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May was the wettest month for U.S. in 121 years of record-keeping

Rainfall totals across the U.S. in May. (National Weather Service)

When it rains, it pours — and in the month of May, it rained a lot. So much, in fact, that May was the wettest month on record for the United States in NOAA’s 121 years of record-keeping.

According to the report, an average of 4.36 inches of rain fell across the Lower 48, which is 1.45 inches above average. Of course, the majority of May’s torrential precipitation fell in Texas and Oklahoma, which both set their wettest months on record. Colorado also saw its wettest May on record last month.

[Texas and Oklahoma rains, by the numbers]

Long-lived, soaking rainstorms with heavy downpours were the norm in the southern Plains last month. These mesoscale convective systems — or “MCS” — tend to produce extreme rainfall totals over a large area, sweeping across multiple states over many hours. The complexes often develop after sunset and persist well into the dark of night, making them all the more dangerous. May’s flash flooding proved to be deadly for dozens of people in Texas and Oklahoma.

Southeast Oklahoma received as much as 28 inches of rain in the month, according to measurements from the Oklahoma Mesonet. Averaged across the Sooner State, 14.40 inches of rain fell, making May the wettest month on record for Oklahoma. Oklahoma City tallied 20.47 inches, which put May at the top of the list for wettest months in the state capital, as well.

[May’s torrential rainfall supercharged by climate trends]

Down south, 8.81 inches fell on average across the entire state of Texas, which made May the wettest month on record for the state — blowing away the previous record of June 2004 by over 2 inches. The rest of the top five statewide records are all within less than a ¼-inch of each other.

[Alaska just had its hottest May in 91 years]

An incredible 8 million acre-feet of water flowed into Texas reservoirs in May — enough to meet New York City’s water needs for seven years, according to The Washington Post’s Wonkblog. Of course, what ends up in the reservoirs is just a fraction of the total that fell. NBC in Dallas-Forth Worth says that the Texas rainfall total would be enough to supply the world’s population with drinking water for over 27 years.

The often excessive May rainfall also brought much-needed relief from an intense multi-year drought. As of Thursday, the U.S. Drought Monitor reported that just 24.6 percent of the contiguous U.S. was in drought, and NOAA says that this is the smallest drought footprint the United States has seen since February 2011. In fall 2011, nearly 80 percent of Oklahoma and Texas were in exceptional drought — the most severe category on the drought scale.

In early May, as Oklahoma and Texas were just beginning to rack up their extreme rainfall totals, we noted how the pattern was in line with a strengthening El Niño in the tropical Pacific, which tends to deliver very moist air to the southern United States and also with climate projections for increased precipitation:

The connection between a warming world and an increase in extreme rainfall events — the kind you only see every 10 or more years — is rooted in rudimentary atmospheric science. As air temperature increases, it can “hold” more water vapor. This means that, even if global weather patterns and the driving force behind storms doesn’t change, it’s more likely we will see more heavy precipitation events, whether it be mid-latitude cyclones in the spring, hurricanes in the summer or even snowy winter storms.
According to the U.S. National Climate Assessment, days of heavy rainfall are becoming more frequent across the country, and the amount of rain that’s falling in the heaviest events is on the rise, as well.
“Nearly all studies to date published in the peer-reviewed literature agree that extreme precipitation event number and intensity have risen, when averaged over the United States,” the National Climate Assessment states. Though scientists are confident that this trend will continue, the only remaining uncertainty is in the forecast models’ ability to determine exactly how that change will play out over time, and what regions will be affected the most.

More Texas-Oklahoma flood coverage:

May rainfall busted the Texas drought. Big time.

Houston drone video reveals extent of major flooding

May rains super-charged by strengthening climate patterns

May 6: Oklahoma City sets record for wettest day in May

May 14: Two days of torrential rain cause flooding nightmare in Houston

May 24: Oklahoma City sets record for wettest May

May 26: Houston is under water (again)