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 Niño events like 1983 and 1998 can drive extreme wet winters over the contiguous U.S. These strong El Niños 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 Niño 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.

我们确定了导致美国本土极端潮湿冬季的物理因素，并检查了 2019 年冬季（2018 年 12 月至 2019 年 2 月）的先决条件，当时发生了创纪录的降水，导致密苏里河和密西西比河沿岸发生了数十亿美元的洪水灾害。模型和观测用于确定可能导致极端潮湿冬季实用预报技巧的缓慢变化强迫的影响。大气模型表明，在 1983 年和 1998 年等强东太平洋厄尔尼诺事件期间，海面温度会导致美国本土出现极端潮湿的冬季这些强烈的厄尔尼诺现象将美国本土降水的分布转移到更潮湿的条件下，平均润湿率为 1.5-2.0 标准年际变率的偏差。转向更潮湿的条件导致 2019 年观测到的潮湿冬季的可能性增加了五倍。 在更长的时间尺度上，观测表明美国连续的冬季降水在上个世纪增加了。对历史耦合模型模拟的分析表明，上个世纪人为强迫向更湿润的条件转变，年际变率的标准差为 0.2-0.4。虽然增加了 2019 年等极端潮湿冬季的风险，但这种影响在任何特定冬季都是有限的可预测性来源。具体就 2019 年而言，美国冬季极端潮湿的风险的预处理因素很弱或不存在，几乎没有提供实用的预警。那个冬天持续的中太平洋厄尔尼诺现象并没有显着改变潮湿的风险，