  In late June, 2021, a devastating heatwave affected the US Pacific Northwest
and western Canada, breaking numerous all-time temperature records by large
margins and directly causing hundreds of fatalities. The observed 2021 daily
maximum temperature across much of the U.S. Pacific Northwest exceeded upper
bound estimates obtained from single-station temperature records even after
accounting for anthropogenic climate change, meaning that the event could not
have been predicted under standard univariate extreme value analysis
assumptions. In this work, we utilize a flexible spatial extremes model that
considers all stations across the Pacific Northwest domain and accounts for the
fact that many stations simultaneously experience extreme temperatures. Our
analysis incorporates the effects of anthropogenic forcing and natural climate
variability in order to better characterize time-varying changes in the
distribution of daily temperature extremes. We show that greenhouse gas
forcing, drought conditions and large-scale atmospheric modes of variability
all have significant impact on summertime maximum temperatures in this region.
Our model represents a significant improvement over corresponding
single-station analysis, and our posterior medians of the upper bounds are able
to anticipate more than 96% of the observed 2021 high station temperatures
after properly accounting for extremal dependence.
