Real-Time Insights on the November 25–27 Atmospheric River Event

On the heels of the November 19–23 atmospheric river that impacted northern California and southwestern Oregon, a second system affected central California from November 25–27. Driven by a deep low-pressure system off the Pacific Northwest coast, this storm delivered widespread rainfall, strong winds, and heavy snowfall in the Sierra Nevada. Precipitation peaked on November 25–26, with mountain rain totals exceeding 3 inches and snowfall above 7,000 feet reaching up to 36 inches in areas like Sequoia and Kings Canyon National Parks. Winds gusted up to 55 mph in the Kern County desert, while the San Joaquin Valley saw moderate precipitation. The system exited on November 27, leaving dry conditions in its wake.‍

Event Analysis

We can estimate the impact of the November 25-27 storm on statewide water year (WY) accumulation and the Core Supply regions (CAP Regions 2, 4, and 6). As noted in previous reports, this analysis is based on an abbreviated dataset—44 stations for Statewide totals and 13 stations for Core Supply regions.  These figures are highly preliminary. 

• Core Supply Regions: The November 25-27 storm added 4.18 percentage points to total precipitation accumulation, bringing the Core Supply to 20.07% of the WY average. Combined with the November 19–23 event, the total increase was 14.49 percentage points.
• Statewide Totals: The state as a whole gained 1.68 percentage points from November 25-27, bringing total WY accumulation to 10.50%. Combined with the previous event (November 19-23), the total statewide increase was 7.57 percentage points—about half the gain seen in the Core Supply regions.

With this limited dataset, we can discern regional patterns in a relative sense, but the percentages should not be taken as precise measures of precipitation received. Keeping this in mind:

Regional Highlights

• Region 6 (KTKK): Precipitation was concentrated in Region 6, where preliminary data suggests an event accumulation of around 13 percentage points for November 25-27. Bakersfield recorded nearly 1 inch of rain, representing 16% of its WY average—the largest single-station percent increase among the 44 analyzed.
  • Caveat: Region 6 had only one reporting station for this event. To create a more representative estimate, we included nearby data from Hanford and Fresno, which balanced the influence of Bakersfield's contribution. We believe this adjustment provides a total more reflective of actual conditions. PRISM images below further illustrate the precipitation patterns in this region.
• Region 4 (SNJQ, TBVL) and Region 5 (CENT): These regions saw similar gains of 5.81 and 4.49 percentage points, respectively, from November 25-27. In Region 4, a strong north-south gradient emerged across the six reporting stations, with Stockton receiving just 0.01 inches and totals increasing steadily to the south: Modesto (0.10 inches), Madera (0.45 inches), Merced (0.73 inches), Fresno (1.14 inches), and Hanford (1.06 inches).

• Region 1 (LKLA): Once more, no data was reported within Region 1 (LKLA) for this event, but precipitation was concentrated outside of this region.‍
• Region 7 (SCAL, MJVE): Southern California saw minimal gains from November 25-27, with only a 0.27 percentage point increase. However, PRISM data shows significant precipitation near Mount Whitney, on the border of Regions 6 and 7. Next month’s comprehensive update may refine Region 7's official totals.

‍Visual Analysis

The images below, sourced from California Water Watch, show PRISM data for WY-to-date precipitation as a percentage of WY-to-date averages and compare conditions before the atmospheric river event (through November 24, 2024, left) and after its conclusion (November 28, 2024, right). CAP regions are roughly overlaid, illustrating the spatial distribution of precipitation relative to regional boundaries. These images highlight concentrated accumulation near the intersection of Regions 4, 5, and 6, while Regions 1–3 remained largely unchanged.

Looking Ahead‍

With the recent storms behind us, conditions have quieted. Stay tuned for our upcoming monthly update, where we’ll provide comprehensive data through the end of November and insights into what lies ahead.

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