2021 heat wave in the Pacific Northwest and Western Canada
The 2021 Western North America heat wave was an extreme heat wave that affected much of Western North America from late June through mid-July 2021. The heat wave affected Northern California, Idaho, Western Nevada, Oregon, and Washington in the United States, as well as British Columbia, and in its latter phase, Alberta, Manitoba, the Northwest Territories, Saskatchewan, and Yukon, all in Canada. It also affected inland regions of Central and Southern California, Nevada, and Montana, though the temperature anomalies were not as extreme as in the regions farther north.
The heat wave was characterized as a heat dome because of the extreme temperatures and the exceptionally strong ridge centered over the area, whose probability of formation was linked to the effects of climate change by multiple studies. It resulted in some of the highest temperatures ever recorded in the region, including the highest temperature ever measured in Canada at 49.6 °C (121.3 °F), as well as the highest temperatures in British Columbia, in the Northwest Territories, in the state of Washington as well as a tied record in Oregon. The record-high temperatures associated with the heat wave stretched from Oregon to northern Manitoba, and daily highs were set as far east as Labrador and as far southwest as Southern California.
The extreme heat sparked numerous, extensive wildfires, some reaching hundreds of square kilometers in area. The eponymous Lytton wildfire destroyed the village of Lytton, British Columbia, the day after the city set the record high temperature for Canada. Extreme heat also damaged road and rail infrastructure, forced closures of businesses, disrupted cultural events, and melted snowcaps, in some cases resulting in flooding. The heat wave also caused extensive damage to agriculture across the region, resulting in substantial loss of crop yield and the death of 651,000 farm animals. The National Oceanic and Atmospheric Administration (NOAA) estimated that the heat wave caused at least $8.9 billion (2021 USD) in damages in the USA.
The death toll has been estimated to exceed 1,400 people, with at least 808 deaths estimated in western Canada. The Chief Coroner of British Columbia reported that 619 deaths were recorded due to heat exposure in the week from June 25 to July 1. Confirmed deaths in the United States included at least 116 in Oregon (of which 72 were in Multnomah County, which includes almost all of Portland), at least 112 in Washington, and one death in Idaho. An analysis by "The New York Times" suggests that around 600 excess deaths occurred the week the heat wave passed through Washington and Oregon.
Meteorological history.
On June 23, the United States National Weather Service warned of an approaching heat wave in the Pacific Northwest, whose origins could be traced to torrential rains in China. There, the warm, moist air rose and was eventually entrained by the jet stream, which transported it east over cooler waters. When that air current encountered an upper-level high-pressure zone, also called a ridge, it started to significantly deform, being forced to accommodate the high-pressure area south of the jet stream's meander. At the same time, the Southwestern United States was enduring an intense drought which caused higher-than-average temperatures, leading to a similar heat wave earlier in June. The remnants of this heat wave then moved north to the Pacific Northwest. Six days later, Environment Canada issued a heat warning for Alberta, Saskatchewan, British Columbia, Manitoba, Yukon, and Northwest Territories.
These conditions made way for a massive Rex block. In this situation, a high-pressure area stays in place for an extended period and does not let cyclones pass through it, which otherwise could have cooled the region; in this particular case, the high-pressure area was sandwiched between two stationary lows, which prevented the high-pressure region from moving. As the Pacific Northwest experienced severe drought conditions, the already warm air heated more quickly than usual, which intensified the ridge so strongly it caused a heat dome. The downslope winds from the Cascades and other mountain ranges further warmed the air in the valleys.
After the heat dome hovered over British Columbia and the Northwestern United States for a few days, it began to move eastward, bringing relief to the Pacific coast but breaking records east of the Rocky Mountains, particularly in the northern parts of the Prairie provinces. Sweltering conditions were observed as far east as Northwest Ontario. By July 4–5, the remnants of the heat dome crossed Hudson Bay and, weakened somewhat by the cool waters, entered Quebec, and after that, Labrador, briefly triggering temperatures of around . 
Climate change and additional factors.
Climate change in Canada and in the United States are widely considered to have worsened the heat wave's unprecedented intensity and duration. According to a rapid attribution analysis, the heat wave would have been highly unlikely in the absence of anthropogenic climate change, which increased the likelihood of such a heat wave at least 150-fold. A further study in Nature Climate Change estimated that its occurrence was projected to increase rapidly with further global warming, possibly becoming a 10-year event in a climate warmed 2°C relative to pre-industrial levels. 
Broader climate shifts may have also contributed to worsening the heat wave; data provided by Verisk shows that the Pacific Northwest is among the fastest-heating regions in the continental United States and southern Canada. The urban heat island effect could have further exacerbated the impact in cities. Based on historic data, several meteorologists noted that this phenomenon should be expected to occur only once over a thousand or several thousand years, while David Sauchyn, a scientist at the University of Regina, said that climate models had been predicting a heat wave of a similar intensity to happen in late 2020s at the earliest.
A paper in "Communications Earth &amp; Environment" found that the heat dome in the Pacific Northwest was 59% longer (27 days duration versus 17 days), 34% larger, and had 6% higher maximum amplitude, comparing the observed event and the event after detrending. Overall the strength of the heat dome was 86% greater than the same event would have been without background warming.
Temperature records.
Much of the Pacific Northwest, normally known for its temperate weather in June, experienced temperature anomalies of above average during this heat wave. Ground temperatures in some locations were measured to be excessively high - in Wenatchee, Washington, it reached , while the pavement at an intersection in Portland, Oregon reached . The heat wave, combined with other extreme weather occurrences elsewhere, yielded the hottest June on record in North America. It also contributed to the hottest June on record locally and regionally for many locations in Canada and the USA.
Canada.
The highest temperatures of the heat wave were registered in British Columbia, but areas as far east as Ontario were affected by the event. As of June 29,[ [update]] 2021, 103 all-time heat records were set across Western Canada.
British Columbia.
Temperature in Lytton
On June 27, there were 59 weather stations in B.C. that set records for hottest temperatures recorded. These were largely beaten in the following days (Kamloops, for instance, registered on June 28 and on June 29, the peak temperature recorded in a major population center in the region).
On June 28, records were set in Squamish, British Columbia at , Abbotsford at , Port Alberni at and Victoria at . 
On June 29, the temperature in Lytton, British Columbia, hit , the highest temperature ever recorded in Canada, although a nearby more modern station reported that the extreme was 1 °C lower. The stations were temporarily isolated by the Lytton wildfire the next day. The record occurred after consecutively setting new record highs of on June 27 and on June 28. It is also the highest temperature ever recorded north of 45°N, the highest temperature in the U.S. or Canada recorded outside the Desert Southwest, and higher than the absolute maximum temperatures of Europe or South America.
Alberta.
In Alberta, the highest heat was observed in the period from June 29 to July 1. Banff , Beaverlodge , Cochrane , Fort McMurray , Jasper , Grande Prairie , Hendrickson Creek , Nordegg , Red Earth Creek all saw the strongest heat ever measured in these communities, most after breaking all-time records of the previous day.
Calgary noted on June 29 and July 1, which stopped just 0.2 °C (0.4 °F) short of the highest observed temperature ever and beating the all-time records for June and July. Edmonton also saw temperatures approach the absolute maxima – the city centre registered on June 30, while the Edmonton International Airport, near Leduc, hovered around from June 29 to July 1. 
Territories.
In the Northwest Territories, on June 28, Nahanni Butte set a regional record at . Two days later, Fort Smith, just north of the Alberta border, reached , which beat the previous all-time territorial record, registered in the same place in 1941. It was also the new highest reliably recorded temperature above 60 degrees latitude.
Yukon was largely bypassed by the heat wave, but on June 28, some areas in the territory went over , including Whitehorse () and Teslin (), both of which were daily records.
Saskatchewan.
Saskatchewan's heat records were mainly concentrated in the northern parts of the province. Stony Rapids saw the mercury reach on June 30, an all-time high. Record temperatures were also set at Key Lake Airport and Collins Bay Airport on July 1, both at , as well as in Uranium City (). July highs were also observed in these settlements on the first day of the month, breaking 26 daily records across the provence.
Elsewhere, the heat arrived slightly later. Saskatoon reached on July 1 and beat the monthly record the following day, at , a tenth of degree below the all-time record. La Ronge registered an all-time high with a reading of . Regina, like most of the southern part of the province was spared the extremes reached in the northern parts, with a maximum of on July 2.
Manitoba.
In a similar way to Saskatchewan, the majority of heat records (including all-time highs) were noted in the northern parts of the province. The remote community of Tadoule Lake observed a high of , beating the previous all-time record by . A record high was also observed at Lynn Lake (), while Churchill, on the shore of Hudson Bay, registered , the highest temperature for July. 25 daily records were set on July 3, including in Winnipeg.
Ontario.
Northwestern Ontario did not see all-time high records beaten, but several daily records were pushed higher: on July 3, Thunder Bay saw a high of , Geraldton saw , while the hottest temperature was in Pickle Lake, at .
Newfoundland and Labrador.
Weakened by interaction with the Hudson Bay waters, the heat wave still managed to beat some daily records in Newfoundland and Labrador. Hopedale registered on July 5, while Happy Valley-Goose Bay reached the following day.
United States.
The heat wave broke numerous records by large margins, particularly in the Pacific Northwest. Several large cities, including Portland, Seattle, and Spokane, experienced high temperatures far exceeding and low temperatures higher than the area's normal daily high temperatures. The heat wave beat Washington's all-time heat record in Hanford () and tied one for Oregon ( at two places, including at Pelton Dam). The same temperature was noted in Peshastin in Chelan County, Washington, where temperatures soared to on June 29, slightly surpassing the previous all-time high for the state.
Oregon.
On June 26, Portland broke its previous all-time record high temperature of , set in July 1965 and August 1981, with a temperature of . It topped that record again on June 27, with a temperature of , and on the following day, the temperature increased further to . 
Salem, Oregon, reached on June 26, its record high temperature for June. It then hit degrees on June 27, breaking the record for the highest temperature ever recorded in that city, which was previously . Salem then exceeded the previous day's record temperature on June 28, with a maximum temperature of . However, not all the regions of the mid-Willamette Valley experienced extreme heat on June 28. Regions south of Salem, for example, did not see highs above mid-90s Fahrenheit on that day, likely due to cooler ocean air in the area.
The Willamette Valley also experienced extreme overnight temperature drops (twice the size of normal fluctuations) due to cooler air coming from the ocean – Portland cooled a record during the night, while Salem almost approached its all-time largest temperature swing, from to .
Washington.
Between 1894 when records began and June 2021, temperatures over were only recorded three times; however, Seattle-Tacoma International Airport recorded