Meteorologists may soon give 50-day notice for extreme heat

WASHINGTON (AP) – Meteorologists may soon be able to predict likelihood of extreme heat in the Eastern United States as much as 50 days in advance, according to a new study.

A team of scientists looking for links between heat waves and the water temperatures in the north central Pacific found a correlation strong enough they said it could be used to make “skillful prediction” of extra warm and dry weather about seven weeks before the temperatures rise. And they will start trying to forecast Eastern heat waves starting in May, said climate scientist Karen McKinnon, lead author of a study published Monday in Nature Geoscience.

Such warnings can help farmers, utilities and big cities prepare for periods when they’ll need more irrigation, more electrical power generation, and more cooling centers for the poor and elderly, experts said.

“There are many possible health, economic and other impacts of these hot temperatures and advanced warning can only help,” said Jonathan Overpeck of the University of Arizona, who wasn’t part of the study but praised it.

The team is looking at Eastern U.S. temperatures when at least 80 weather stations are at least 11.7 degrees warmer than normal for that day – basically, the top 15 percent of warmest summer peak temperatures going back to 1982.

The key is a large area in the Pacific north of Hawaii (which is not where El Nino forms). When the southern part of the region is unusually warm and the northeast of that region is unusually cold, the conditions are what McKinnon and colleagues call the Pacific Extreme Pattern. They looked at that pattern in summers from 1982 through 2015 and found that when they saw the most glaring PEP signal, the odds of a heat wave increase to one in four at 50 days out. By 40 days out, the odds have increased to one in two.

What’s likely happening is a train of waves moving from the PEP region to the United States with high pressure systems parked over the Eastern United States. High pressure systems in summer usually bring clear, dry and warm weather.

The scientists concentrated on the East – basically east of the Mississippi River, plus Iowa and minus Florida – because heat waves tend to hit the region as a whole, and that’s an area where high heat and little rain have consequences for farming and for a large population. The team also only looked at the peak 60 days in summer, which in a leap year like 2016 run from June 23 to Aug. 21.

“The physical mechanisms underpinning the results of their study make sense,” said Victor Gensini at the College of DuPage, who is working on long-range tornado forecasting but wasn’t part of this team. “It is exciting that there is some skill demonstrated as far out as 50 days as this also gives us hope for linkages to other extreme weather events.”

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Nature Geoscience: http://www.nature.com/ngeo