The recent, record-setting heat wave appears to have been triggered by a little-noticed patch of storm activity off the western coast of Central America.

Descriptions abound of the so-called heat dome, a zone of high atmospheric pressure that pushed warm air down over the central and eastern United States, then held it for a sweltering week.

But with the average evening news weather map ending at U.S. borders, the heat dome seemed to come from nowhere. Of course that’s not the case.

“Subsiding” — descending — “air over the United States is associated with the heat wave. Where is the air subsiding from? There has be upwards motion somewhere,” said Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research.

Global rainfall anomalies between July 14 and July 19. Areas of unusually high precipitation are blue. NOAA

Asked what might have caused the heat wave, Trenberth pointed to data gathered by the National Weather Service’s Climate Prediction Center. The most tantalizing clue is contained in a map of global rainfall activity between July 14 and July 19, just as the heat dome came down.

The rainfall map shows heavy precipitation in the eastern Pacific, centered roughly off the coast of Guatemala. Heavy rains drive warm surface air upwards.

“If there’s upwards motion there, there must be downwards motion somewhere else. The odds are, that’s related to what’s going on over North America,” said Trenberth.

Indeed, maps of convection activity and wind velocities in the run-up to the heat wave hint at atmospheric currents that could have carried the warm eastern Pacific air into the central United States, where currents pushing down from Canada helped lock the hot air in place.

Enhanced (blue) and suppressed (red) convection activity in the 30 days preceding and including the July U.S heat wave. NOAA

Trenberth said the heat wave, while record-setting in its extremes, was not especially uncommon and shouldn’t be interpreted as climate change.

“This is largely associated with weather,” he said, contrasting it with the Russian heat wave of 2010, which was likely triggered by a climate-change–related rise in northern Indian Ocean sea temperatures.

“They had similar temperatures, but it got stuck in place for two months,” Trenberth said. “What would happen if the heat wave was stuck for two months instead of a week? We weren’t in quite the same situation.”

Top image: Temperatures in the United States on July 18. (NOAA)

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