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 over a thousand fatalities. This unprecedented event was unforeseeable even after accounting for anthropogenic climate change, making it impossible to explain its abnormality or quantify the probability of a similar event in the future. Furthermore, the observed 2021 daily maximum temperature across much of the Pacific Northwest exceeded upper bound estimates obtained from single-station temperature records, 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. Nonetheless, our model is still unable to anticipate the 2021 high temperatures even after properly accounting for extremal dependence, reiterating the uniqueness and unpredictability of the 2021 heatwave in the Pacific Northwest.