Warmer and Wetter Weather On The Way

Warmer and Wetter Weather On The Way

After two weeks of subfreezing temperatures, snowfall, and even sleet and freezing rain for folks living in Bellingham and near the Columbia River Gorge, a major pattern change towards warmer and wetter weather is in store for us. Although I like the cold (and LOVE the snow that sometimes comes with it), I’m excited to transition back to a much more active pattern that promises to bring gobs of snow to the mountains, heavy rain to the lowlands, and blustery winds just about everywhere.

Nationwide, we are coming off of one of the most prolific arctic outbreaks in recent memory. We were plenty cold here in the Pacific Northwest, but things were downright frigid once you went a bit further east. Fellow WeatherTogether Blogger Mark Ingalls wrote a fantastic post showing that much of the nation has actually been colder than the location of the Curiosity Rover on Mars over the past week. Even though the Curiousity Rover is located close to the Martian equator, it’s truly incredible that a significant portion of the country has been colder than an area nearly 50 million miles further away from the sun with an atmosphere only 0.6% as thick as Earth’s.

The culprit behind our arctic outbreak is also every news station’s favorite winter weather buzzword: the “polar vortex.”

A Sharknado Sequel?

But the “polar vortex” wasn’t just conjured up by a creative journalist some blisteringly cold winter day; it is an actual phenomenon in scientific literature that only found its way into the mainstream media over the past couple years. The polar vortex is an area of low pressure and very cold air that is generally kept locked up in the arctic by the jet stream – the current of strong winds in the upper troposphere that steer the paths of storms across the midlatitudes. But sometimes, a strong ridge of high pressure will push the jet stream poleward, disrupting the polar vortex and weakening it. As the vortex weakens, it loses its staying power in the arctic and can actually slide down into the midlatitudes courtesy of a large trough in the jet stream, bringing true arctic air down to the Lower 48.

 

Strong polar vortex (left) vs weak polar vortex (left) during the 2013-2014 winter. On January 5, 2014, Green Bay in Wisconsin recorded a temperature of -18 degrees F.
Credit: NASA

The graphic below shows the 500 millibar height anomalies in meters over the Northern Hemisphere so far this month. Millibars are a unit of pressure, and the 500 mb level corresponds to approximately 18,000 feet in the atmosphere. The “warm colors” indicate where ridges in the atmosphere are, while the colder colors indicate the locations of troughs, some of which are polar vortices. As you can see, a massive ridge of high pressure over Alaska weakened the polar vortex and sent it plunging southward. 

Though the Intermountain West and Upper Midwest bore the brunt of this arctic outbreak, the Pacific Northwest got a piece too.

Credit: ESRL Physical Sciences Division

Thankfully for those weary of the polar vortex, the Pacific Storm Train is revving up right off our coast, and will help bring much milder air to the entire region. As the satellite picture/model analysis below shows, the ridge of high pressure near Alaska has been replaced by a strong area of low pressure, and the jet stream is oriented east-west, not north-south. This helps keep cold air locked up in the arctic and brings more active weather to the West Coast.

Infrared satellite picture with modeled 500mb heights in meters (green lines) and 500 mb temperature in Celsius (red lines)
Credit: University of Washington Atmospheric Sciences

Though a ridge of high pressure off the coast of California is helping direct the majority of the action to our north, we are seeing an increase in cloud cover and a few areas of light precipitation off the coast and to the north associated with a weak warm front clipping our area.

Credit: UW Atmospheric Sciences

 

There is not a lot of moisture associated with this warm front, but because temperatures are still in the 30s around the region, the precipitation that does fall could start as a rain-snow mix or even plain snow. This is particularly true for the Northern Interior, where a north wind blows and temperatures are still below freezing in many locations. Any accumulations should be light – under a half inch for most folks, even those up north. The National Weather Service previously had a winter weather advisory for 1-2 inches of snow for Skagit and Whatcom Counties, but it was canceled this afternoon due to the warm front being weaker than predicted.

Credit: National Weather Service Mesowest Network

Later tonight into Monday morning, the warm front will move east and the tail end of the cold front will sweep across the region. Expect breezy southerly winds with this feature, particularly along the coast, Hood Canal, and north of Seattle. A low-end wind advisory is in effect for these locations form 10 pm tonight to 10 am Monday morning with the potential for winds gusting to 45 mph and local power outages.

Valid 04:00 am PST, Mon 19 Dec 2016 – 24hr Fcst
Credit: UW Atmospheric Sciences

Another, stronger system arrives Monday night and Tuesday. You can see this system in the above graphic around 150 degrees W, but it will strengthen significantly before making landfall. Models have been inconsistent with the location of this feature – most take it to Vancouver Island, but others take it much further south towards the Washington/Oregon border. If this system makes landfall to our north, we’ll deal with another round of blustery winds, and if it makes landfall just to our north (for example, it crosses the Olympic Peninsula), we’ll have to worry about the potential for stronger winds and a high wind warning.

This will be a very wet system. Models currently show Sea-Tac getting up to an inch of rain from 4 am Monday through 4 am Tuesday, with up to 5-10 inches of rain for the Northern Coast Range of Oregon and the Northern Oregon/Southern Washington Cascades. Snow levels will rise up to 2,500 feet in the Northern Cascades and 5-6000 feet around the Washington/Oregon border, but easterly flow through the passes will keep snow going in those areas.

Valid 04:00 am PST, Tue 20 Dec 2016 – 48hr Fcst
Credit: University of Washington Atmospheric Sciences

48-hour snow totals are very impressive: the Cascades and Olympics see 1-3 feet, with even more on the volcanoes. Mt. Rainier in particular gets walloped!

Valid 04:00 pm PST, Tue 20 Dec 2016 – 60 Fcst
Credit: UW Atmospheric Sciences

Things calm down on Wednesday after Tuesday’s storm, but yet another front arrives on Thursday, bringing more wind, rain, and mountain snow. Expect showers and sunbreaks for Friday before yet another front moves in on Saturday with cooler temperatures. Some models have hinted at a White Christmas Eve, but most just give us a chilly rain with low snow levels.

The far extended looks like more of the same, with near average temperatures and slightly above average precipitation. Snow will continue to pile up in the mountains, so be sure to get those skis waxed and head up to the hills when you have a chance!

Happy Holidays,
Charlie

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