Publication Date June 17, 2016 | Washington Post

How global warming may push the Southwest heatwave past an extremely rare threshold

United States
The Southwest is going to sizzle this weekend. The data tell us this it’s happening more often. Photo: Matt York / AP
The Southwest is going to sizzle this weekend. The data tell us this it’s happening more often. Photo: Matt York / AP

The evidence is piling up that global warming has likely intensified heat waves in recent decades. And, looking at the data itself, the historic hot weather forecast for the Southwest on Sunday and Monday fits squarely into this emerging story line.

Records are forecast to be shattered across southern Arizona, Nevada and southeast California. In a few cases, June heat records and even all-time hottest temperature records are in jeopardy...

The culprit is for this heat wave is an enormous, sprawling high-pressure heat dome that will span from roughly San Diego to Kansas City on Monday. The way we measure the intensity of a high pressure ridge is “geopotential height.” It’s the height at which 500 millibars is measured in the atmosphere (pressure decreases as you go up).

This weekend, 500-millibar geopotential height is expected to be outside our normal climate range, according to large group of computer model simulations. In other words, the models predict this ridge will be more extreme than anything observed in this particular climate data set that dates back to 1979...

Ryan Maue, a meteorologist at WeatherBell Analytics, has found these extremely intense heat domes have become more frequent in recent decades in the West. In a dataset he analyzed dating back to 1958, he found almost all of the high-powered heat domes have occurred since 1983 — with the overwhelming majority of them occurring since 1990.

The warming climate creates a background environment more favorable for such extreme events, Maue believes. “I’d surmise that the [6,000-meter] threshold — while an arbitrary big round number — is now more easily exceeded"

Maue’s analysis is consistent with an analysis performed by Richard Grumm, Science Operations Officer at the National Weather Service office in State College, and Anne Balogh of Penn State University. Grumm and Balogh investigated the frequency of what they call “warm pockets” across the Northern Hemisphere in June, July and August. Grumm and Balogh found a higher frequency of warmer air and larger ridges.