Large parts of the US are at risk of major flooding in the coming weeks due to recent heavy rain and snow.
Almost half the country is at risk from these floods, which will also affect large parts of the Mississippi River Basin, with flooding forecast along the river between Minneapolis and St. Louis, according to a National Oceanic and Atmospheric Administration (NOAA) news release. .
In California in particular, the record-breaking snowpack in the Sierra Nevada will melt and allow water to flow into the valleys.
“Approximately 44% of the United States is at risk of flooding this spring,” Ed Clark, director of NOAA’s National Water Center, said in a statement. “California’s historic snowpack coupled with spring rains increases the potential for spring flooding.”
The NOAA forecast puts 146 million people at risk of flooding, including 1.4 million from major flooding.
Floods often occur after heavy rains, big waves or snowmelt because rivers burst their banks or the ground is so saturated that water can no longer sink into the ground and instead flows over the ground. Floods kill more people in the United States each year than tornadoes, hurricanes or lightning, according to NOAA.
Rainfall was very heavy in the west, with numerous atmospheric river storms hitting California. Last week, flooding caused by the heavy rains killed at least two people.
Snow in the Sierra Nevada broke records this week, with stations in the southern Sierra reporting snowpack 257 percent higher than the average for the day. Data from the California Department of Water Resources shows snow cover has surpassed the previous record year of 1982-1983.
When this snow melts, the water will run down the mountains. Because California has been in a mega-drought for over 20 years, the soil is very dry and hard, making it difficult to absorb water. If the entire snowpack melts at once, it can cause flooding at lower elevations.
“It’s important to remember that the timing and type of precipitation — rain or snow — is critical to avoiding a drought,” said Jacob Petersen-Perlman, an expert in water resource geography and an assistant professor at East Carolina University, before news week. “Timing is also a factor: All the rain at once means much of it is draining into the ocean rather than filling California’s reservoirs.”
NOAA’s forecast for Mississippi River Basin flooding is also due to higher-than-average snow cover in the mountains combined with poorer soil moisture absorption due to the scorching hot summer under drought conditions.
“There’s been a tremendous amount of snow this year, which is very encouraging,” said Donald Bader, Lake Shasta area manager at the US Bureau of Reclamation news week in January. “That will come later in the spring when it gets warmer, so we can anticipate that and add that to ours [reservoir] Storage volume because we know we have that extra storage up there in the mountains.”
In California, the large amount of rainfall has helped ease the state’s drought somewhat, with US Drought Monitor data showing 44.66 percent of the state is free of drought conditions, nearly doubling from a week earlier.
Even so, experts say it will take several years of continuous rain and snow to fully restore the state to pre-drought conditions as aquifers need to be fully replenished.
“It’s going to be several years of above-average rainfall — both rain and snow at appropriate times of the year,” said Lara Fowler, environmental and energy advocate and interim director of the Penn State Sustainability Institute at Penn State University, previously news week.
She continued, “As soil moisture and surface water supplies are depleted, increasing groundwater pumping has also led to a drop in groundwater levels in many places. This groundwater contributes to the ground flow in streams. So not only surface water – streams/rivers – need to be replenished, but also soil moisture and water tables, the latter of which can be replenished very slowly.
“It’s like we’ve exhausted our bank accounts and it’s going to take quite a while to rebuild. A year of good snow cover helps a lot, but it’s definitely not enough,” said Fowler.
Meanwhile, climate change is expected to change weather patterns across the country and could lead to worsening flooding.
“As climate change progresses, we expect less frequent but more intense rain and snow events,” said Rick Relyea, director of the Darrin Fresh Water Institute at Rensselaer Polytechnic Institute news week at the beginning of the year. “Such events can cause flash flooding, making it difficult for water to slowly infiltrate the ground to replenish underground aquifers.”
“Warmer storms mean more precipitation is rain rather than snow. Mountain snow that persists in winter and melts in spring/summer provides seasonal water storage,” said Roger Bales, a distinguished professor of engineering at the University of California, Merced, and an associate professor at UC Berkeley, previously recounted news week.
This can mean that although more water falls, this is not stored in reservoirs or groundwater, but is washed into the ocean as floodwaters.
Additionally, increased air temperatures due to climate change may cause snowpack to melt faster, potentially worsening flooding in California and other areas.
“As the big snowstorms we’re used to becoming big rainstorms in a warmer climate, we need to use more capacity behind the levees for flood control and leave less for seasonal water storage,” Bales said.
“We also get less downhill runoff in a warmer climate because growing seasons are longer and vegetation uses more water. Wildfires and drought-induced tree deaths are feedback loops that partially offset this. So it’s about quantity, timing, storage and more,” he said.
Do you have a tip on a science story that news week should cover? Do you have a question about the flood? Let us know at firstname.lastname@example.org.