The governors of Nebraska, South Dakota and Wisconsin have declared emergencies, and Iowa’s governor has issued a disaster proclamation. At least two people in Nebraska have died in the floodwaters, and two others are missing.

Hundreds of families have fled their homes, especially in the Mississippi and Missouri flood plains, where levees were breached in many areas. Offutt Air Force Base, outside Omaha, Nebraska, said that one-third of the base was underwater Sunday. Even the National Weather Service said it had to evacuate its offices in Omaha on Friday because of rising water.

What touched off the flooding?

Rain was the immediate cause. The Weather Service’s Omaha office recorded 1.37 inches last week. More fell to the north and west, with Norfolk, Nebraska, getting 2.27 inches Tuesday, Wednesday and Thursday.

That is not all that much by itself, though it is more than the area usually gets at this time of year: Omaha, which averages less than 1 inch of rain in all of March, already has had 2.15 inches this month.

How did that cause so much flooding?

The devastating effect came mainly from what the rain fell upon: a snow-covered region that was unable to absorb much of the blow.

“A lot of it stems from the fall flooding in September and October,” said Mindy Beerends, a senior meteorologist at the Des Moines, Iowa, office of the National Weather Service. “The soil was saturated in the fall.”

That moisture stayed in the ground all winter, deep-frozen, while snow piled up on top of it, she said — and then, “on Wednesday and Thursday, warm air moved in, and we got rain, and the snow melted.”

“The higher-than-average precipitation, combined with warm temperatures, snowmelt and the frozen ground, was a perfect storm for flooding,” Beerends added. “The ingredients were in place.”

The flat, frozen land, unable to soak in much of the water, spread it fast and furious, the way liquid would spread across a tiled floor. And the runoff quickly filled many rivers and streams to overflowing.

“The ground was like concrete,” said Kevin Low, a hydrologist at the service’s Missouri Basin River Forecast Center. “In January, temperatures took a nose dive, and we’ve had deeply frozen ground all the way south into Missouri.”

Did anyone see this coming?

Forecasters knew that the snowy winter was building up the potential for spring flooding. In February, the Army Corps of Engineers lowered the reservoirs along the Missouri River to try to make room for excess runoff.

The snowpack began to melt the weekend of March 9 and 10, leading to widespread snowmelt of 2 to 3 inches in many places and 4 inches in isolated spots, according to the National Weather Service. Some places had the equivalent of 6 inches of water in 24 hours, Low said.

“This would not have been historic if we had not had the 2 inches of rain on top of that,” he said. “We would have had some minor-to-moderate flooding. Instead, we have 30 locations that hit records in central and eastern Nebraska, and northwest Iowa, including two locations on the Missouri River itself.”

Under models of climate change, a warmer atmosphere can hold more water, and thus the likelihood of intense inundation increases.

What’s the forecast now?

River levels have peaked in many areas and have started to recede, but parts of Nebraska, Iowa, Minnesota, Wisconsin and South Dakota still face “historic to catastrophic flooding,” the National Weather Service said Monday.

Warm weather will continue to melt snow this week across the vast area drained by the Mississippi and Missouri rivers, from the northern Great Basin to the Northern Rockies, and water levels downstream in Missouri are expected to keep rising for several more days.

This article originally appeared in The New York Times.