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Fall 2016: The Warmest in U.S. Weather History

Climate

By Bob Henson

The autumn of 2016 was the warmest ever observed in records going back to 1895 for the 48 contiguous U.S. states, according to data released Wednesday by NOAA's National Centers for Environmental Information (NCEI). The nation's average September-to-November temperature of 57.63 F was a full 1.05 F above the previous autumn record, set way back in 1963 and it was 4.08 F above the 20th-century average (see Figure 1). The record-setting margin of more than 1 F is a hefty one for a temperature record that spans an entire season and a landmass as large as the 48 contiguous states. For comparison, the second, third, fourth, fifth, sixth and seventh-warmest U.S. autumns are all clustered within 1 F of each other, as are the six coldest autumns on record.

Pushing this past autumn to the top of the temperature pack were the third-warmest October and third-warmest November on record, along with the ninth-warmest September. Eight states along a swath from New Mexico to Michigan saw their warmest autumn on record, and every contiguous state except for California, Nevada, Oregon and Washington had a top-ten warmest autumn (see Figure 2).

Figure 1. Autumn 2016 was by far the warmest on record for the contiguous U.S. NOAA / NCEI

Figure 2. Statewide rankings for average temperature during September—November 2016, as compared to each September—November period since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest.NOAA / NCEI

Record Highs Outpace Record Lows in November by an Unprecedented Margin

Although it wasn't the warmest November on record, last month transcended all other months in modern U.S. weather history by the outsized presence of record highs to record lows. According to preliminary NOAA data compiled through Wednesday, November saw 4,544 daily record highs set or tied and just 94 daily record lows set or tied—a ratio of more than 48 to 1! This is the largest such ratio for any month in U.S. data going back to the 1920s, according to independent meteorologist Guy Walton, who has tracked U.S. records for many years. Because many U.S. reporting stations came on line in the 1890s, the occurrence of records did not stabilize until around the 1920s.

The total of 94 daily record lows in November is remarkably sparse—the lowest value for any month since September 1922. In general, it's best to use warm-to-cold ratios rather than comparing the raw number of records over time, observed Deke Arndt, the chief of NOAA/NCEI's Climate Monitoring Branch. "Because the number and lifetimes of weather stations has varied over time, comparing raw numbers of records doesn't completely capture the signal. Using a ratio of warm-to-cold records helps account for these effects," Arndt told Mashable's Andrew Freedman. Each decade since the 1980s has seen more daily record highs than lows, with the ratio increasing to nearly two-to-one during the 2000s and just over two-to-one for the 2010s through November 2016.

Although 2016 is running just behind 2012 as the warmest year in U.S. records thus far, the widespread chill expected to continue through at least mid-December has a good chance of keeping this year from outpacing 2012.

Figure 3. Statewide rankings for average precipitation during September - November 2016, as compared to each September—November period since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest.NOAA / NCEI

Big Regional Contrasts in Precipitation

November was the 25th driest on record, with the average 48-state precipitation total of 1.73" a half inch below the 20th-century mean of 2.23". For the autumn as a whole, precipitation came in very close to the long-term mean when averaged across the 48 contiguous states, with 2016 placing the 65th wettest autumn in 122 years of data. However, that "normal" value obscures some major regional differences, as seen in Figure 3 (above). It was the wettest autumn on record in Washington, as a powerful Pacific jet brought vast amounts of moisture into the Pacific Northwest. States from California to the upper Midwest were all much wetter than average this autumn. The jet stream largely passed Colorado's mountains, though, leaving the state with its eighth driest autumn on record.

The other major wet zone this autumn was along the Southeast coast, as Hurricane Matthew dumped enormous amounts of rain and set a number of local rainfall and storm-surge records. The resulting floods were catastrophic across parts of inland North Carolina, where the rains pushed rivers to record crests and flooded an estimated 100,000 structures, producing at least $1.5 billion in damage. Of the 49 deaths attributed to Matthew, 28 occurred in North Carolina. Both North and South Carolina came in with their 19th wettest autumns on record, although those values are skewed somewhat by the failure of Matthew to drench western parts of those states and by the largely dry weather before and after Matthew.

Drought and Fire Take Their Toll in the Southeast

A vicious drought sank its claws deeper into parts of the South this autumn, with Louisiana, Mississippi, Alabama, Tennessee and Kentucky all seeing a top-ten driest autumn. Combined with near-record warmth, the dryness primed forests across the central and southern Appalachians for widespread wildfires that culminated in deadly disaster in the Gatlinburg, Tennessee, area lats month. The death toll in Gatlinburg now stands at 14, with more than 1,400 structures damaged or destroyed and a damage toll estimated at more than $100 million by the insurance broker Aon Benfield. As of Dec. 1, most of Mississippi, Alabama, Georgia, and Tennessee were in extreme or exceptional drought on the weekly U.S. Drought Monitor. Generous rains of 1" - 3" across much of this region over the past week will help put a dent in the drought, although much more moisture is needed to recoup widespread annual deficits on the order of 12" to 20" (even more in some locations).

Reposted with permission from our media associate Weather Underground.

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