Weather? Climate change? Why the drought is persisting and growing.

In late July 2011 the US drought had a parched core centered in the southern Plains and stretched along much of the southern tier. About 30 percent of the continental US was experiencing drought at varying degrees of severity, and another 11 percent was considered abnormally dry.

This year it has overspread most of the country.

Sixty-three percent of the continental US is now drought-stricken, a figure that grows to 79 percent when abnormally dry areas are included, according to the latest figures from the National Drought Mitigation Center in Lincoln, Neb.

"It's kind of like the blob in the old Steve McQueen movie,” says Martin Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory.

Summer monsoons have brought some relief to the Southwest and up into the Rocky Mountain states. Early tropical storms also eased dryness along the Gulf of Mexico and into the southeastern corner of the country.

But forecasters are calling for the drought to expand its boundaries through the end of October to cover more of Michigan, Minnesota, and the Dakotas, while to the east drought is expected to move deeper into Pennsylvania and West Virginia.

Several factors have contributed to the expanded drought, meteorologists say.

The lingering aftereffects of two years' worth of colder-than-normal sea-surface temperatures in the eastern tropical Pacific – a condition known as La Niña – set the stage. La Niña events drive average storm tracks farther north than usual as they snake across North America. La Niña also tends to encourage hurricane formation in the Atlantic and Caribbean, but with a couple exceptions, that hasn't helped the southern tier much.

[Editor's note: The original version of this story incorrectly described the effect that La Niña has on hurricane formation.]

But for some parts of the United States, some researchers add, the dryness encouraged by this natural climate cycle appears to be reinforcing a longer-term drying that is consistent with climate models gauging the effects of global warming. For the West in particular, conditions may be setting up for what researchers call a “megadrought” by the end of the century.

For the Midwest, the recent onset of drought was sudden.

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