Could America's Wettest Winter of 2018-19 Have Been Anticipated?

The winter (December-February) of 2018-19 was unusually wet and stormy over most of the contiguous United States which subsequently set the stage for record flooding in spring. Precipitation ranked in the upper tercile over nearly all regions of the contiguous United States, and the nationally averaged precipitation ranked wettest for the 124-yr long instrumental record. The resulting springtime flooding over major river basins, including the Missouri and Mississippi, led to economic losses that exceeded one billion dollars.

We test whether the risk for such an extreme wet winter over the United States was elevated in 2018-19, and may thus have been potentially predictable. Did, for instance, slowly evolving boundary forcing such as tropical east Pacific sea surface temperature (SST) fluctuations linked to El Niño and the progressive increase in radiative forcing linked to anthropogenic climate change act to condition the event's likelihood?

We present results from ensembles of atmospheric model simulations forced by prescribed boundary conditions to argue that the extreme wet United States winter of 2018-19 could not have been anticipated based on the observed SST conditions alone. Furthermore, parallel suites of atmospheric model simulations with and without effects of global warming indicate that human influences also failed to materially alter the risk of an extreme precipitation pattern as occurred in 2018-19.

As a final assessment of predictability, we also present analysis of fully coupled multi-model prediction systems that were initialized in November 2018. These also failed to predict an increase in the risk for wet winter conditions over the United States. Sequences of one-month precipitation forecasts initialized at the beginning of December, January and February did, however, indicate an increase in the likelihood of a precipitation pattern like winter 2018-19. We argue that initial value information of the atmosphere, but not of the SST boundary forcing nor of the anthropogenic radiative forcing, were key to skillfully anticipating America's wettest winter.