The wettest 12 months in U.S. history
Precipitation over the last year (May 2018 to April) in the United States has been extraordinary. An average of 36.2 inches has fallen over the Lower 48, the first time it has topped 36 inches over a 12-month period in over 120 years of record-keeping. This amount is more than six inches above average, according to Weather Underground meteorologist Bob Henson, who first reported the record.
Heavy precipitation extremes have been a staple of U.S. climate conditions over the past year whether Hurricane Florence’s record-setting rain in the Carolinas, the wettest year on record in Washington, Baltimore and much of the Mid-Atlantic, a sopping wet winter in California, and recent flooding in the central United States.
Except for the Pacific Northwest and large swaths of New Mexico and Colorado, almost the entire contiguous United States has been wetter than normal over the last 12 months.
The record precipitation over the last 12 months fits into a long-term increasing trend, an expectation as climate warming intensifies rainfall. Since the late 1800s, annual precipitation averaged over the Lower 48 states has risen from 29 to 31 inches, Henson noted.
Drought disappears, in a flash
Following the widespread rains, drought affects just 2 percent of the country — about the smallest area since the federal government began official monitoring in 2000.
The sudden lack of drought is remarkable considering that in January 2018, nearly 40 percent of the nation suffered from a lack of precipitation. But since then, storm after storm has eaten away at the dry conditions.
Much of drought relief came in a span of just three months this winter. As recently as December, over 20 percent of the nation still suffered from a shortage of precipitation. Then, the Lower 48 posted its wettest winter on record, and the territory affected by drought shrank sharply.
Nowhere has precipitation fortunes changed as quickly or as radically as in California. In December, 75 percent of the state was in drought. That amount sank to zero after a stormy winter. More than 20 atmospheric rivers, streams of moisture from the Pacific, bombarded the state. Plant life exploded, and the state enjoyed a magnificent “super bloom” in the spring.
El Niño, the episodic warming of waters in tropical Pacific Ocean, is certainly a major driver of the rainfall recovery. It tends to pump subtropical moisture toward the West Coast and nudge the prevailing storm track across the southern part of the country. Here weather systems can draw moisture from not only the Pacific but also the Gulf of Mexico. NOAA projects that El Niño could persist into next winter, so there is no reason to expect drought to quickly rebound.
Incessant rains: Too much of a good thing?
NOAA tweeted Wednesday that the historic lack of drought is “good news.” But while dwindling drought is a positive for places such as California that were desperate for water, the spring’s excessive rainfall has spurred historic flooding in parts of the central United States. In its spring outlook, NOAA warned that flooding could be unprecedented in some areas, and that has played out.
Recall the March “bomb cyclone” that became a $4 billion flooding disaster in Nebraska, Iowa, Missouri, South Dakota, Minnesota and Wisconsin? Since then, additional historic flooding has hit the upper Midwest and Mississippi River basin, with the river level in Davenport, Iowa, reaching its highest crest recorded.
More than 100 river and stream gauges, mostly in the central United States, are reporting moderate to major flooding.
A powerful spring weather system this week has triggered a new round of flooding in several states in the central and southern United States, and more is expected.
“Our meteorologists are predicting rainfall amounts of 5 to 10 inches from Southeast Texas into portions of Arkansas, Louisiana, and Mississippi through the weekend with locally higher amounts approaching a foot,” the National Weather Service tweeted Wednesday. “This intense rainfall will lead to dangerous flooding."
Over the next two weeks, at least, the Weather Service projects wetter-than-normal conditions for large parts of the country. So concerns will continue to center on too much water rather than too little.
But if this recent and rapid drought recovery has taught us anything, it’s that precipitation patterns can change quickly. This can especially be the case in a warming climate in which precipitation extremes — both drought and heavy downpours — are intensified. At some point, drought conditions are sure to return and perhaps with little notice.