It Took More Than a ‘Heat Dome’ to Turn Portland Into an Oven - Bloomberg

It Took More Than a ‘Heat Dome’ to Turn Portland Into an Oven - Bloomberg

It Took More Than a ‘Heat Dome’ to Turn Portland Into an Oven

 Meteorologist Clifford Mass has the best explanation of the heat wave—and the rapid cool-off that followed.

June 30, 2021, 10:00 AM GMT+1

Residents at a cooling center during a heatwave in Portland, Oregon, on June 28.

 

As a physics major at Cornell in the 1970s, Clifford Mass helped famed astronomer Carl Sagan develop an atmospheric model for Mars. In the decades since, armed with a doctorate in atmospheric science from the University of Washington, he has become the go-to meteorologist when the weather goes nuts in the usually mild Pacific Northwest.

 

So it’s no surprise that Mass produced the most thorough explanation for the stunning heat wave that hit the region over the past few days. The “heat dome” that everyone keeps talking about is part of the story, but only part, as Mass explained in a series of podcasts and blogs.

 

Understanding what causes extreme weather like the heat wave is essential, as Mass says, because early and accurate forecasts can allow for preparations that save lives. (The National Oceanic and Atmospheric Administration says 291 weather and climate disasters in the U.S. have caused $1 billion or more in damages or costs, for a total of $1.9 trillion in 2021 dollars.)

 

The big story on June 29 was how quickly the extreme heat leaked away. As this chart of National Weather Service data shows, the temperature at Portland International Airport fell from 115 degrees on June 28 at 3:53 p.m. to 63 degrees on June 29 at 5:53 a.m. That’s a drop of 52 degrees in 14 hours—almost a Martian rate of cooling.

 

 

Still, the record temperatures won’t soon be forgotten. As Mass put it in a June 25 podcast looking ahead to the heat wave, “The pieces that need to come together to make this amazing event are pieces that can happen by themselves but rarely have occurred simultaneously.” Or, as he said in an accompanying blog post,  “it is like throwing several dice and having all of them come up with sixes.”

 

The ridge of high pressure in the upper atmosphere—the heat dome, as it’s called—was a necessary but insufficient condition. After all, if a high-pressure ridge was all it took, bouts of extreme heat would be common occurrences.

 

Another essential factor was that the ridge of high pressure was matched by a trough of low pressure just offshore. Both were probably the result of “a tropical disturbance that went northward” and “banged into the jet stream,” Mass said on his podcast.

 

The big pressure differential between the ridge and the nearby trough produced strong winds, which brought warm air from the desert southwest, Mass said. Topping it all off, as the flow of air came down the western slope of the Cascades mountain range, it became warmed by compression. Though Mass didn’t mention it in his podcast, that’s the same phenomenon—adiabatic heating—that raises the temperature of the scorching Santa Ana winds in southern California.

 

In a follow-up blog post on June 29, Mass marveled that “Seattle now has a higher record maximum temperature than Miami, Atlanta, Washington, D.C.,  or Chicago,” while “Portland's record high exceeded that of Houston, Austin, or San Diego.”  He explained the abrupt cooling by saying that “a thin layer of marine air surged in last night.” Relief at last.