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Still, even when a wave number-5 system is present in the atmosphere, the overall risk of a US heat wave is still small, she adds.

The existence of a strong wave number-5 pattern increases the chances of a heat wave to only 6% — not enough to allow meaningful warnings.

The next step toward eventually producing that kind of advance forecast is to pinpoint the conditions that create wave number-5 patterns, says Schubert.

The analysis revealed that major heat waves were four times more likely to develop after a strong wave number-5 system formed at mid-latitudes.