When record breaking heat waves should not surprise: skewness, heavy
tails and implications for risk assessment
Abstract
Extreme heat waves beset western North America during 2021, including a
46.7°C (116°F) observation in Portland, Oregon, an astonishing 5°C above
the previous record. Using Portland as an example we provide evidence
for a latent risk of extreme heat waves in the Pacific Northwest (PNW)
and along the west coast of the United States where a maritime climate
and its intrinsic variations yield a positive skewness in summertime
daily maximum temperatures. A generalized Pareto extreme value analysis
yields a heavy tailed distribution with a return period of 300-1000
years, indicating that, while rare, the event was possible, contrary to
prior claims that the event was “virtually impossible”. We demonstrate
that the extreme temperatures can be explained by the coincident extreme
values of geopotential heights, and that the relationship between
heights and extreme temperatures has not materially changed over the
observational record. The dynamical nature of the event along with
recent developments in stochastic theory justifies the use of skewed and
heavy-tailed distributions which may provide the basis for a more
proactive approach to managing the risk of future events.