Why a West Coast Winter Storm's Birthplace has a Big Impact on Who Gets Snow

Chris Dolce
Published: February 7, 2019
Three general origins for winter storms on the West Coast.

The birthplace of a winter storm can make a big difference for where and how much snow it produces, and the West Coast is the ultimate example of why this is true.

Winter storms can move into the West from several different locations depending on the jet stream pattern. Those birthplaces, or origin areas, dictate whether snow can blanket valleys floors or remains pinned to the higher elevations, as illustrated in this explainer graphic tweeted by the National Weather Service (NWS) in Sacramento.

Birthplace One, Western Canada: The Coldest Storms

The coldest storms dive southward from out of western Canada into the Northwest, and although they don't contain high amounts of moisture, can produce snowfall in lower elevation foothills as well as valleys.

Winter Storm Lucian was an example of this type of storm in the first few days of February 2019. It brought several inches of snow to the Seattle metro area and also blanketed parts of the Oregon and Washington coast as well as the hills around California's Bay Area.

Seattle only averages 5 to 6 inches of snow each winter season, despite its relatively high latitude in the U.S. That's due to its location near the Pacific Ocean which helps prevent persistent subfreezing temperatures.

But when a storm tracks southward from western Canada, it helps to pull colder air from the Canadian Rockies through gaps in the Cascade Mountains and into the lowlands of the Pacific Northwest.

Birthplace Two, Gulf of Alaska: Cool, but Not the Coldest

Another birthplace that provides cold winter storms to the West Coast is the Gulf of Alaska.

Although not as cold as the type of storm in scenario one above, these systems can still produce snow at fairly low elevations in the mountains. Snow levels are typically 3,000 feet or higher in Northern California when this type of storm occurs, according to the NWS.

As you would expect, the northerly latitude starting point is what makes this storm scenario cold, but that is modified some by the milder influence of the Pacific Ocean.

Birthplace Three, Pacific Ocean: Warm and Snowy

Storms tend to be warmer and wetter when they originate farther south over the Pacific Ocean.

Example of an atmospheric river, highlighted in green, forecast by a computer model to landfall on the West Coast of the U.S. on March 5, 2016.

These storms sometimes tap into a so-called atmospheric river. An atmospheric river is a thin, but long plume of moisture in the atmosphere that stretches from the tropics or subtropics into higher latitudes.

While the atmospheric river packs plenty of moisture for snow, it's also filled with milder air which raises the elevation at which snow falls. In California's Sierra Nevada, this often means you have to be 7,000 feet or higher in elevation to see the white stuff.

The three scenarios above give a general sense of the type of winter storms that occur on the West Coast, but there are others that don't completely fall into those buckets.

For example, a warmer Pacific storm can sometimes meet up with preexisting arctic air in the Pacific Northwest. That may cause snow or ice initially before potentially changing to rain as the cold air is forced out.

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