Scientists have discovered a possible early warning sign of heat waves: When a halo of five strong and persistent high-pressure systems crowd the midlatitudes of the Northern Hemisphere (see image; high pressure areas outlined in red, low pressure in blue), the chances of a heat wave striking the continental United States are up to four times higher than average, the researchers report today in Nature Geoscience. The team used a worldwide climate model that incorporated normal month-to-month variations in sea surface temperatures and sea ice coverage, among other climate factors, to simulate 12,000 years’ worth of weather. During the summer months of that period, and within an area roughly equivalent to the continental United States, there were more than 5900 heat wave events, including a total of more than 16,000 heat wave days (defined as days when the high temperature reached the top 2.5% of readings for that date across 10% or more of the continental United States). For the 2300 isolated heat waves (ones that had no heat wave days in the preceding 3 weeks) that occurred during the simulation, a higher-than-normal proportion were heralded by the halo of high-pressure systems . The distinctive weather pattern didn’t precede all heat waves, so other factors are also at play, the researchers note. Also, the occasional halo of high-pressure systems doesn’t seem to be linked to abnormally high temperatures in the tropics or to any particular climate cycle, such as El Niño. Instead, the researchers say, the configuration seems to be the result of natural variations in the atmosphere’s weather patterns. Nevertheless, the new finding provides hope that scientists can recognize 2 to 3 weeks in advance the conditions that make deadly heat waves more likely, an early warning substantially beyond the 10-day forecasts meteorologists typically muster.