Wettest 12 Months in U.S. History—Again

June 6, 2019, 1:31 PM EDT

Above: Flood waters surround homes in Fort Smith, Ark., on Wednesday, May 29, 2019. The Arkansas crested more than 2.5 feet above the previous all-time record at Van Buren, Arkansas, across the river from Fort Smith. Image credit: AP Photo/Hannah Grabenstein.

Propelled by a two-week siege of widespread severe weather and heavy rain in late May, the contiguous U.S. has once again broken its record for the wettest year-long span in data going back to 1895. According to the monthly U.S. climate summary released Thursday from the NOAA National Centers for Environmental Information, last month was the second-wettest month in U.S. history, with the nationally averaged total of 4.41” just behind the 4.44” recorded in May 2015. All other months in U.S. precipitation annals have been no wetter than 4.24”.

The year to date also ranks as the wettest January-to-May period in U.S. history. The nationally averaged total of 15.71” is well above the previous record of 15.13” from Jan.-May 1983.  In fact, the difference of 0.58” is almost twice as big as the difference between any other two Jan.-May periods in the 125-year dataset, when arranged from driest to wettest.

Precipitation averaged across the continguous U.S. for the period Jan.-May for all years going back to 1895
Figure 1. Precipitation averaged across the continguous U.S. for the period Jan.-May for all years going back to 1895, with 2019 standing head and shoulders above all prior years to date. Image credit: NOAA/NCEI.

Four states—Illinois, Kansas, Nevada and South Dakota—are having their wettest year on record to date through May, as noted by weather.com's Brian Donegan.

The sodden May pushed the period June 2018-May 2019 into a clear first place for contiguous U.S. precipitation among all year-long time spans going back to 1895.  Here is the new Top Ten list:

37.68”     June 2018–May 2019
36.20”      May 2018–Apr. 2019
35.95”      May 2015–Apr. 2016
35.78”      Apr. 2015–Mar. 2016
35.73”      Mar. 2018–Feb. 2019
35.63”      Feb. 1973–Jan. 1974
35.49”      Apr. 2018–Mar. 2019
35.47”      Jun. 1982–May 1983
35.42”      May 1982–Apr. 1983
35.35”      Mar. 1973–Feb. 1974

Remarkably, the last twelve months beat the previous record (set just a month ago) by a full 1.48”. None of the other margins on this top-ten list are greater than 0.25”, which speaks to the exceptional nature of just how wet it’s been nationwide. The period includes the wettest winter in U.S. history.

Another piece of evidence: for the last two weeks of May, the U.S. Drought Monitor reported no areas of severe, extreme, or exceptional drought (categories D2-D4) in the contiguous U.S.—the first “0.00” result in the 19-year-plus history of the monitor. In this week’s monitor, released Thursday, small D2 areas were added in far western Washington and parts of southeast Georgia and South Carolina. This amount of D2 coverage (0.51%) is still the lowest observed in monitor history prior to 2019.

Will it keep going?

It’s possible that the record for the wettest year-long span in U.S. history will be broken yet again a month from now. In order for that to happen, our current month would need to come in wetter than June 2018. That month was only the 40th wettest June in U.S. history, with 3.16”.

Of course, there’s no telling how much rain this entire month will produce when averaged nationwide, but some moisture-laden weather features this week have gotten things rolling. The slow-moving disturbance dubbed 91L never became a named system in the western Gulf, yet its circulation and moisture have led to torrential rain from southeast Texas into Louisiana. More than 9” fell south of Houston in just six hours on Wednesday morning. At least one person was killed on Thursday from flash flooding in the Baton Rouge area of southeast Louisiana. Major flash flooding also hit central Oklahoma on Thursday.

The slow-moving upper low generating the rain in Oklahoma will join forces with the Gulf Coast moisture, leading to a dreary weekend across the South with widespread heavy rain and potential flash flooding. 

Projected precipitation for the seven-day period from 8 am EDT Thursday, June 6, 2018, through Thursday, June 13
Figure 2. Projected precipitation for the seven-day period from 8 am EDT Thursday, June 6, 2018, through Thursday, June 13. Image credit: NOAA/NWS/WPC.

NOAA’s latest 8-to-14-day outlook shows wetter-than-average conditions over the bulk of the nation, with only the Pacific Northwest projected to be drier than average. NOAA’s monthly outlook for June, issued on May 16, called for better-than-usual odds of wetter-than-average weather over most of the western and central U.S., with typical odds elsewhere.

Wet patterns this widespread and strong are often surprisingly resilient, in part because the soaked landscape and vegetation continue to pump moisture into the overlying atmosphere to support subsequent rain. The weak El Niño also remains in place, which supports continued wet conditions over much of the nation. With all this in mind, persistence may be as good a forecast as any, and I won’t be shocked if we’re reporting on another 12-month precipitation record about a month from now.

May in review

Three states—Kansas, Nebraska, and Missouri—saw their wettest May on record in 2019, and twelve other states had a top-ten-wettest May. Only four of the contiguous U.S. states, all in the Southeast, were significantly drier than average in May.

Statewide rankings for average precipitation for May 2019, as compared to each May since records began in 1895
Figure 3. Statewide rankings for average precipitation for May 2019, as compared to each May since records began in 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 125 the wettest. Image credit: NOAA/NCEI.
Statewide rankings for average temperature for May 2019, as compared to each May since records began in 1895
Figure 4. Statewide rankings for average temperature for May 2019, as compared to each May since records began in 1895. Darker shades of red indicate higher rankings for heat, with 1 denoting the coldest month on record and 125 the warmest. Image credit: NOAA/NCEI.

With a cold upper low and widespread precipitation tamping down temperatures across the western and central U.S. for much of May, nationally averaged temperatures were on the cool side, despite some impressive local and regional heat. Last month was the 37th coolest May in contiguous U.S. recordkeeping, and the coolest May since 2011. The year to date (Jan.-May) is close to average in temperature—the 65th coolest of 125 years of recordkeeping, and the coolest Jan.-to-May period since 2014.

Florida saw its warmest May on record, and nine other states—all in the Southeast, except for Washington state—ended up with a top-ten-warmest May. Meanwhile, five states covering a wide expense from the Southwest to the Northern Plains saw a top-ten-coolest May.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

bob.henson@weather.com

@bhensonweather

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