Friday, February 12, 2021

Why Portland, OR and Seattle, WA Get Snow and Ice Storms

National Weather Service Winter Alerts Issued Thursday, February 11, 2021 for parts of Washington and Oregon. Note the Blizzard Warning for the tiny area on the map, The Columbia River Gorge.

Rough winter weather is being forecast for the Pacific Northwest. In fact, Blizzard Warnings were issued, not for the mountains, but the Columbia River Gorge only a few miles from Portland.  In Portland and Seattle Winter Storm Warnings were posted. Don't these cities normally just gets clouds and rain in winter?  Yes, for the most part they do, however under specific weather patterns, they can get their share of winter weather.  

I find mountain weather so very interesting. Combine the largest ocean in the world on one side of these Pacific Northwest cities and some of the tallest and snowiest mountains in North America to the east and you get a wonderful natural laboratory to study the weather!

Portland, Oregon and Seattle, Washington are located on the Pacific Northwest Coast in between the warm and moist Pacific Ocean to the west and the towering Cascade Mountains to the east.  Because of their proximity to the coast and their sea level elevation, both cities receive very little in the way of winter weather.  In fact, Portland receives only 4.3 inches of snow annually and Seattle gets just a bit more at 6.8 inches.

You will want to look closely at two features on the map above, The Fraser River Valley in southern British Columbia, Canada up north and the Columbia Rover Gorge in the southern part of the map.  These two geographic features play a big role in wintry weather for Seattle and Portland. 

The biggest reason these two locations do not get much snow is that it is just too warm long the coast to see much in the way of frozen precipitation. Essentially, The Cascades act as a gigantic wall to keep the cold air that comes down out of Canada to the east of the mountains, protecting the West Coast from frequent outbreaks of really cold air.  

Down along the coast, at sea-level there aren’t many ways to get cold enough for snow or ice.  Just east of the coast however, it snows “to beat the band” in the Cascades because higher elevations alone produce a much colder environment, one that is much more favorable for snow.  The temperature drops a little more than 3 degrees for every 1,000 ft. elevation. If you go up 5-10 thousand feet that's a 15 to 30 degree drop in temperature, more than enough in the winter to make snow.  The extra lift created by prevailing westerly onshore winds heading up the slopes of these 10,000+ ft. mountains also wrings out more precipitation from the moisture-laden air. 

It’s crazy when you realize that Seattle is only about 50 miles as the crow flies from Mt Rainier. The mountain tops out at 14,411 ft. and at the Paradise Ranger Station, elevation 5400 ft., they average 639 inches of snow each winter. Back in the winter of 1971-72 they picked up 1,122 inches of snow, a world record at that time, while Seattle averages 6.8" of snow.

Wow!! What a difference 50 miles makes. This may be one of the greatest differences in annual snowfall over such a small distance. 

So, what does it take to get snowfall in locations like Seattle and Portland, its neighbor to the south? Well, you need a way to get cold air across the Cascades and into those coastal areas. A few times each winter season, the large-scale weather pattern sets up to provide a way to get that dense, cold air across the mountains.  Those conditions begin with the presence of an Arctic High Pressure system over western Canada that pushes very cold air down along the east side of the western mountain ranges.


In Seattle and locales around the Olympic Peninsula, that cold air can come through gaps in the mountains that are to the east.  Under conditions with strong High Pressure east of the Cascades and Low Pressure just off the Pacific coast, a pressure gradient develops from east to west. The wind wants to blow from high to low pressure, like letting air out of a balloon. The really cold and dense air from east of the Cascades wants to get west, but it’s so dense it is tough to get over the mountains.  However, where there are gaps in the mountains, it offers a perfect gateway for the air to move through, sometimes all the way to the coast.  

The Fraser River Valley, just across the Canadian border in British Columbia serves as a significant gateway for that arctic air to pass through and it is often responsible for pre-conditioning the temperature of the lower atmosphere with cold air to produce snowfall.


Portland has some of the same characteristics as Seattle, but is located about 100 miles south.  Instead of getting cold air through the Fraser River Outflow, Portland is right at the mouth of the Columbia River, which runs from east to west through the Cascades and down to the coast. 

The gorge is the only near sea level gap through the Cascades. Its average width is about 3 miles at river level.  It stretches 120 miles back to the east and the western entrance is less than 15 miles from Portland.  The crest of the Cascades lies about 45 mi east of Portland.  

One of the most beautiful places in the world in my book, The Columbia River Gorge features some amazing weather, including strong winds that allow for some great wind surfing. In winter, maybe not as inviting.


In fact, Portland Oregon gets a majority of its wintry weather when the winds are out of an easterly direction. On a broad sense of course, you would be hard pressed to get snow or ice from the westerly winds coming off that warm ocean.  However, similar to Washington State, a large pressure gradient between arctic air and strong High Pressure to the east of the Cascades and Low pressure off the Pacific Coast results in a flow of cold, dense air from east to west through the gorge.  

The graph below shows, without a doubt, the dependence on wind direction to produce wintry precipitation in Portland.  When the low-level winds are from the east and the mid-level flow is from the west, it becomes the “perfect storm” of sorts for snow or ice.  Snow will occur under conditions when the atmosphere is also cold aloft.  Typically, that comes with closed upper levels “cold-core” Lows.  However, if the air aloft is warm and the easterly gap winds are below freezing, then precipitation formed way up in the clouds in the form of snow melts as it drops through the warmer mid-levels then refreezes as it hits the surface where the temperatures are below the freezing mark. 

The distribution of annual snowfall and freezing rain vs. wind direction clearly shows just how important the role of easterly winds is for wintry weather in Portland, Oregon. 

The soundings below, taken at Salem Oregon, show the difference in the temperature throughout the atmosphere for snow vs. ice events in the Lowlands in Oregon to the west of the mountains including Portland. Notice the above-freezing layer in the sounding on the left, perfect for melting snow which then freezes on contact where surface temperatures are well below freezing. The sounding on the right shows above freezing surface temperatures but the atmosphere quickly cools below freezing a few hundred feet off the ground. That temperature and moisture profile results in heavy, wet snow. 

No, they don’t get too many snowstorms or ice storms in the Pacific Northwest, but they are more common than you might think in places like Seattle and Portland. You just need the proper ingredients to come together in the wrong place at the wrong time, and Voila, you have the recipe for winter weather.

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