January and Beyond
The Northern Sierra, San Joaquin, and Tulare Basin precipitation indices for WY 2024-25 and the two analog water years are shown below. The pink box marks the month of January. Precipitation nearly flatlined throughout the month. However, to the right of the box, you can see evidence of the February storms. In last month’s report, you’ll recall our expectation of wet months ahead. This has shown to be true – and we look forward to analyzing the full NWS dataset in next month’s report.
Research & Development Update:
Timing
Understanding when precipitation will occur is critical for operational planning. As we mentioned in the fall, Weather Tools has been developing a method to forecast seasonal precipitation timing across California’s major hydrologic domains. This new approach provides projected precipitation ranges for three key seasonal periods: October–November, December–February, and March–May.
In December, we shared preliminary expectations for each region, and now, with updated February data, our forecasts continue to closely align with observed precipitation trends. Early results from this effort are encouraging (see plot).
We look forward to evaluating the full-season results and refining this approach to provide even greater insight and confidence in future precipitation timing forecasts.
Gradient Forecasts
Back in October, Rob noted the CAP model output on his whiteboard (right), marking important orographic features and the model indices at our various station locations. Based on our experience, it was clear to us that the model was indicating a precipitation decrease to the south of a line from our Half Moon Bay site to our Modesto site. October’s whiteboard notes captured a hypothesis—now, months later, the rain gauge is putting it to the test.
The CWW map (left) shows water year precipitation as a percentage of average through February 24. An approximate line from our Half Moon Bay site through Modesto is shown. The coastal gradient is evident; southern coastal areas have received notably less precipitation than their northern counterparts. Similarly, our valley gradient prediction is holding, with the southern Central Valley showing significantly lower precipitation totals compared to northern sections. As the water year progresses, we look forward to further evaluating the performance of our gradient forecast.