Preconditions for extreme wet winters over the contiguous United States

We identify physical factors leading to extreme wet winters over the contiguous U.S. and examine whether preconditions operated during winter 2019 (December 2018 to February 2019) when record precipitation occurred that led to billion-dollar flood disasters along the Missouri and Mississippi Rivers. Models and observations are used to determine the effect of slow-varying forcing that may lead to practical forecast skill for extreme wet winters. Atmospheric models indicate that sea surface temperatures during strong eastern Pacific El Nino events like 1983 and 1998 can drive extreme wet winters over the contiguous U.S. These strong El Ni (n) over tilde os shift the distribution of contiguous U.S. precipitation to wetter conditions with a mean wetting of 1.5-2.0 standard deviations of the interannual variability. The shift to wetter conditions leads to a fivefold increase in the probability of wet winters of the magnitude observed in 2019. On longer timescales, observations indicate contiguous U.S. winter precipitation has increased over the last century. Analysis of historical coupled model simulations indicate anthropogenically-forced shifts to wetter conditions over the last century of 0.2-0.4 standard deviations of the interannual variability. While increasing the risk of extreme wet winters like 2019, this effect is a limited source of predictability during any particular winter. Concerning 2019 specifically, preconditioning factors of the risk for extreme contiguous U.S. winter wetness were weak or absent and offered little practical early warning. The ongoing central Pacific El Nino that winter did not significantly alter the risk of the wetness, and thus the extreme 2019 conditions are judged not to have been a seasonal forecast of opportunity.

Automated summary

This study identifies physical factors leading to extreme wet winters in the contiguous U.S., mainly driven by sea surface temperature changes during strong eastern Pacific El Nino events. Analysis shows that these strong El Nino events can shift the distribution of precipitation to wetter conditions over the contiguous U.S., increasing the probability of wet winters. Additionally, observations over the past century indicate an increase in winter precipitation in the contiguous U.S., with a portion of this increase attributed to anthropogenic forcing.