Wettest 12 Months in U.S. History, Yet Again

Topping a remarkable record that was set just a month earlier, the year-long period ending in June was the wettest 12-month span in U.S. records that go back to 1895. For the 48 contiguous U.S. states, precipitation averaged 37.86” over the period from July 2018 to June 2019. The previous record for any 12-month period was 37.68”, recorded from June 2018 to May 2019. No other year-long spans on record have averaged more than 37 inches for the entire nation.

What’s more, going back another month, the period from May 2018 to April 2019 recorded 36.20”, putting it in third place.

Another period of sustained wetness in 2015-16, including a peak in spring 2015, led to the fourth and fifth wettest year-long spans on record. Sixth place is now held by April 2015 – March 2016. Put another way, the six wettest spans among all of the 1494 overlapping year-long spans since January 1895—and seven of the eight wettest—have occurred in the last five years. Here is the current top-ten list:

37.86"      July 2018–June 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
35.42”      May 1982–Apr. 1983

Figure 1. The last 12 months were the wettest July-to-June period by far in U.S. records dating back to 1895. The total of 37.86 inches is more than 3 inches more than the previous July-to-June record. The last 12 months are also the wettest of any year-long span in U.S. records. Image credit: NOAA/NCEI.

Behind these record-wet year-long spans, various parts of the nation have seen a wide spectrum of eye-popping precipitation events, each lasting days to weeks. These have been separated by less-eventful periods, but the overall skew toward wetness is what’s led to the impressive 12-month records above. Except for March, each of the last 12 months have been wetter than average for the nation as a whole. Five of those months were among the top-ten wettest for those months. The full rundown:

July 2018:  43rd wettest (tie)
August 2018: 20th wettest (tie)
September 2018:  2nd wettest
October 2018: 6th wettest
November 2018: 29th wettest
December 2018: 7th wettest
January 2019: 33rd wettest
February 2019: 2nd wettest
March 2019: 89th wettest
April 2019: 11th wettest
May 2019: 2nd wettest
June 2019: 27th wettest

Another data point: The last two weeks of May were the first in the 19-year history of the weekly U.S. Drought Monitor without any areas flagged as experiencing D2, D3, or D4 drought (severe, extreme, or exceptional). As of July 2, the D2-to-D4 area was a mere 0.59% of the contiguous U.S. No values below 1% had been observed in the history of the monitor prior to March 2019.

Impacts from coast to coast

Flood-related issues have plagued large parts of the U.S. over the past year, the result of multiple periods of very heavy rain and snow. Some of the highlights:

• Hurricane Florence brought massive rainfall to the mid-Atlantic in September, including the heaviest totals ever recorded in a tropical cyclone in both North and South Carolina.
• Flash-flood episodes dotted the Upper Midwest last fall, as the region saw its third wettest autumn on record.
• More than a dozen locations from California to Michigan saw their snowiest February on record, coupled with unusually cold temperatures.
• The “bomb cyclone” of mid-March caused record flooding and widespread destruction over parts of the Central Plains and Midwest, as a burst of heavy rain triggered snowmelt and major ice jams. All-time high river crests were reported at more than 40 locations.
• A siege of severe weather and torrential rain in late May produced intense flash flooding over parts of the Southern and Central Plains, followed by several all-time-high crests along the Arkansas River in Oklahoma and Arkansas. Kansas, Missouri, and Nebraska had their wettest May on record.
• The relentless moisture of the past few months has also led to records for the longest span of flood conditions at several points along the Mississippi River. Hundreds of thousands of acres in western Mississippi have been under water for months as a result of backwater flooding in the Mississippi delta near Yazoo City.

What next?

As shown above, July 2018 wasn’t a particularly wet month, so if July 2019 turns out to be even slightly above average, we can expect the next 12-month span (August 2018–July 2019) to break the year-long precipitation record that we’ve just set. The string of records may end at that point, though, unless it remains markedly wet from August onward.

Moist patterns can be surprisingly tenacious, for at least a couple of reasons: continental-scale upper-level patterns that can hold in place for months (such as those related to El Niño), and regional and local positive feedbacks that allow a moist landscape to pump water back into the atmosphere, helping set the stage for additional rain. As human-produced greenhouses warm our climate, they’re stoking evaporation from the ocean, helping to make the most intense rainfall events even heavier.

There are hints of change in the Great Wet of 2019 on the horizon, though. For one, the weak El Niño in place since last autumn appears to be on its last legs. Cooling has been evident across the tropical Pacific over the last month, and recent runs of NOAA’s CFSv2 model bring the tropical Pacific into cool-neutral territory by late summer.

One wild card that remains is tropical activity in the Atlantic. Even if it’s not strong enough to become a named storm, a single slow-moving tropical system can still produce widespread, destructive regional rainfall that can push up the national average (see Jon Erdman’s weather.com roundup for several examples since 2015).

Figure 2. Infrared picture of showers and thunderstorms associated with a disturbance moving into the northeast Gulf of Mexico as of 2112Z (5:12 pm EDT) Tuesday, July 9, 2019. Image credit: tropicaltidbits.com.

Tropical cyclone still likely to form in the Gulf this week

Speaking of the tropics, we’re keeping a close eye on the disturbance dubbed 92L, now moving off the Florida Panhandle into the Gulf of Mexico. Intense thunderstorms were blossoming on Wednesday afternoon around this disturbance, with additional convection off the west coast of Florida. The evolution of convection tonight will help determine how far offshore 92L consolidates, and in turn how strong a system it might be as it moves toward the Texas and Louisiana coasts later this week.

In its 2 pm EDT tropical weather outlook on Tuesday, National Hurricane Center gave 92L an 70% chance of developing into at least a depression by Thursday afternoon, and models are suggesting a tropical storm (which would be called Barry) is likely to form. Regardless of how strong it gets, this system could easily produce rainfall amounts of 10" or more along and near the upper Gulf Coast, especially over southern Louisiana.

Figure 3. Precipitation forecast for the seven-day period from 7 pm EDT Tuesday, July 9, 2018, to 7 pm EDT Tuesday, July 16. Image credit: NOAA/NWS Weather Prediction Center.

See today’s earlier post from Jeff Masters for more on 92L. We’ll have our next update on Wednesday.