Extreme precipitation poses a major challenge for local governments, including the City and County of San Francisco, California, as flooding can damage and destroy infrastructure and property. As the climate continues to warm, reliable future precipitation projections are needed to provide the best possible information to decision makers. However, future changes in the magnitude of extreme precipitation are uncertain, as current state-of-the-art global climate models are typically run at relatively coarse horizontal resolutions that require the use of convective parameterization and have difficulty simulating observed extreme rainfall rates. Here, we performed ensembles of convection-permitting regional climate model simulations to investigate how five historically impactful extreme precipitation events over the San Francisco Bay Area could change if similar events occurred in future climates. We found that changes in storm-total precipitation depend strongly on storm type. Precipitation associated with an atmospheric river (AR) accompanied by an extratropical cyclone (ETC) is projected to increase at a rate exceeding (by up to 1.5 times) the theoretical Clausius Clapeyron scaling of 6–7% per °C warming. On the other hand, future precipitation changes are weak or negative for events characterized by an AR only, despite increases in precipitable water and integrated vapor transport that are similar to those of the co-occurring AR and ETC events. The differences in the sign of future precipitation change between AR-only events and co-occurring AR and ETC events is instead linked with changes in mid-tropospheric vertical velocity. Given that the majority of observed ARs are associated with an ETC, this research has important implications for future precipitation impacts over the Bay Area, as it indicates that storm-total precipitation associated with the most common type of storm event may increase by up to 26–37% in 2100 relative to historical.

极端降水对包括加利福尼亚州旧金山市和县在内的地方政府构成重大挑战，因为洪水会破坏和破坏基础设施和财产。随着气候继续变暖，需要可靠的未来降水预测，以便为决策者提供尽可能好的信息。然而，极端降水量的未来变化是不确定的，因为当前最先进的全球气候模型通常以相对粗糙的水平分辨率运行，需要使用对流参数化并且难以模拟观测到的极端降雨率。这里，我们进行了允许对流的区域气候模型模拟的集合，以研究如果未来气候中发生类似事件，旧金山湾区的五次具有历史影响的极端降水事件将如何变化。我们发现风暴总降水量的变化很大程度上取决于风暴类型。伴随着温带气旋 (ETC) 的大气河流 (AR) 的降水预计将以超过（高达 1.5 倍）理论克劳修斯·克拉佩龙（Clausius Clapeyron）每升温 6% 至 7% 的速度增加。另一方面，尽管与同时发生的 AR 和 ETC 事件相似的可降水量和综合蒸汽输送增加，但未来降水变化对于仅以 AR 为特征的事件来说是微弱的或负面的。仅 AR 事件与同时发生的 AR 和 ETC 事件之间未来降水变化符号的差异与对流层中部垂直速度的变化有关。鉴于大多数观测到的 AR 与 ETC 相关，这项研究对湾区未来的降水影响具有重要意义，因为它表明与最常见的风暴事件类型相关的风暴总降水量可能增加多达 26相对于历史数据，2100 年为 –37%。