Second Wettest Year in U.S. History; Exceptional Warmth in Alaska and Florida

Capping a spectacularly soggy period that spanned parts of two calendar years, the contiguous United States saw its second wettest year on record in 2019, according to NOAA’s annual summary issued on Wednesday. The national average temperature wasn’t especially hot by recent standards, but there were landmark heat extremes on either end of the nation, in Alaska and in Florida.

Figure 1. Precipitation averages for overlapping two-year calendar periods from 1895 through 2019. The period 2018-19 was the wettest of these, with 69.43” coming in well above the previous record of 68.62” from 1982-83. Image credit: NOAA/NCEI.

The annual average precipitation for 2019 of 34.78” came in just 0.18” shy of the record-wet year of 1973. However, the last 24 months (see Figure 1 above) easily set a record for the wettest two-year calendar span in data going back to 1895. Moreover, eight of the ten wettest 12-month spans on record for the U.S. fell within 2018 and 2019:

37.86"  July 2018–June 2019
37.73”  August 2018–July 2019
37.68”  June 2018–May 2019
37.55”  September 2018–August 2019
36.45”  October 2018–September 2019
36.21”  November 2018–October 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

These records were driven by sustained bouts of precipitation that extended from fall 2018 well into 2019. The resulting floods were prolonged and devastating, especially across the Midwest in late March, along the Arkansas River in May and June, and throughout the Mississippi Valley over much of the year. Depending on how the next few weeks unfold, major flooding could recur in the Midwest in spring 2020, based on moisture already in the largely frozen soil along with seasonal outlooks for a wet winter and early spring.

Floods unrelated to tropical cyclones accounted for only 3 of the 14 billion-dollar disasters tallied by NOAA in 2019, but they were responsible for almost half (44%) of the total cost of these events. Adjusted for inflation, this year’s tally of billion-dollar disasters was tied for fourth highest in the NOAA database, which goes back to 1980.

In five states—Michigan, Minnesota, North and South Dakota, and Wisconsin—2019 was the wettest year on record. Fifteen other states from Nevada to Rhode Island saw a top-ten-wettest year. On the other end of the spectrum, the state of Washington had its ninth driest year.

Figure 2. Statewide rankings for average precipitation in 2019, as compared to each year since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest year on record and 125 the wettest. Image credit: NOAA/NCEI.

The nation came in near average for precipitation in December (53rd wettest on record). However, some of the states hardest hit by excessive rain and snow in the past two years—including Michigan, Minnesota, Wisconsin, and South Dakota as well as Connecticut, Massachusetts, Rhode Island, and South Carolina—had a top-ten-wettest December. Meanwhile, Louisiana had its eighth driest December on record.

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

In contrast to near-record global heat, temperature wasn’t a standout in 2019 when averaged across the contiguous United States. It was the 34th warmest year in records that go back 125 years, and the coolest year since 2014. The year closes out a decade that saw nationally averaged temperature whipsaw back and forth from intense warmth (including the hottest year in U.S. history, 2012, and the next three warmest years on record, 2015–2017) to more typical values. Temperatures in the 2010s averaged more than 2°F above the values seen a century earlier, in the 1910s.

Figure 4. Average U.S. temperature, 1895-2019. Image credit: NOAA/NCEI.

For the year as a whole, unusual warmth in 2019 was concentrated in the South. Every state from Louisiana eastward to Florida, and northward to Kentucky, West Virginia, and New Jersey, saw a top-ten-warmest year. It was the warmest year on record for Georgia and North Carolina. Chilliness in the yearly average was centered in the northern Great Plains, but no state had a top-ten-coldest year.

Figure 5. Statewide rankings for average temperature in 2019 as compared to each calendar year since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest year on record and 125 the warmest. Image credit: NOAA/NCEI.

December ended up as the sixth warmest in U.S. history, and that warmth was unusually well distributed. No single state had its warmest December, and only Missouri, Oklahoma, and Tennessee ended up in the top ten, but all 48 contiguous states came in warmer than average, and only four contiguous states fell outside of the warmest third on record, as denoted in light orange below.

Figure 6. Statewide rankings for average temperature for December 2019, as compared to each December 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.

Warmest year on record in Alaska

The exceptional heat that dominated most of the year in Alaska—accompanied by periods of record-warm water and record-low sea ice extent in the Bering and Chukchi seas—led to the warmest year on record for the nation’s most northerly state. At 32.2°F (0.1°C), this was the first time in nearly 100 years of recordkeeping that the statewide annual average came in above the freezing mark.

First above freezing year statewide for Alaska according to NCEI. #akwx pic.twitter.com/x6vY8LHCNq

— Brian Brettschneider (@Climatologist49) January 8, 2020

The town of Utqiaġvik (formerly Barrow), on Alaska’s north coast, had its warmest year on record—if you can call it warm. The average of 20.8°F made last year the first in Utqiaġvik’s 101 years of recordkeeping to end up above 20°F.

Marathon lives up to its name: hottest annual average on record for a U.S. city

At the opposite corner of the fifty U.S. states, the town of Marathon, Florida, located about halfway between Key Largo and Key West, earned a dubious honor in 2019. Marathon’s annual average temperature for the year was a stifling 81.7°F (27.6°C). “This beats by a very big margin every yearly temperature ever recorded in any of the 50 U.S. states,” said weather records expert Maximiliano Herrera. The previous record annual average for any U.S. city of 80.5°F (26.9°C) was set at Marathon in 2015.

In fact, Herrera pointed out, Marathon came in above any annual average on record anywhere in The Bahamas, and just 0.1°C behind the 2019 average in San Juan, Puerto Rico. This year was warmer in Marathon than in Iquitos, the biggest city in the Peruvian Amazon and very near the equator.

In this extraordinary year, Marathon:

• set monthly-average heat records for February, April, May, June and October
• notched 131 days that failed to get below 80°F
• set 149 daily heat records (71 warm maxima, 78 warm minima)
• recorded its hottest temperature on record in February, March, April, and June

Official temperature records in Marathon only extend back to 1952, but the magnitude of this year’s record-stomping heat is stunning nonetheless. Not to be outdone, Key West also saw its warmest year on record, with an average of 80.3°F (26.8°C)—and the data in Key West go back to 1874. This is the first year in its 146-year database that Key West has seen a yearly average of 80°F or more.

As was the case with Alaska, the warmth in South Florida was accompanied and supported by record-warm waters nearby, in this case across and near the Florida Straits.

#SummerIsComing. Passion vine is blooming, tomatoes are getting bigger, and monarchs are devouring everything in sight. #Miami #MiamiHeat #Zone10b #NeverendingSummer pic.twitter.com/4pga23bFOs

— Brian McNoldy (@BMcNoldy) January 1, 2020

Ratio of U.S. record highs to record lows ratchets up again in the 2010s

The number of daily record highs to record lows in the United States, which has increased every decade since the 1970s, got another boost in the 2010s (defined as 2010-2019), according to NOAA data analyzed by independent meteorologist Guy Walton. Based on the data available through January 1, Walton found that the decadal ratio of record highs to lows will likely come in at around 2 to 1. That compares to 1.9 to 1 in the 2000s, 1.4 to 1 in the 1990s, and 1.2 to 1 in the 1980s.

Figure 7. Departures from 1-to-1 in the ratio of record daily highs to record daily lows for the United States. In a stationary climate (e.g., one not being pushed warmer or cooler by changes in greenhouse gases or other factors), one would expect the departures to be near zero over time. Image credit: Guy Walton.

Final data for December is still trickling in, but Walton expects 2019 to end up with a ratio narrowly on the warm side, as six months dominated by cold records were counterbalanced by a swarm of heat records in August and September. The only years this decade to rank decidedly on the cold side in terms of records were 2013 and 2014.

A 2009 paper cowritten by Walton and led by Gerald Meehl (National Center for Atmospheric Research) and Claudia Tebaldi (now at the Joint Global Change Research Institute) projected that the record-high to record-low ratio would continue to increase in the coming decades, perhaps reaching 20 to 1 by midcentury and 50 to 1 by late in this century assuming that greenhouse emissions follow a midrange scenario. It’s possible that the model is overestimating future warming based on its handling of 20th century trends, the authors noted, while adding: “Under any future scenario that involves increases of anthropogenic greenhouse gases and corresponding increases in temperature, the ratio of record high maximum to record low minimum temperatures will continue to increase above the current value.” (A follow-up paper in 2015 lowered the estimated future ratio to about 15:1 if U.S. temperatures warm by 3°C.)