Death Valley heat in Kansas? How the end of June got so hot.

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."

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