Extraordinary Tibetan Plateau early-winter heating drove record-breaking winter precipitation in California and...

California and adjacent regions received record-breaking precipitation in the winters of 2016-2017 and 2022-2023, causing extraordinary damage and severe societal impacts. However, subseasonal to seasonal predictive skill for Californian winter precipitation has remained persistently low. Furthermore, these extreme hydroclimate events occurred during La Niña conditions, which are traditionally associated with dry conditions in California. A fundamental lack of predictability has been suggested. This study shows that anomalous early-winter heating over the Tibetan Plateau (TP) played a key role in driving the extreme precipitation based on observational analyses and Earth system model experiments. In the control simulation, the model failed to reproduce the observed very warm 2-m air temperature anomaly over the TP and the extreme precipitation over California. After improving the temperature initialization with a mask over the TP, the model reproduced most of the observed TP 2-m temperature anomaly and successfully generated about 56% (January 2017) and 38% (March 2023) of the observed extreme precipitation anomalies over California and adjacent regions. The TP heating modulated a Tibetan Plateau-Rocky Mountain wave train, which in turn affected atmospheric rivers and triggered Rossby wave breaking over the northeastern Pacific and the western coast of North America. Both processes are well known as major contributors to extreme precipitation in the western United States. These results suggest that the two catastrophic winter precipitation events were predictable from remote land-surface thermal conditions and identify high-elevation terrestrial temperature anomalies as a previously unidentified source of subseasonal to seasonal predictability for winter extreme hydroclimate events.