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.
"It's been kind of a flash drought," says David Miskus, a senior meteorologist specializing in agricultural weather forecasting at NOAA's Climate Prediction Center in Camp Springs, Md. When you look back to February of this year, there was no drought at all in the Midwest Corn Belt area.
But the winter was unusually warm and dry and was capped by a period in early March when daytime high temperatures in several Midwestern cities ranged from the upper 70s to the mid-80s.
The heat, combined with sparse snow cover, set the stage for drier soils heading into the growing season. Drier soils also meant less evaporation to ease the warmth.