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US Heat wave, why the end of June is so hot

Pete SpottsThe Christian Science Monitor

Across the US, high-temperature records are falling like beads of sweat, thanks in part to back-to-back La Niñas and a current jet-stream pattern that is steering storm systems coming off the Pacific well up into Canada.

These records appear to be falling into step with a longer-term trend in which record highs are being set more often than record lows for each decade since the 1970s – a trend many climate researchers have attributed to global warming.

As June 2012 draws to a close, it feels more like mid-July or August to people in wide swaths of the country.

Between June 27 and June 28, 32 communities stretching from Colorado to Indiana posted the highest temperatures on record ever for their locations – with a handful tying or topping records set only a few days before, according to data kept by the National Climatic Data Center in Asheville, N.C.

Norton Dam, Kan., for instance, recorded an all-time record of 118 degrees F. on Thursday, two degrees above Death Valley's July average. The 118-degree reading shattered Norton Dam's previous record of 113 degrees F. – set just three days before.

More than 350 sites across a broad swath of the continent's interior have posted daily record highs since June 27, with heat advisories on Friday covering all or parts of 23 states from Kansas east to the Carolinas and into the Northeast, and from Wisconsin south to Mississippi and Alabama.

At the same time, other parts of the country are reporting record lows for this time of year.

Anyone looking for relief might put the Northwest on their itinerary. Over the same two-day period, 57 locations, largely clustered in Washington state and northeastern Oregon, posted at least one daily high temperature that tied or beat the lowest for the date on which it was measured. Waterville, Wash., posted the biggest drop among the group – a high of 51 degrees on Wednesday, nine degrees below the previous record-low high of 60 degrees on June 27, 1946.

And it's all coming out of a spring that was the warmest on record in the US, bringing a heat wave to the center of the country in March the likes of which the US hasn't seen since 1910. Indeed, Spring 2012 in the US was 2 degrees warmer than the previous record-holder, the spring of 1910.

One reason for the seemingly relentless high temperatures is the presence of a broad ridge of high pressure inching its way across the continent, forecasters say. With skies generally clear, sunlight has a clear path to travel on its way to baking what in many places is an already parched surface.

As of Tuesday, a broad swath of the US was experiencing either severe or extreme drought, according to the National Drought Mitigation Center, based at the University of Nebraska in Lincoln.

The vast majority of the region stretching from southern Texas north into Nebraska and across to eastern California is experiencing severe to extreme drought conditions that in some cases have lasted for more than a year. Similar conditions cover a patch of the country from Arkansas northeastward through parts of Missouri, Illinois, Kentucky, and Indiana. A similar patter is persisting in the Southeast from eastern Mississippi through Georgia and into South Carolina, with some areas there experiencing exceptional drought conditions.

Conditions seem to be mimicking last years, with a slight geographic shift, says Klaus Wolter, a researcher who specialized in regional climate forecasting at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.

"Last year we had all that heat in Texas and Oklahoma. This year, things seem to be shifted a bit further west," he says.

Back-to-back years of La Niña conditions have set the stage, he says.

La Niña refers to one half of a see-saw pattern in ocean temperatures and atmospheric pressure along the tropical Pacific. During La Nina, tropical Pacific waters off the coasts of Central and South America become colder than normal, while waters in the western tropical Pacific become warmer than normal. During an El Niño event, the temperature patterns reverse.

Both La Niña and El Niño affect atmospheric circulation patterns in the tropics and beyond.

When La Niña prevails, the polar jet stream – a high speed river of air that steers storms across the continent – get pushed farther north than usual, taking storms that move off the Pacific with it. This dries out much of the US southern tier and areas up into the southern Rockies.

Over the past two winters, the US has been affected by back-to-back La Ninas, although the second one was weaker. And while forecasters now expect an El Niño to emerge during the second half of the year, atmospheric circulation patterns can be slow to make the shift, Dr. Wolter says.

Increasingly early snowmelt also leaves the soil drier heading into the warm season. With the landscape across much of the US already deprived of moisture, the region's temperatures rise higher because there is little or no evaporation from the soil to moderate the heat.

"So I'm not surprised we're setting some extreme records," he says.

The patterns that have dried out much of the Mountain states and southern tier also steer storms across the Pacific Northwest before they head into Canada. That accounts for the record low high temperatures there.

It's the same pattern that set up conditions for the extreme warmth in March 1910, Wolter says, suggesting that "once in a hundred years, Mother Nature plays some cards it has played before."

Wolter's biggest concern for the next few months is the potential for smoke from the large fires now burning in Colorado to offset the benefits from the Southwest's summer monsoons, which an on-coming El Niño can drive well into Colorado and beyond.

The tiny soot particles that make up the smokey plumes serve as tiny seeds around which rain can form. But the more particles that are present, the smaller the drops. Monsoons could fizzle by drizzle, instead of bringing badly needed rain.

Over the longer term, researchers need to tease out the causes for the slow pace at which the high pressure has been moving across the continent, he says. Forecasters attribute this to atmospheric blocking patterns, which can cause weather patterns to stall.

Such was the case in Russia in 2010, when a record-smashing heat wave gripped western Russia for five weeks.

"That was an extraordinary block," he says. "You can get a block for one week or two; that's garden variety. We see this all the time. To see it for five weeks is very unusual."

Researchers analyzing the event afterward estimated that there was an 80 percent chance that global warming produced the event – an effect researchers have dubbed "loading the dice."

But Wolter, who focuses his research on regional forecasting, notes that little is known about how the atmosphere sets itself up for such blocking patterns. Similar patterns in winter can lead to record-breaking winter snows accumulations as well.

Getting a better handle on the mechanisms is vital if forecasters hope to predict them, he says. "If you think of the impact of these extreme events, like last year's heat wave in Texas, it's huge."