Heatwave · Western Canada and the north-western United States
The heat dome over the Pacific Northwest
At the end of June 2021 a ridge of high pressure stalled over the Pacific Northwest, a region where summer days above 30 °C are rare and many homes have no air conditioning. Portland reached 46.7 °C and Seattle 42.2 °C. In the village of Lytton, in the canyon of the Fraser river, the thermometer read 46.6 °C, then 47.9, then 49.6: three national records in three days, the last one 4.6 °C above the record set in 1937. On 30 June a fire destroyed most of the village.
25 June – 1 July 2021 · British Columbia, Washington, Oregon · ERA5
600+
Heat deaths in British Columbia
Sudden deaths the coroner attributed to the heat, reported over the following weeks.
150×
Rarer without climate change
At least; the heat would have been virtually impossible without human-caused warming.S1
What happened
From 25 June the heat built day by day. Records fell across Washington, Oregon and British Columbia, not by fractions of a degree but by several degrees. Roads buckled, power cables sagged, and hospitals filled with heat stroke. In British Columbia alone more than 600 people died of the heat, most of them older people alone at home.
On 30 June, the day after its record, a fire swept through Lytton in minutes, driven by a hot wind; two people died and most of the village was destroyed.
The dome
A heat dome is a ridge of high pressure aloft that stalls. Under it, air sinks, warms as it is compressed, and clear skies let the sun heat the ground for long June days. In 2021 the ridge was among the strongest ever seen in the region: the 500 hPa surface, normally near 5,700 m in summer, rose to around 6,000 m over British Columbia. A plume of hot air fed by a cut-off low upstream added to the heat.S1
Lytton and the grid
−12.8°C
The gap between ERA5's afternoon high at its grid point near Lytton (36.8 °C) and the thermometer in the village (49.6 °C) on 29 June. The grid cell averages mountains more than 1,000 m higher than the village on the canyon floor, where the hottest air collects.
| Day | Station | ERA5 grid point |
|---|---|---|
| Jun 27 | 46.6 | 34.4 |
| Jun 28 | 47.9 | 35.2 |
| Jun 29 | 49.6 | 36.8 |
The reanalysis still shows the event clearly: the grid's afternoon highs rise by more than 2 °C over three days, and the heat at 850 hPa is extreme over the whole region. But for the number on a village thermometer, a station is needed.
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References
News
- N1CBC News. For 3rd straight day, B.C. village smashes record for highest Canadian temperature at 49.6 C. 29 June 2021.Lytton's three national records.
Studies and guidelines
- S1Philip, S. Y., et al. (2022). Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in June 2021. Earth System Dynamics, 13, 1689–1713.How rare the heat was and how much climate change made it more likely.