Wettest 12 Months in U.S. History

The 12 months ending in April 2019 were the wettest year-long period in U.S. records going back to 1895, according to the monthly U.S. climate summary issued Wednesday by the NOAA National Centers for Environmental Information. Averaged across the contiguous U.S., the total of 36.20” made the period from May 2018 to April 2019 the first year-long span ever to top 36”. The old record for any 12-month period was 35.78”, from April 2015 to March 2016.

Given the fierce drought-related impacts of the 2010s—including multiple deadly wildfire disasters from Tennessee to California—it may seem a bit counterintuitive that the nation has actually been getting wetter overall. Across the contiguous U.S., average yearly precipitation has risen by about 2” over the past century, from around 29” to just over 31” (see Figure 1). For the entire nation, including Alaska and Hawaii, precipitation increased by about 4% in the period from 1901 to 2015, according to the U.S. National Assessment.

Figure 1. Annual precipitation across the contiguous U.S. has increased by about 7% over the past century. Blue bar shows the linear increase since 1895, while the red curve is a smoothed version of the year-to-year numbers in green. When averaged over running four-year periods (not shown), the past four years are the wettest on record for the contiguous U.S. Image credit: NOAA/NCEI.

Of course, the averages above obscure a lot of regional and temporal variability, and the devil of drought impact lies in those details. U.S. climate is famously variable from year to year, decade to decade, and region to region (see Figure 2). As human-produced greenhouse gases boost temperatures over the long haul, both globally and nationally, the most intense precipitation episodes are getting even heavier, while the intense droughts that do occur in places like California are increasingly “hot” droughts, where the heat pulls moisture from vegetation and the landscape more effectively. We may see similar tendencies toward hot droughts in other parts of the U.S. as the climate continues to warm. The upshot is  that drought impacts can intensify in a warming world even in places where the long-term precipitation average, across both wet and dry periods, is unchanged or even rising slightly.

A 2018 study found that California’s wet season is likely to get compressed into a shorter window, likely leading to precipitation “whiplash” between wet winters and hot, dry summers.

Figure 2. Seasonal changes in precipitation over the United States. Changes are the average for present-day (1986–2015) minus the average for the first half of the last century (1901–1960 for the contiguous United States, 1925–1960 for Alaska and Hawai‘i) divided by the average for the first half of the century. Image credit: Fig. 7.1, Chapter 7, Climate Science Special Report, U.S. National Climate Assessment.

A few soggy periods dominate the wettest U.S. years on record

The ten wettest 12-month spans in the contiguous U.S. (see below) are clustered around several periods when major events pushed the numbers up in a big way. Over the past year, we’ve had Hurricane Florence in the Southeast, as well as a remarkably soggy autumn in the Midwest and the nation’s wettest winter on record. In 2015-16, we saw the wettest months on record in both Texas and Oklahoma (May 2015) and the Southeast flooding related to offshore Hurricane Joaquin (October 2015).

The top-ten list also reflects the impact of the major El Niño of 1972-73 and the wet Midwest winter and spring that followed, culminating in large-scale snowmelt and massive flooding along the Missouri and Mississippi River. The El Niño of 1982-83, the strongest on record at the time, also enters into the list, and it’s worth noting that a comparably strong El Niño was in play during 2015-16.

The Ten Wettest 12-Month Periods in Contiguous U.S. History

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
35.33”      Feb. 2018–Jan. 2019

The Wettest 12-Month Periods by Starting and Ending Month

                        Most recent                     Record

Jan-Dec          34.62” (3^rd wettest)         34.96” (1973)
Feb-Jan          35.33” (2^nd wettest)        35.63” (1973-74)
Mar-Feb          35.73” (1^st wettest)         35.73” (2018-19)
Apr-Mar          35.49” (2^nd wettest)        35.78” (2015-16)
May-Apr         36.20” (1^st wettest)         36.20” (2018-19)
Jun-May          30.05” (65^th wettest)       35.47” (1982-83)
Jul-Jun             30.13” (66^th wettest)       35.11” (1982-83)
Aug-Jul            30.18” (67^th wettest)       35.01” (1972-73)
Sep-Aug          29.75” (72^nd wettest)      34.43” (1972-73)
Oct-Sep           31.05” (44^th wettest)      35.15” (1972-73)
Nov-Oct           31.87” (28^th wettest)       34.77” (1972-73)
Dec-Nov          32.96” (11^th wettest)       35.08” (1982-83)

Figure 3. Statewide rankings for average precipitation for April 2019, as compared to each April 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.

April keeps the wet streak going

Last month was the 11th-wettest April in U.S. records. The moisture was spread fairly evenly across the nation, with the only states running drier than average right in the middle: Colorado, Kansas, and Nebraska. Eight states had a top-ten-wettest April, but none made it into the top three.

Figure 4. Statewide rankings for average temperature for April 2019, as compared to each April since records began in 1895. Darker shades of orange indicate warmer areas, with 1 denoting the coldest month on record and 125 the warmest. Image credit: NOAA/NCEI.

Temperatures were on the warm side, with last month ranking as the 22nd-warmest April on record. As with moisture, the mildness was well distributed from coast to coast. A brisk train of frontal systems moving across the country helped to keep the heat from intensifying to record monthly levels, although it was the second warmest April on record in Delaware and Maryland and the third warmest in California, New Jersey, and Virginia. Ten other states had a top-ten warmest April.