How bad could it get? Future AR Flooding Scenarios in the San Francisco Bay Area

In February 2017, localized flooding occurred in the southern San Francisco Bay Area (SFBA) as an Atmospheric River (AR) impacted the region. The flooding happened despite a 24-hour rain accumulation of just 2”, implying that other factors (e.g., soil moisture saturation, flood control, public awareness and decision making) were implicated. The event prompts us to ask however: How bad could it get? Specifically within the next 20-50 years, as the climate crisis continues to unfold. In a warming climate, ARs will bring more integrated water onshore, a result of the Clausius-Clapeyron relationship. Hence high-end AR events will potentially generate more rainfall. Given an “ideal” synoptic situation, we answer the question “How much more rainfall” via WRF simulations of such an event.

A “worst case scenario” would include elements such as: a developing synoptic-scale storm offshore and approaching the SFBA; ideal jet dynamics, with the SFBA under the left-front or right-rear portion of the synoptic jet; an AR incident on the region with enhanced moisture content (e.g., high-end IVT values); and synoptics such that the AR stays over the SFBA for 24-36 hours.

We examine first idealized simulations of this scenario, using simulations of recent AR events but with the moisture plume artificially enhanced. We will also search the IPCC database for future AR events, and use these to frame our WRF simulations.