The continental United States recently finished its soggiest 12 months in 124 years of modern recordkeeping. The results are visible in satellite measurements of fresh water.
From May 1, 2018, to April 30, 2019, the lower 48 states collectively averaged 36.20 inches (919.48 millimeters) of precipitation, a full 6.25 inches (158.75 mm) above the mean. The previous record (April 2015 to March 2016) was 35.95 inches. According to the National Centers for Environmental Information, ten U.S. states had their wettest 12 months, and three others were in the top three. Many of them were clustered in the Mid-Atlantic and Midwest regions.
According to the May 21 report from the U.S. Drought Monitor, just 2.72 percent of the contiguous U.S. was in drought, among the lowest levels in two decades of records. California is completely out of drought for the first time since 2011. As recently as February 2018, one-third of the United States was in drought.
The map above shows how groundwater has responded to the unusually wet year. The colors depict the wetness percentile; that is, how the amount of groundwater on May 13, 2019, compares to all Mays from 1948 to 2012. Blue areas have more abundant groundwater than usual for the time of year, and orange and red areas have less. The map is based on multiple types of meteorological data (precipitation, temperature, etc.) integrated within an advanced computer model developed by scientists at NASA’s Goddard Space Flight Center.
The second map shows soil moisture anomalies, or how much the water content near the land surface was above or below the norm on May 11–13, 2019. The measurements are derived from data collected by the Soil Moisture Active Passive (SMAP) mission, the first NASA satellite dedicated to measuring the water content of soils. SMAP’s radiometer can detect water in the top 5 centimeters (2 inches) of the ground. Scientists use that surface layer data in a hydrologic model to estimate how much water is present even deeper in the root zone, which is important for agriculture.
Much of the East and Midwest had an extremely damp autumn in 2018; land-falling category 5 hurricanes Michael and Florence dropped copious amounts of rainfall in the late summer; and California has been soaked by sporadic atmospheric river events and the effects of a mild El Niño. But there is no one explanation for the extreme precipitation of the past year. It does, however, fit with long-term increases in overall precipitation and with heavy rainfall events in our changing climate.
“I do not have an explanation for the weather systems that caused the heavy precipitation, but sea surface temperatures in the western Atlantic and Gulf of Mexico have been generally well above normal over the past year. This has surely added to the atmospheric water vapor content available to the precipitating weather systems,” said Ken Kunkel, a climatologist with the National Oceanic and Atmospheric Administration. “The pattern of precipitation over the past 12 months indicates general wetness over most parts of the U.S. but does not match projections of the future, which show increases mostly in the northern U.S. Thus, the recent wetness probably has explanations in addition to, or instead of, just anthropogenic forcing.”
In the Fourth National Climate Assessment, released by the U.S. Global Change Research Program in 2018, scientists reported: “a national average increase of 4 percent in annual precipitation since 1901 is mostly a result of large increases in the fall season. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901…The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century. Mesoscale convective systems (organized clusters of thunderstorms) in the central United States are expected to continue to increase in number and intensity in the future.”
Writing for The Washington Post, meteorologist Jason Samenow reflected on a record-setting year of rain in the nation’s capital: “The historic rainfall over the past year is somewhat of a random occurrence. It is mostly a result of weather patterns that have frequently arranged themselves, by chance, in an optimal way to squeeze water from the sky. Yet, at the same time, this record-wet year has occurred against a longer-term backdrop of climate warming and increasing precipitation extremes. In other words, climate change probably intensified the rain and increased the chance it would become a record breaker.”
NASA Earth Observatory images by Lauren Dauphin and Joshua Stevens using soil moisture data from the NASA-USDA SMAP team and using GRACE data from The National Drought Mitigation Center at the University of Nebraska-Lincoln, and rainfall data from The Iowa Environmental Mesonet The Iowa Environmental Mesonet (IEM). Story by Mike Carlowicz.