, you’ve heard about the snow forecast for California with as much as 10 feet of snow possible in the Sierra Mountain Range (like Lake Tahoe and Mammoth Lakes).

Here’s the American Model Forecast which dumps a mere 99″ of snow. Check out the highlights:

I've highlighted 66", 84" and 99" of snow forecast

But these are just forecasts based on the snow ratio which is generally considered to be 10:1. In other words, an inch of rain would produce 10 inches of snow. This is just an estimate, as snow can amount to less under windy conditions (and blizzard gusts to 70 mph are expected), but amount to more under very cold conditions, which you’ll get in the mountains. The Midwest often gets a 12:1 ratio and can get a 20:1 ratio depending on other variables which is one reason forecasting the depth of snow is so difficult!

Here is more on snow ratio.

Here is the warning from the NWS office in Sacramento for blizzards, whiteout conditions and travel delays:

From the NWS office in Sacaramento

So what about the rain?

Given the ratio above is based on the rain, where it does not snow California will see devastating flooding.

This warning comes from The Weather Prediction Center:

Over the next three days, as much as 10 to 15 inches of rain could fall along the central California coast roughly between Monterey and Santa Barbara. This amount of rain in a 72-hour period is very rare for this region, with an annual exceedance probability of only 2 percent.”

The ‘atmospheric river’

The culprit behind this is known as an “atmospheric river,” and it is exactly like it sounds: a wind-driven column of moisture dumping huge amounts of water vapor in one spot. The best way to envision this is to just look at it:

You can see the big LOW spinning off the west coast driving that river of moisture

It’s easy to compare this to what we normally call a “feeder band” associated with a tropical system and that is part of the difference. These atmospheric rivers are NOT tropical but yet drop just as much rain!

Here is a final graphic explaining more and one remarkable takeaway is that these “rivers” can produce a water equivalent 15 times the average flow at the mouth of the Mississippi River!

Here's are some facts on this phenomena. Elements of this image furnished by NASA

All of this is going to end up producing mudslides in burn-scarred areas of California where those wildfires occurred, along with flooding and blizzards. A real mess. My heart goes West.

Enjoy our benign weather while it lasts. You know it never does!

Frank

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