Warm Nights Tip the Scales as Summer Grinds On

By: Bob Henson , 5:11PM,GMT on August 11,2016




 

Data for July 2016 make it clear that this summer is worming its way into the nation’s warmest batch on record, thanks in large part to consistently sultry nights in many areas. Meteorological summer so far--June plus July--has been the fifth warmest for the contiguous U.S. in 122 years of recordkeeping, according to the July climate report released on Thursday by NOAA’s National Centers for Environmental Information (NCEI). Ahead of 2016 at this point are the scorching Dust Bowl summers of 1934 (#4) and 1936 (#1) along with the recent 2006 (#3) and 2012 (#2). The toasty summer so far is the result of the nation’s warmest June on record followed by its 14th warmest July. Last month’s warmth was focused across the nation’s southern and eastern halves (see Figure 1), with New Mexico and Florida each recording their hottest July on record. Fourteen other states made it into their top ten warmest for July.


Figure 1. Statewide rankings for average temperature during July 2016, as compared to each July 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.

Warmest nights on record for June-July
It’s the muggy nights that are imprinting themselves on the psyche of millions of Americans this summer. The average daily minimum for the contiguous U.S. was the warmest on record for June and July combined: 60.57°F, beating out 2015, 2010, 2002, and 2006. (See Figure 2.) Warmer nights are a hallmark of a climate being heated by added greenhouse gases, and it’s long been recognized that nights should generally warm more than days, and winters more than summers, as climate change proceeds. Of the eight June-July periods with the warmest average daily temperatures (including both highs and lows), four are from Dust Bowl years. However, the eight years with the warmest average daily minimum temperatures are all from the 21st century. Urban heat islands are no doubt helping to increase overnight lows in large metropolitan areas; however, the nationwide extent of the trend toward warm nights goes well beyond this effect.

For the year to date through July, the U.S. has seen 15,061 daily record highs and just 2709 record daily lows, according to data compiled by meteorologist Guy Walton. In an email, Walton told me that “2016 is in a race with 2012 for that dubious distinction of having the highest ratio of record daily highs to record daily lows since 1920.”


Figure 2. The average daily minimum temperature for the contiguous U.S. is at record-warm values for the summer of 2016 thus far (June plus July). Image credit: NOAA/NCEI.

Wilting in Washington, D.C.
Several U.S. locations have been setting records for the most consecutive nights above unpleasant thresholds of warmth. Reagan Washington National Airport made it five weeks--July 5 through August 8--without once getting below 70°F. In records dating back to 1872, that stretch of 35 days above 70°F beats the previous record of 32 days notched from July 15 to August 15, 1980. This summer, the city has set only one record high--100°F on July 25--but there were three record-warm lows in a row: 81°F on July 25, 80°F on July 26, and 81°F on July 27. The next several days will test the patience of D.C. residents, as lows are expected to hang in the upper 70s and afternoons warm into the mid to upper 90s.

The warm nights extend well beyond the contiguous 48 states. As of Wednesday, Anchorage, Alaska, had gone 59 days without dipping below 50°F, topping the record string of 53 days set from June 23 to August 14, 2013. The city also saw a run of 18 consecutive days above 55°F this summer (July 13-30), which doubles the old record of 9 days (July 16-24, 1984). In Fairbanks, the stretch of 41 nights above 50°F this summer (June 24 to August 3) smashed the old record of 32 nights (June 25 to July 26, 1975). These are just a few examples of the exceptional mildness bathing Alaska for months now. Eric Holthaus (Slate) points out an ominous milestone: Alaska’s average for the year to date of 33.9°F is the first time in records going back to 1925 that the January-to-July period has topped 32°F. The freezing mark carries extra physical and psychological significance in a state like Alaska, where entire ways of life are based on the reliable presence of ice for most of the year.


Figure 3. Statewide rankings for average precipitation during July 2016, as compared to each July 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.

Wet to the north, dry to the south
After an unusually dry June across the contiguous U.S., July produced generous rains across much of the nation's northern tier, while leaving most of the Northeast and the nation's southern half on the dry side. It was the second-driest July on record for Georgia and the third-driest for Florida, with Wyoming and New Mexico also coming in among their top-ten driest. July was among the ten wettest on record in five states: Illinois, Kentucky, Minnesota, Missouri, and North Dakota. The weekly U.S. Drought Monitor issued on Thursday morning shows that more than 40% of California remains in extreme or exceptional drought, a situation that is unlikely to change before the 2016-17 wet season arrives (if then). Much of the state remains extremely vulnerable to fast-growing wildfire, especially over the next several months. Wildland fire potential is also expected to increase across the South this autumn, according to the latest outlooks from the National Interagency Coordination Center. (Torrential rains scattered over parts of the upper Gulf Coast this week will help tamp down the immediate drought and fire risk in those areas.)

The wet-north/dry-south tendency evident in July may be a foreshadowing of La Niña influence to come (see Figure 4 below). The strong El Niño of 2015-16 is giving way to borderline La Niña conditions, with the weekly index of sea-surface temperatures in the Niño3.4 region hanging near the -0.5°C threshold of La Niña over the past month. NOAA maintained a La Niña Watch in its monthly ENSO discussion issued Thursday, although the event is projected to be relatively weak if it does take shape. NOAA is giving a 55-60% chance of La Niña being present this fall and winter and a negligible chance of El Niño (below 10%). Because La Niña typically leads to a more consolidated jet stream, it often leaves southern parts of the 48 states on the dry side of upper-level flow as storm systems whip across the nation’s heartland, keeping northern areas more moist.


Figure 4. Typical effects produced by La Niña (the cold phase of the El Niño–Southern Oscillation) during northern winter, December through February. The most common effects are largely but not totally the opposite of what typically occurs during El Niño. Image credit: NOAA/NWS/CPC.

NOAA ups its Atlantic hurricane outlook for 2016
In an update released Thursday, NOAA increased the totals that it had projected in May for the 2016 Atlantic hurricane season. The agency is now calling for a 70% chance of 12-17 named storms, 5-8 hurricanes, and 2-4 major hurricanes. These numbers compare to the May outlook of 10-16 named storms, 4-8 hurricanes, and 1-4 major hurricanes.

“We’ve raised the numbers because some conditions now in place are indicative of a more active hurricane season, such as El Niño ending, weaker vertical wind shear and weaker trade winds over the central tropical Atlantic, and a stronger west African monsoon,” said Dr. Gerry Bell, lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center. “However, less conducive ocean temperature patterns in both the Atlantic and eastern subtropical North Pacific, combined with stronger wind shear and sinking motion in the atmosphere over the Caribbean Sea, are expected to prevent the season from becoming extremely active.”

The NOAA update lines up closely with the most recent forecast from Colorado State University, issued on August 5. Several other forecast groups have gone bigger for this season, as you can see from the forecast compilation graphics now being produced by CSU.

For the time being, the global tropics remain quiet. No areas of interest were highlighted on Thursday morning by the National Hurricane Center for the Atlantic. In the East Pacific, NHC is giving Invest 94E, located a few hundred miles southwest of Baja California, a 20% chance of development between two and five days from now as it moves west over open water. The only active tropical cyclone on Earth was Tropical Storm Conson, spinning harmlessly in the Northwest Pacific about 1000 miles west-northwest of Wake Island. Conson should remain a tropical storm as it gradually accelerates north-northwestward toward Japan and Korea, perhaps crossing the Kuril Islands. Longer-range models suggest that the Northwest Pacific could spring to life in the next few days, with several simultaneous tropical cyclones possible. Models also suggest the potential for a tropical cyclone in the North Bay of Bengal at some point next week. Meanwhile, as Jeff Masters pointed out yesterday, the Madden-Julian Oscillation favors an active couple of weeks ahead over the Western Pacific and relatively quiet conditions over the East Pacific and Atlantic.

We’ll have a new post by Friday afternoon.

Bob Henson