California's First Atmospheric River Brings Rare Meteorological Trifecta

The first major atmospheric river of the season brought a remarkable meteorological trifecta to Northern California: a bomb cyclone, an atmospheric river, and the Fujiwhara Effect—a phenomenon where two adjacent storms rotate around each other, pulling in moisture and amplifying their intensity. This rare combination resulted in a highly dynamic storm that produced significant precipitation totals. Santa Rosa recorded its wettest day on record (6.92 inches), and some areas of the North Bay saw precipitation exceed 20 inches over just a few days.

At Weather Tools, we exist to answer our customers' questions about precipitation and the resulting inflow: How much? Where? And when? This event underscores the importance of being able to answer the “where” and “when” questions, as atmospheric rivers are, by definition, narrow and capable of delivering a powerful punch. Our CAP product and the CRAFT inflow forecast are designed to answer the “how much” question, providing reliable estimates to support critical decision-making. However, we recognize that understanding where and when precipitation will occur is equally essential.

Where?

The California Water Watch (CWW) precipitation estimates highlight November's impact, comparing accumulation through October (left) with November (right). Key takeaways include the significant accumulation from atmospheric rivers in Northern California and the pronounced North/South precipitation gradient.

While such gradients are common due to California's geography, Weather Tools has historically lacked the station coverage to analyze and forecast them effectively. With our growing station network, we are now gathering valuable data and expanding our predictive capabilities to include precipitation gradients. Although it's too early to integrate this data into our official forecast, initial insights from these stations and model outputs are promising. Although this data is not yet operational, it demonstrates immense potential for future water management applications:

• Coastal Gradient: Forecast indices from our stations in Fort Bragg and Half Moon Bay indicated that southern coastal regions could be up to 25% drier than their northern counterparts.

• Valley Gradient: Similarly, data from our Modesto and Clovis weather stations suggested that the Central Valley’s southern regions may experience up to 30% less precipitation than the northern regions.‍

• Broader Gradient Pattern: The forecast indices also suggested a drying trend south of a line from Half Moon Bay to Modesto and that has certainly been observed in the season's precipitation data to date.

When?

‍Understanding the timing of precipitation is critical, particularly for operational decisions. Until now, timing has been inferred from remaining precipitation needed to reach forecast totals, climatology, and water year analogs. While this provided some important insights, there was room for improvement.

Weather Tools is developing a method to forecast expected precipitation during specific seasonal periods: October–November, December–February, and March–May. Precipitation ranges will be available for the 8-Station (Southern Cascades & Northern Sierra), 5-Station (Central Sierra Nevada), and 6-Station (Southern Sierra Nevada) domains, each with an uncertainty range of ±17.5% of the forecasted value. 

The information that follows is highly preliminary:

8-Station Domain – Southern Cascades & Northern Sierra

• Oct–Nov: Expect 25% of CAP forecast (10.58” – 15.07”)
  • Actual: 14.2” (27.7% of CAP)
• Dec–Feb: Expect 40% of CAP forecast (16.93” – 24.11”)
• Mar–May: Expect 35% of CAP forecast (14.81” – 21.10”)

5-Station Domain – Central Sierra Nevada

• Oct–Nov: Expect 15% of CAP forecast (4.79” – 6.82”)
  • Actual: 4.4” (11.4% of CAP)
• Dec–Feb: Expect 40% of CAP forecast (12.77” – 18.19”)
• Mar–May: Expect 45% of CAP forecast (14.37” – 20.46”)

6-Station Domain – Southern Sierra Nevada

• Oct–Nov: Expect 15% of CAP forecast (3.38” – 4.81”)
  • Actual: 4.9” (18.0% of CAP)
• Dec–Feb: Expect 35% of CAP forecast (7.88” – 11.23”)
• Mar–May: Expect 50% of CAP forecast (11.26” – 16.04”)

These early results are encouraging. For example, actual precipitation through Oct–Nov for the 8-Station domain (27.7% of the CAP forecast) closely aligns with the forecasted range of 25%. Similarly, while the 5 and 6-Station domains recorded numbers just outside of the forecasted range (slightly below and slightly above, respectively), the results remain within the predicted range of variability.

Additionally, this data suggests that the spring period (March–May) may contribute an increasing percentage of water year precipitation as we move south.  March–May precipitation is forecasted to contribute up to 50% of the water year total for the Southern Sierra, compared to 35% for the Northern Sierra.

Although this is our first year testing a seasonal precipitation timing forecast, the consistency between the initial forecasts and early results gives us confidence in the value this method will bring to water management decisions.

Looking Ahead

Precipitation is on the way—and likely starting just as this report reaches your inbox. For some areas of California, December’s precipitation totals could double by the time this system moves through. We’ll provide an ad hoc update to quantify totals at the event’s conclusion.

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