Not long after we crossed the threshold into autumn last week, leaving the fall equinox behind and watching the days grow shorter, an extreme cold wave hit the West. Records were smashed all around the region. Montana was hit by blizzards and several feet of snow. And in Washington, some of the higher elevations saw almost that much. Spokane got its earliest snow ever.
“We had a situation, that, if it had happened in November or December, we would have been digging out, here in Seattle,” says KNKX weather expert Cliff Mass.
He says the main reason for the cold was a blast of Arctic air brought in by an area of low pressure aloft known as an upper-level trough. It combined with high pressure over the Pacific Ocean and caused strong, icy cold to flow in from the North.
“We got substantially cooler than normal, but it was even colder as you got into Eastern Washington and particularly into Montana, where temperatures dropped in some places into the teens,” Mass says.
At the same time, the East Coast experienced an extreme autumn heat wave. Cities from Atlanta to Washington,D.C. to Atlantic City saw temperatures soar to the upper 90s. Mass says it got even hotter further south.
“In Alabama and Mississippi, there were some places getting to 100 degrees or more. They were breaking not only daily records, but they were breaking monthly records -- these are big records,” Mass says. “The temperatures back East have been extraordinary.”
LINKING THE EXTREMES
And he says the extremes are connected. While the Western U.S. has a strong upper level trough, the East is beneath the opposite: an upper-level ridge. That’s an area of high pressure aloft that pushes warm air into the region and keeps the cold out.
The two are “intimately connected” by waves in the upper atmosphere and an undulation of the jet stream, Mass says.
“You can think of a rope – you know if you have a rope and you shake it up and down and it has these wave patterns in it? The atmosphere is like that. And that wave produced these tremendously different anomalies: cold in the West, but warm in the East.”
Mass says one up side to the cold and storminess that chilled the West is that it seems to be causing the marine heat wave off the West Coast to dissipate. Mass calls it "Blob Jr."
“I just looked this morning and the blob right off our coast has really weakened,” Mass says. “Sea surface temperatures are near normal right off the Washington, Oregon and California coasts – that’s been a big change.”
He says the weekend ahead will offer at least one lovely autumm day (Sunday,) but after that, more cold is headed our way.
Friday: “the rainiest and cloudiest day of the weekend.” Showers on and off all day and a Puget Sound Convergence Zone later, producing heavy rain north of Seattle Friday evening. Highs in the mid-60s. Lows in the 40s.
Saturday: an improving trend begins. Showers in the morning, gone by lunchtime. High temperatures in the 60s, lows in the 40s.
Sunday: “the best day of the weekend.” Crisp, sunny, mild mid-60s. “It will be dry. It will be the most perfect fall day you can imagine."
But he says enjoy it because the winter weather is coming back.
Monday: clouds and showers come back later in the day and another upper level trough brings back those cold temps. More new snow in the mountains likely. The cold wave pattern we just saw last week returns, extending into next week.
Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.