WASHINGTON (AP) ― Meteorologists may have found a way to predict some killer heat waves up to three weeks in advance. Now, the best they can do is about 10 days.

An earlier warning would help cities prepare for the heat wave, arrange to open up cooling centers and check on the elderly, said Gerald Meehl, co-author of a study that describes the forecasting clue.

“It gives you a little bit of a heads up of what’s coming,” he said.

The key may be a certain pattern of high and low pressure spots across the globe high in the sky. When that pattern shows up, the chances double for a prolonged and intense heat wave in the eastern two-thirds of the United States, according to the study published in the journal Nature Geoscience.

This could predict some types of heat waves but not all, meteorologists said. The study’s authors said they think the pattern occurred before last year’s heat wave in much of the central United States, but they still need more work to confirm it.

The researchers at the National Center for Atmospheric Research in Boulder, Colorado, looked at heat waves that lasted at least a week and were about 5 to 8 degrees warmer than normal. In any given summer week, the odds of a heat wave like this happening are usually only about 1 in 67 in the U.S.

They did thousands of computer simulations and discovered that when high pressure and low pressure systems line up in a specific pattern, it foreshadows heat to come in about 15 to 20 days. Scientists call this pattern wave No. 5.

The weather on the ground at the time of the pattern really doesn’t matter; it can be rainy, dry, hot or cold, said study lead author Haiyan Teng, a scientist at the research center. The same pattern that signals a U.S. heat wave also indicates different extreme weather in other parts of the globe, like heavy rains, she said

This wave pattern was seen before the 1980 heat wave that was blamed for 1,250 deaths and pushed temperatures over 100 degrees in Dallas every day, said Randall Dole, a senior scientist at the National Oceanic and Atmospheric Administration. Triple-digit temperatures in Dallas persisted for a record 42 consecutive days that year. Dole, who wasn’t part of the study, said the science behind the study is sound, significant and may be practical after extensive testing.