U.S. Endures its Sultriest Summer Nights on Record

The broiling summer of 2016 placed fifth hottest among the 122 summers since records began in 1895 for the contiguous U.S., according to NOAA analyses released on Thursday. Even more impressive, this past summer (June through August) saw the highest average minimum temperature on record--certainly no surprise to people across the country who endured one muggy night after another. The average daily minimum for June through August 2016 was a balmy 60.81°F, beating the record of 60.70°F set in 2010. The average daily summer low in the contiguous 48 states has climbed about 1.4°F in the last century. That’s double the increase of 0.7°F in the average daily summer high.


Figure 1. Average daily minimum temperatures for the contiguous U.S. for each summer from 1895 to 2016. Image credit: NOAA/NCEI

Wetter, hotter American summer
In a climate warmed by increasing greenhouse gases, computer models and basic theory agree that nights should warm more quickly than days. In large part this is because of increasing atmospheric moisture that keeps nighttime temperatures up, even when that moisture doesn’t actually produce rain. Climate Central has a handy site that allows you to calculate the ongoing increase in summer dew point temperatures for more than 100 cities around the nation. In larger cities, there’s another human-produced climate effect in the mix: the urban heat island, whereby the pavement and buildings of metro areas heat up by day and release the heat by night. The urban heat island is only a small part of the global temperature picture, since rural areas and oceans are heating up as well.

California, Connecticut, and Rhode Island each saw their warmest summer on record in 2016, while many other states had a top-ten warmest summer. Some U.S. cities set records for their longest stretches above a certain temperature, and a few also saw their warmest summer overall, as noted by Chris Burt in a WU blog earlier this week. Among those setting all-time records for average summer temperature, including both daily highs and daily lows:

Anchorage, Alaska: 60.7° (previous 60.2° in 2015)
Charleston, South Carolina: 84.1° (previous 83.4° in 2011)
Las Vegas, Nevada: 93.1° (previous 92.5° in 2007)

See Chris’s post for other examples.


Figure 2. Statewide rankings for average temperature during June-August 2016, as compared to each June-August since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.


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


Figure 4. Corn stalks were stunted by lack of rain in this Barnstead, New Hampshire, field on September 2, 2016. The drought in southern New England and dry spells this summer further north mean that fall foliage could come earlier this year and not last as long in some areas. Image credit: AP Photo/Jim Cole.

Boston sees its driest summer on record, while the central U.S. was soaked
Large parts of the western and eastern U.S. had to deal with a scarcity of rain along with the heat this summer. It was the 24th wettest out of the past 122 summers nationwide, but that is largely due to the moisture surplus over the central U.S., centered on the Mississippi Valley. Eight states in this corridor had a top-ten wettest summer. Meanwile, the rest of the nation saw precious little rain overall. Wyoming, South Carolina, and Massachusetts all had top-ten driest summers. It’s been especially parched in New England, where parts of eastern Massachusetts and southern New Hampshire are now in extreme drought, according to Thursday’s U.S. Drought Monitor. Boston had its driest summer in records going back to 1872. Only 3.92” fell from June through August, breaking the record of 3.97” set in 1957.

August was both very warm (17th hottest) and very wet (2nd wettest) for the contiguous U.S. as a whole. Eight states saw their hottest August on record: Pennsylvania, Maryland, Delaware, New Jersey, New York, Massachusetts, Rhode Island, and Connecticut. One state had its wettest August. Unsurprisingly, that was Louisiana, where a slow-moving low pressure system dumped massive amounts of rain and triggered unprecedented flooding in and near Baton Rouge. According to the August 2016 Catastrophe Report from insurance broker Aon Benfield, damage from the flood was estimated at $10 - $15 billion, which will likely make it the second most expensive non-hurricane related flood in U.S. history, behind the $35 billion in damage from the summer 1993 flooding in the Midwest.


Figure 5. Mud-covered belongings line the floor of a home after floodwater receded on August 17, 2016, in Denham Springs, Louisiana. Image credit: Brendan Smialowski/AFP/Getty Images.

Climate change made the Louisiana floods more likely
The mammoth rainfall in August that dumped 10” - 30” on a large part of southern Louisiana was probably twice as likely to occur because of the influence of human-produced greenhouse gases, according to a rapid-response study released on Tuesday. Researchers at Climate Central, NOAA, Princeton University, and the Royal Netherlands Meteorological Institute analyzed output from several major climate model runs, covering both past and future climates, to see how this type of heavy-rain event has changed across the Central Gulf Coast from east Texas to western Florida.

Such an event was calculated to be at least 40% more likely—and probably twice as likely—due to the influence of climate change. Even in today’s climate, a rainfall of this size and scope remains a rarity, one that would now be expected once every 550 years at any particular point in the region, the study found. However, the expected frequency of getting such an event somewhere within the entire Gulf Coast study region is now about every 25-30 years—that is, frequent enough to play into infrastructure, building, and insurance decisions. Put another way, the intensity of a typical 25-30 year rainfall event in this region has gone up by at least 10%. When it comes to flooding, a small increase in water level can lead to a huge increase in impact.

This week’s report is part of the World Weather Attribution project, coordinated by Climate Central. The project brings in researchers around the globe to carry out prompt analyses of high-profile weather events and their potential links to climate change. Such studies have typically taken a year or two to complete, so the project has zeroed in on ways to produce these studies more quickly, while an event is still high on the public’s mind. For example, scientists can extract relevant detail from recent comprehensive model runs, rather than carrying out entirely new model runs from scratch.

Bernadette Woods Placky (Climate Central) mentioned another benefit in an email: “People usually assess and rebuild immediately after an event. This information can help inform that process, from the public to insurance to emergency managers to other decision makers. When we wait 1-2 years for this information, the plans are already finished or in the works.”

We’ll be back with our next update by Friday afternoon.

Bob Henson