Long ago, Peruvian fisherman experienced a change in the ocean circulation in the equatorial Pacific Ocean, which resulted in poorer than normal fishing conditions. It always seemed to occur in December, so they called it El Niño ("the boy" or "the Christ child.")

If it seems like you hear more about El Niño all the time – you're right. The impacts are increasingly evident, partly because more people occupy areas where El Niño's effects are felt.

Also, more thorough and accurate meteorological measurements are demystifying the event. "Scientists have gotten better at making El Niño predictable," says John Nielsen-Gammon, climatologist at Texas A&M.

"There's some evidence that the effect is being enhanced in North America," says Elwynn Taylor, Iowa State University meteorology professor. "In the 1960s, El Niño had little or no impact outside the tropics. That was during a global-cooling period (1940 to 1972). In 1972, a warming period began, and El Niño started having significant impact throughout the globe." This natural warming cycle will likely continue through 2030.

El Niño is on the scene again, though computer models suggest it will be weak to moderate. Surface-water temperatures in the equatorial Pacific are above normal, and drought conditions have developed in South America, Australia and Southeast Asia – all El Niño indicators. In the Northern Hemisphere, the first effect is usually drought in Mexico.

But something brewing in the stratosphere may affect the United States' weather this summer more than El Niño. "Stratospheric winds change direction about every 13 months – this is called the Quasibiennial Oscillation (QBO)," says Art Douglas, Creighton University professor. The QBO has changed from east to west.

"A west-phase QBO favors a strong monsoon in Mexico, but dry weather to the north from Montana to the central Midwest. El Niño typically favors a dry Mexico and a wet Midwest," notes Douglas. "But given the current El Niño's weak nature, the QBO will likely dominant this summer's weather pattern." That means the odds are greater that the Midwest will see a dry summer.

When El Niño rules, precipitation tends to increase in California, the southern Rockies and Gulf Coast states. El Niño forces the jet stream and the accompanying storms south. Meanwhile, the Pacific Northwest tends to be dry.

"The Northwest is likely to see a dry fall. It could be wet from mid- to late-winter from southern California into Texas," says Nielsen-Gammon.

El Niño is strongly correlated with weather from Arizona to central Alberta; the Rocky Mountain States are an in-between area, with the switchover in Utah.

Warmer than normal temperatures are more strongly correlated with El Niño than precipitation. "You can expect to hear about dramatically warmer temperatures across the northern United States," says Neilsen-Gammon. "The farther south you go, the cooler the temperatures tend to be." The same goes for moisture.

El Niño has a stronger effect on winter weather than summer. "Most people consider El Niño insurance against widespread drought," says Taylor. It impacts temperature more than precipitation, but it does impact precipitation in the Corn Belt – usually in August when precipitation is scarce, he points out.

The continental United States has just experienced its 14th driest and fifth warmest winter in 107 years. Many states are experiencing drought conditions – from the departing La Niña. El Niño next winter may offer some relief.

But there are numerous kinds of drought, notes Taylor. According to the crop-moisture index, some areas are showing marginal drought levels, otherwise moisture is near normal. The big deficit is in reservoir water supplies.

Drought discussions may become more frequent, because in the last seven years the North Atlantic and North Pacific Oceans have shifted sea-surface temperature patterns (SST). Today, the North Pacific is cool and the North Atlantic is warm.

"For the next 15 to 20 years, this new locked-climate pattern between the North Atlantic and North Pacific should be an important factor shaping regional U.S. temperature and precipitation trends," says Douglas. "The United States will likely see more recurrent drought from the Southwest into the Plains and Gulf Coast regions."

The SST is like background information in the stock market," says Douglas. It means the United States is heading into 10, 20 or 30 years of warmer and drier conditions.

Suzanne Bopp is freelance writer