U.S. Heat Builds: Parts of Southwest May Approach All-Time June Highs

Earth’s warmest year on record so far will make its presence felt in North America during the latter half of June. A massive dome of high pressure at upper levels will take shape across the United States this weekend and persist through next week. Subsidence beneath the high pressure will warm the atmosphere and help temperatures to soar well above average over large parts of the nation. The most immediate concern is across the Southern Plains and Mississippi Valley, where very moist low-level air combined with the building heat has led to heat advisories across many areas for Wednesday and/or Thursday. Residents of St. Louis, MO, may see their fifth consecutive day of heat index readings above 100°F by Thursday, and heat index values near or above 105°F are predicted on Wednesday and Thursday in Kansas City, MO; Tulsa, OK; Little Rock, AR; and Dallas-Fort Worth, TX. Only slight relief is expected toward the weekend. As an upper-level low traverses the north side of the ridge early next week, another burst of intense heat may develop ahead of it over the Northern Rockies and shift eastward across the Plains and Midwest.

Meanwhile, the deserts of southern California, Nevada, and Arizona--which largely missed out on the rains of El Niño--are now heating up quickly. Late June is typically the hottest time of the year in southeast New Mexico, southern New Mexico, and far west Texas (see Figure 3 below). It usually takes till July for the deserts of southwest AZ and southern CA to hit their peak of summer heat, so the upcoming heat wave is an early one there. An excessive heat watch is already in effect across parts of the far Southwest, where temperatures are expected to peak above the century mark on Sunday and/or Monday in such cities as Tucson, Phoenix, Las Vegas, and Los Angeles. Figure 2 shows the latest WU forecasts compared with record highs for June and for any month. Phoenix’s hottest high on record prior to the first day of summer is 115°F, so Sunday is likely to break that mark. In Los Angeles, record heat this time of year depends hugely on local effects, such as the presence or absence of offshore winds, as well as the large-scale factors that will be present this weekend.



Figure 1. National Weather Service alerts on Wednesday morning, June 15, 2016, included a heat advisory for parts of the south-central US (orange) and an excessive heat watch for parts of the Desert Southwest (dark red). Image credit: NOAA/NWS.


Figure 2. WU temperatures for several major southwestern U.S. cities issued on Wednesday morning for Sunday and Monday, June 19-20, 2016, compared with record readings for June and for any time of the year. *Note: NWS records (NOWData) show 117°F at Tucson International Airport on June 26, 1990, as the highest official temperature for the Tucson area, going back to September 1894. An even higher reading of 118°F at the Tucson Magnetic Observatory was recorded on June 27, 1990, according to weather records researcher Maximiliano Herrera. However, based on his knowledge of the instrumentation in use in 1990 (an aspirator later found to be problematic), Herrera believes that both of these temperatures are most likely slight overestimates, and that the most reliable all-time records for Tucson are the 115°F readings at the Tucson NWS office on June 19, 1960; June 26, 1990; and July 28, 1995.


How does the National Weather Service classify dangerous heat?
Every NWS office except those in Alaska has its own set of criteria for issuing heat watches and warnings, based on local climatology. An excessive heat warning is used in the most dangerous situations, when vulnerable people can experience serious health effects. The national standard (which can be adjusted by local offices) is to issue an excessive heat warning when the heat index is expected to be at least 115°F for at least three hours, and/or at least 105°F for three days. A heat advisory is issued at a lower threshold, typically for a heat index of 105°F together with nighttime lows at or above 80°F. The thresholds for a heat advisory may be even lower for multiday heat waves (due to the cumulative effect of intense heat) or for events striking early in the season (see Figure 3 below). An excessive heat watch generally indicates that the excessive heat warning criteria may be met within the next 24 to 48 hours, again varying by location. For example, because the Los Angeles and San Diego NWS offices employ differing thresholds, the excessive heat watch for this weekend does not include Los Angeles County, even though temperatures will be comparable on either side of the county’s eastern border.


Figure 3. Average date of peak summertime temperature varies from late June near El Paso, TX, to late August along parts of the TX and LA Gulf Coast. Sunlight is most intense near the summer solstice (usually June 20 or 21), which heats up dry areas quickly, but the peak summer heat often takes a few weeks to build in relatively moist areas, especially the south-central states. Image credit: NOAA/NWS. For more on this topic, see the related post by Christopher Burt, “Warmest Days of the Year for the U.S.”.


The challenge of air travel in fierce Desert Southwest heat
The upcoming scorcher could give the great heat wave of June 1990 a run for its money in some areas. On June 26, 1990, Los Angeles notched an all-time June record of 112°F, while Tucson and Phoenix set all-time highs for any date with almost-unfathomable readings of 117°F and 122°F, respectively. Temperatures in this range can actually impede air travel, since the near-ground atmosphere becomes so thin that some types of aircraft cannot obtain sufficient lift to take off. Extreme heat can also damage the internal components of some aircraft. Phoenix’s Sky Harbor Airport halted all takeoffs as a precaution during the all-time record heat of June 1990. Urban legend has it that the closure was due to the tarmac melting, but an airport spokesperson confirmed to KJZZ radio last year that it was uncertainty about aircraft performance at that temperature that actually led to the closure. More recently, on June 29, 2013, 18 US Airways flights were cancelled because the regional aircraft involved had been certified for takeoff in temperatures of no more than 118°F, according to the UK Telegraph. Temperatures on that day hit 119°F in Phoenix, the city’s third-highest reading on record.

Hot times at high latitudes
Global temperatures continue to run at a near-record pace as we transition from the recently deceased El Niño toward a potential La Niña. We’ll have a full report on May’s global climate with the release of NOAA’s monthly summary later this week, including another bumper crop of all-time local heat records. Two of the most striking heat reports in the last several weeks have come from opposite ends of the globe.

On May 27, the daily low temperature at Esperanza Base, on the outer end of the Antarctic Peninsula, was 8.8°C (47.8°F). According to weather records researcher Maximiliano Herrera, this appears to be the warmest daily low on record anywhere in Antarctica, including the Antarctic Peninsula, King George Island, and other islands lying below the 60°S latitude that are considered part of the continent by the Antarctic Treaty. (WU weather historian Christopher Burt has a post on the all-time high of 17.5°C (63.5°F), set at Esperanza in March 2015.) Herrera, who maintains a comprehensive list of extreme temperature records for every nation in the world on his website, has not found any other examples of daily lows in Antarctica any milder than 6.5°C. “For a continental record, this was smashed by an amazing margin,” he added.


Figure 4. The extent of melting on the surface of the Greenland Ice Sheet hit dramatic seasonal peaks in April, May, and early June 2016, most recently approaching the 40% mark. Image credit: NSIDC.


Only a few days after the mild night at Esperanza, a major burst of early-season warmth swept across southwest Greenland. According to the Dutch meteorological agency DMI, temperatures at the Nuuk airport hit 24.8°C (76.6°F) on Friday, June 9. This is the warmest reading ever observed in Greenland for the month of June, easily topping the 23.3°C (73.9°F) set on June 15, 2014, in Kangerlussuaq, about 200 miles north of Nuuk. Downslope winds blowing offshore from the mountains of southwest Greenland helped warm the atmosphere in the Nuuk area (the same process that drives foehn and chinook warming). Nuuk’s record high resulted from this downslope warming on top of already-unusual mildness throughout the depth of the atmosphere over much of Greenland. This extreme early-June warmth led to the third noteworthy spike of the year in surface melting atop Greenland’s ice sheet, following major melt events in April and May (see Figure 4 above).

Still waiting on our first Eastern Pacific tropical storm
The tropics remain fairly quiet, with the only system currently being tracked--Invest 93E in the eastern North Pacific--expected to organize only slowly over the next few days as it moves away from the Mexican coastline. In its 8:00 AM EDT tropical weather outlook, the National Hurricane Center gives 93E a 20% chance of development by Friday and a 50% chance by Monday. If it does become a tropical storm, it will be Agatha, the first named storm of the Eastern Pacific season. Meanwhile, a surface low associated with an upper-level impulse moving toward the East Coast may undergo some organization this weekend well off the East Coast, although models have largely backed off on potential tropical/subtropical development as surface high pressure is projected to dominate the Northwest Atlantic.

We'll be back with a new post on Thursday.

Bob Henson


Figure 5. Infrared satellite image showing an area of disturbed weather off the southern coast of Mexico (Invest 93) as of 1500Z (11:00 AM EDT) Wednesday, June 15, 2016. Image credit: NASA/MSFC Earth Science Office.