From WunderBlog (Bob Henson):
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.
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.”
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.