New Weather Pattern Predicts Heat Wave 20 Days in Advance

Heat waves can cause major issues for the Northern Hemisphere. In fact, a 2006 heat wave across much of the United States and Canada was blamed for more than 600 deaths in California alone. Now, though, scientists may have found a new way to predict these heat waves. They've identified a distinctive atmospheric wave pattern high above the Northern Hemisphere that can foreshadow the emergence of summertime heat waves more than two weeks in advance.

Actually detecting this wave pattern wasn't easy, though. The scientists had to analyze a 12,000-year simulation of the atmosphere over the Northern Hemisphere. Yet they found that during the times when a distinctive "wavenumber-5" pattern emerged, a major summertime heat wave became far more likely to subsequently build over the U.S.

In order to create this simulation, though, the scientists turned to real data. They looked at relatively modern records dating back to 1948. More specifically, they focused on summertime events in the United States in which daily temperatures reached the top 2.5 percent of weather readings for that date across roughly 10 percent or more of the contiguous U.S.

The wavenumber-5 pattern itself is a sequence of alternating high- and low-pressure systems-five of each. These systems form a ring circling the northern midlatitudes, several miles above the surface. This pattern can lend itself to slow-moving weather features, raising the odds for the stagnant conditions that are often associated with prolonged heat spells.

In fact, the researchers found that during the 20 days leading up to a heat wave in the model results, the five ridges and five troughs in the wavenumber-5 pattern tended to propagate very slowly westward around the globe, moving against the flow of the jet stream itself. And when wavenumber-5 patterns in the model were more amplified, U.S. heat waves became more likely to form about 15 days later.

The findings reveal the potential for actually predicting heat waves further in advance. This could potentially help residents prepare for the scorching temperatures that can sometimes accompany heat waves and make living conditions dangerous. Currently, the scientists are searching for other circulation patterns that may presage other extreme weather events.

"There may be sources of predictability that we are not yet aware of," said Haiyan Teng, one of the researchers, in a news release. "This brings us hope that the likelihood of extreme weather events that are damaging to society can be predicted further in advance."

The findings are published in the journal Nature Geoscience.