Could climate models have captured the record-shattering 2021 Pacific Northwest heatwave - and did they?

Until June 2021, most people would have thought of the Pacific Northwest as one of the last places to worry about heat extremes. The emphasis for climate change impacts in the region has focused much more on water resources than on high temperatures. But this summers heat wave shattered temperature records, in some cases by a substantial margin, and it also shattered the notion that the region can escape the impacts of heat extremes that global warming brings. After an unprecedented extreme event like this, one question to ask is: Could climate models have projected such an event? If models cannot capture events that we observe, this could reveal processes that are missing or represented inaccurately. On the other hand, if they can capture similar events, did they? If model projections can but did not capture such events, this would indicate we should adjust our approach to model experimental design. Finally, if model projections both can and did capture events like this one, this could inform views of climate models as credible. We investigate whether or not a particular climate model (CESM2) could have captured events as hot or hotter than this one using an ensemble boosting storyline approach, in which simulations are re-run with perturbed initialization starting shortly prior to an extreme event. We also investigate whether or not a large ensemble of simulations run prior to the event did capture any similar events.