The U.S. Climate of 2014: Remarkable Hot, Cold, Wet and Dry Extremes

How you experienced the climate of 2014 depended a great deal--by some measures, more than any year in U.S. history--on where in the nation you happened to be. This was made abundantly clear in the full 2014 report on U.S. temperatures and precipitation, released this morning by NOAA's National Climatic Data Center (NCDC). When looking at the entire contiguous 48 states, the annual rankings aren't especially striking: the year placed 34th warmest and 40th wettest out of 120 years of data. The overall warmth comes as no surprise, given that every year since 1996 has placed above the nation's long-term temperature average.

These unremarkable statistics obscure the real story of 2014: the titanic contrast between a parched, scorched West (especially California, where the heat left all-time records in the dust), a very warm New England and Florida, and a much cooler area in between, with some months at or near all-time record lows in states stretching from the Great Lakes to the Gulf Coast.

NCDC's state-by-state map of 2014 temperature rankings (see Figure 1) tells the tale vividly. California, Nevada, and Arizona all saw their hottest year on record, going back to 1895. The year placed among the top-twenty warmest in most of the other western states, as well as in Maine. At the same time, a corridor of seven central states--Louisiana, Arkansas, Missouri, Illinois, Indiana, Wisconsin, and Michigan--saw 2014 place among their top-ten coolest years.


Figure 1. State-by-state rankings for annual average temperature in 2014. A ranking of 1 denotes the coolest year in the 120-year record, while 120 denotes the warmest. Image credit: NOAA National Climatic Data Center.

Another way to look at these contrasts is through the statistical lens of NCDC's Climate Extremes Index. The CEI is made up of five different indicators that show how much of the nation experienced a particular type of weather extreme. Two of the indicators relate to the percentage of the nation experiencing either unusually warm or cold daily highs or unusually warm or cold daily lows (averaged from month to month in both cases). Some 23.2% of the contiguous U.S. qualified as having unusually warm highs for the year, which is the 18th-largest percentage out of the past 120 years. The percentage of the nation experiencing unusually cold highs (18.6%) ranks 21st. What's especially intriguing is that this is the first year on record that both the warm-high and cold-high percentages have exceeded 15%, a sign of how difficult it is to sustain such wildly divergent temperature regimes between the Pacific and Atlantic for an entire year. Overall, using all the elements of the CEI, 2014 ranked as the 9th most extreme year since 1910 (excluding the impact of tropical cyclones), or the 19th most extreme when including the impact of tropical cyclones. Interestingly, for the second year in a row, daily record low minimums occurred more often than daily record high maximums (20,937 vs. 14,122). This trend is unlikely to continue; the opposite occurred for a number of years prior to 2013.

The Midwest and Southern chill established itself early, with a series of cold-air outbreaks that came to be associated with the term "polar vortex". (That phrase's meaning became so mangled in press coverage and popular understanding that it led the American Meteorological Society to update its official definition). Colder-than-average weather persisted across much of the central and east until May and June, which came in above average in most states. Midsummer saw a return to strikingly cool weather across the nation's heartland. The pattern was even more unusual--and pleasant for millions of residents--in that it was accompanied by relatively dry weather. It was the coolest July on record for Arkansas and Indiana, and the second coolest in Illinois, Mississippi, and Missouri.

After the west-to-east contrast eased somewhat in late summer and early autumn, a record-setting Arctic outbreak in November reestablished the cold-east/warm-West pattern once more, leading to the second-coldest November on record for Alabama and Mississippi. Finally, just in time for the holidays, the 48 states got on the same temperature track, with unusual mildness nationwide producing the second-warmest December on record. Alaska joined in as well: the state's 19 first-order weather stations were a collective 7.5°F above average for the month, and Fairbanks saw its second warmest December in its 111-year record, according to the Alaska Climate Research Center. Overall, 2014 was Alaska's warmest year in a 97-year period of record, with an average statewide temperature 4°F above the average for 1971-2000.


Figure 2. State-by-state rankings for annual average precipitation in 2014. A ranking of 1 denotes the driest year in the 120-year record, while 120 denotes the wettest. Image credit: NOAA National Climatic Data Center.

Days of deluge
The national precipitation ranking and the state-by-state maps (see Figure 2) hide some dramatic contrasts as well. Most of the year was extremely dry in California, even though the state ended up near average for total annual precipitation. Elsewhere, intense bouts of precipitation made the headlines in a number of spots. Day after day of extreme rain pushed the June precipitation totals across parts of the Midwest into record-obliterating territory. Sioux Falls, South Dakota, received 13.70" for the month, with more than half of that falling in just three calendar days. The town of Canton broke South Dakota's monthly precipitation record with 19.65".

Several major one-day rainfall events emerged from an extremely moist summer air mass that slathered much of the eastern United States in early August. Detroit experienced its second-heaviest calendar-day rainfall (4.57") on August 11, as did Baltimore on August 12 (6.30"). Even more impressive was the 13.57" that fell at Islip, New York, on August 11-12. The downpour set a new state record for 24-hour rainfall, which is especially noteworthy given that a tropical cyclone was not directly involved. A few weeks later, not to be outdone, Phoenix set an all-time calendar-day rainfall record on September 8 with 3.29", fed by deep moisture from ex-Hurricane Norbert.


Figure 3. A highway in Brentwood, New York, resembles an infinity pool after more than a foot of rain fell across parts of central Long Island on August 11-12, 2014. WunderPhoto credit: Hurricane765.

The NCDC's Climate Extremes Index lends some statistical backing to this anecdotal portrait of deluges. The "extremes in 1-day precipitation" indicator measures how much precipitation for the year fell in calendar days with extreme amounts (equal to the wettest tenth percentile of all days). Some 15.3% of the nation saw a much-above-average number of days fall into this category for 2014. That's a bit less than the 2013 value of 16.3%, but still enough to put it at 11th highest of the past 120 years. Notably, all of the top seven years for this index, and 13 of the top 15 years, have occurred since 1990.

Wet days getting wetter, and droughts getting hotter
The recent uptick in extreme one-day precipitation totals across the nation is consistent with more than a decade of research showing that many parts of the world, including the United States, are seeing their heaviest bouts of rain and snow getting even heavier over time. This conclusion was reinforced on a national and regional scale in the 2014 U.S. National Climate Assessment and on a city-by-city scale in a study by Brian Brettschneider (Boreas Scientific LLC) highlighted by Weather Underground blogger Chris Burt last August. The result is also consistent with the basic concept that a warming planet will see an increase in hydrologic contrasts, as warmer temperatures allow for more water to evaporate from lakes, oceans, and plants--helping boost the output of rainstorms and snowstorms--while sucking more water from already-parched land, intensifying the effects of drought.

This process is vital to keep in mind when taking stock of the California drought, arguably the nation's most catastrophic weather event of the year. Although calendar year 2013 was the state's driest on record, the water year of 2013–14 (July to June) placed third driest. (Water years are the most commonly used index for assessing California precipitation, which occurs mainly in the fall through spring). A NOAA-led study released in December found that the severity of drought conditions over the last three water years--looking only at rainfall--is within the realm of natural variability, with 1974–75 to 1976–77 even drier than the period from 2011–12 to 2013–14. However, the temperatures associated with the more recent drought went well beyond what one would expect from historical analogs (see Figure 4), which has made the impact on ecosystems, agriculture, and people even more severe. The NOAA study acknowledged, "record-setting high temperature that accompanied this recent drought was likely made more extreme due to human-induced global warming." In a similar fashion, the intense Texas drought of 2011 was associated with all-time temperature records established during the brutal, more prolonged droughts of the 1930s and 1950s. As states and regions consider how best to adapt to drought conditions in the future, they would be well advised to consider the possibility that temperatures during drought periods could soar beyond anything observed in more than a century of experience.


Figure 4. The annual average temperature for California in 2014 came in far above the previous record for the last 120 years, and it was roughly 4°F above the 20th-century average. Image credit: NOAA National Climatic Data Center.


Figure 5. A lone weed grows on an unplanted field on August 21, 2014 in Firebaugh, California. As the severe California drought continued for a third straight year, Central California farming communities struggled to survive, with an unemployment rate nearing 40 percent in the towns of Mendota and Firebaugh. Photo credit: Justin Sullivan/Getty Images.

Bob Henson