Real-Time Insights on the November 19-23 Atmospheric River Event

Historically, CAP’s monthly progress reports have a data lag of over a month due to reliance on National Weather Service (NWS) procedures. While necessary for unbiased forecast verification, this lag has proven particularly problematic following significant weather events, when timely information is most needed. To address this, we began using a separate NWS data stream featuring stations with real-time data availability for post-event analysis. While this dataset is smaller in number and doesn't provide a comprehensive picture like the full NWS feed, it does offer a valuable glimpse into event impacts to support decision-making in the interim. From this, we can provide estimates of % Total Water Year (WY) accumulation and evaluate the effect that a given event has had on the water year.

Event Synopsis

A powerful atmospheric river impacted northern California and southwestern Oregon from November 19–23.  Driven by a deep low-pressure system off the Pacific Northwest coast, the storm funneled moist air into the region, causing heavy rainfall, strong winds, and significant mountain snowfall. The two main waves of the event peaked on November 20 and 22, with conditions gradually improving by November 23.

The impacts across the seven CAP regions varied, with significant increases in WY precipitation percentages in some areas and more modest effects in others. We can analyze this event's impacts on the Statewide WY accumulation and the Core Supply (CAP regions 2, 4, and 6). It’s important to note that these estimates are based on data from 44 stations for the Statewide total and 13 stations for the Core Supply regions, with the dataset skewed toward coastal population centers. As such, these figures are highly preliminary.

That said, the Core Supply regions saw a significant increase of 10.31 percentage points, bringing the total Core Supply precipitation accumulation to 14.84% of the total water year average. Meanwhile, the statewide accumulation gained 5.89 percentage points, ending at 8.12% of the total WY accumulation.  If we explore the data regionally:

• Region 1 (LKLA): Unfortunately, no data was reported within Region 1 (LKLA) for this event, but we will provide more information on WY precipitation for this region in our next monthly update. However, PRISM data estimates in the image below offer an idea of the event's impact on this region.

• Regions 2 (SYFA) and 3 (RNSF): These regions experienced significant WY precipitation increases, with event contributions of 16.37% and 16.32%, respectively. Every reporting station in Region 2 recorded event totals exceeding 11%.
• Regions 4, 5, 6, and 7: The event's contributions tapered off in these areas, with increases of 5.93%, 2.99%, 2.69%, and 0.07% of total WY precipitation, respectively.
• Santa Rosa Station (Region 3): This station stood out with a remarkable 40.94% increase from the event, bringing its total to 45.89% of WY precipitation—a stunning jump.‍
• ‍Region 3 Disparity: At first glance, the data suggested significant variability across Region 3 stations, with Santa Rosa leading the region at 40.94% of water year accumulation for the event, while three stations reported an event accumulation of five percentage points or less. However, a closer look at the geographical distribution revealed that five of the seven stations in Region 3 are clustered near the San Francisco Bay Area, where precipitation varied sharply over a small area. The PRISM data below illustrates a steep north-to-south precipitation gradient within this region, with accumulation percentages highest in the north bay and tapering off across the south bay. Outside the immediate Bay Area, the two northernmost stations, Ukiah and Santa Rosa, reported significantly higher accumulations of 24.23% and 40.94%, respectively. Considering the PRISM data estimates and the station locations, it’s likely that the overall water year accumulation for Region 3 was skewed toward the dry side and is likely higher than indicated by this interim report.

The images below from California Water Watch display PRISM data for water year-to-date precipitation as a percentage of water year-to-date average, comparing conditions before the atmospheric river event (left) and toward its conclusion on November 22, 2024 (right). CAP regions are roughly overlaid, offering a clear view of the spatial distribution of precipitation relative to our regional boundaries. These successive images highlight just how significant this event was, with the most notable concentrations of precipitation accumulation in Regions 1, 2, and 3. In Region 1, where NWS data was unavailable, the PRISM data suggests precipitation totals comparable to those observed in Regions 2 and 3, underscoring the widespread impact of this event.

This report provides valuable real-time insights but represents only a partial picture of the event's full impact. With the comprehensive NWS data stream available in next month’s report, we will provide a more robust and detailed assessment, ensuring a more complete understanding of how this atmospheric river contributed to and influenced the current water year.  Stay tuned for additional updates.