Deadly summer heat waves in the eastern United States may be predictable nearly two months before they occur, giving emergency planners and farmers more time to prepare, scientists reported on Monday.

The key to such an advance forecast, scientists said, is the occurrence of a distinctive pattern of water temperatures across a wide stretch of the North Pacific Ocean. While the existence of the pattern does not guarantee that a heat wave will occur, it significantly increases the odds of one happening as much as 50 days later.

From 1999 to 2010, about 620 people died each year, on average, from heat-related illness in the United States, according to the Centers for Disease Control and Prevention. Some heat waves have been especially lethal, like the one in Chicago in 1995 when more than 700 people, most of them old or infirm, died over five days.