U.S. Racks Up Wettest Calendar Year to Date

The Big U.S. Wet of 2018-19 went on cruise control in July, but the year so far managed to hang on as the nation’s wettest calendar year to date in records going back more than a century, NOAA reported on Wednesday. Averaged across the 48 contiguous states, the month of July saw 2.69” of precipitation, which is just below the 20th-century average of 2.78”.

The period from August 2018 to July 2019 came in just shy of the wettest 12-month period on record. Below is the new list of the top-ten wettest 12-month spans in U.S. records going back to 1895. Amazingly, the seven wettest spans among all of the 1495 overlapping year-long spans since January 1895—and eight of the ten wettest—have occurred in the last five years. Even given the fact that a very wet span of a few months will be factored into such listings more than once, this is still remarkable testimony to the power of our warming climate to make extreme rain events even more extreme.

37.86"  July 2018–June 2019
37.73”  August 2018–July 2019
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

Figure 1. Total precipitation averaged across the 48 contiguous U.S. states from January to July, 1895-2019. Image credit: NOAA/NCEI.

Total U.S. precipitation from January to July was 21.79”.  That’s a full 0.51” above the previous January-to-July record of 21.28” set in 1998. No other two Jan-Jul periods are separated by that big a margin, except for the third driest (1931, with 14.92”) and the second driest (1988, with 14.23”).

How likely is it that 2019 will be our wettest calendar year on record?

Given the head start it already has, 2019 has a reasonable chance of breaking the wettest-calendar-year record of 34.96” from 1973.

—The 20^th-century average for August is 2.62”, and if this month comes in right at that amount, then the Jan-Aug total will exceed the record of 24.23” set in 2017.

—The average for September is 2.49”, so if both August and September were to match their 20th-century averages, then the year to date will end up above the record of 26.50” set in 2017.

Figure 2. Statewide rankings for average precipitation for July 2019, as compared to each July 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.

July 2019: Wet in the river valleys, dry in the Southwest

A delayed Southwest monsoon helped keep parts of the southwest U.S. drier than average in July. Above-average rainfall was focused near the Missouri, Mississippi, and Ohio valleys. South Dakota had its third wettest July on record, while no state had a top-ten-driest July. Nationwide, it was the 44th driest July in the 125-year database.

Floods and flash floods were distinctly less prevalent in July than they were in the first half of the year. The month’s biggest precipitation event was Hurricane Barry, whose heaviest rainfall lingered off the Gulf Coast until well after the storm’s center moved ashore in south central Louisiana. Barry produced less rain across Louisiana than expected, but nearly two feet fell in Ragley from July 12 to 16. The storm total of 16.59” in Dierks, Arkansas, set an all-time record for tropical cyclone rainfall in the state, making Arkansas the fifth U.S. state (including Hawaii) to set such a record since 2017.

Figure 3. Statewide rankings for average temperature for July 2019, as compared to each July 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.

Last month was the 27th-hottest July on record for the contiguous U.S., but this hides some distinct regional and temporal patterns. It was a top-ten-hottest July for all of New England, plus Delaware, Maryland, New Jersey, Arizona, and New Mexico. Boston had its hottest month on record, as did Portland, Maine. Meanwhile, some of the areas that were notably wet in July were also on the cool side, especially for our 21st century climate. Arkansas had its 29th-coolest July on record.

Not reflected in this map is Alaska, where July didn't just rewrite the record book—it set the old one on fire. July was the state's hottest month on record, easily besting July 2014. At least 13 locations in Alaska chalked up their hottest month, including Anchorage, which saw its first 90°F on record and blew away its previous hottest-month record from July 2016 by 1.9°F.

UPDATED. Warmest months on record in Alaska (1925-2019). July 2019 easily exceeded the previous record warm month. Also note that June 2019 is also on this list. @AlaskaWx @IARC_Alaska pic.twitter.com/GhQTw1j7Rk

— Brian Brettschneider (@Climatologist49) August 7, 2019