Winter Forecasts for California: Navigating the Uncertainty with CAP

A group of blind men encounters an elephant, each touching a different part: one feels the trunk and claims, “it’s like a snake,” another touches the leg and declares, “it’s like a tree,” while yet another grips the tail and thinks, “it’s a rope.” Each man describes the same elephant differently based on his limited perspective. Much like the blind men and the elephant, people often focus on singular aspects of the water year—specific climate drivers or the occurrence of atmospheric rivers—leading to a fragmented understanding that misses the broader picture.

As we prepare for the winter of 2024-25, our California Annual Precipitation (CAP) forecast projects a near-normal precipitation range of 90-110% of historical averages. This forecast stands out in a landscape of speculation, where many emphasize extremes—either exceptionally wet or dry conditions—and the only consensus seems to be “uncertainty”. Ironically, against a backdrop of phrases like 'uncertainty' and 'extremes,' CAP confidently projects a water year that is likely to be… well… normal.

‍NOAA’s 2024-25 Winter Precipitation Outlook (right) indicates “equal chances” of above- or below-normal precipitation for all but the smallest sliver of California¹. This vague assessment provides no actionable insights for water management and planning.

NOAA’s Climate Prediction Center projects a 71% likelihood of weak La Niña conditions emerging this fall.  La Niña events have historically influenced California’s winter weather with outcomes on both sides of the normal. Notably, during seven of the ten La Niña events this century, drier conditions were observed, with precipitation averaging 30% below normal. Conversely, the wet years that coincided with La Niña saw precipitation levels exceeding averages by a similar margin.

If we look specifically at the CAP data record from 2016-2024, weak La Niña conditions resulted in above normal precipitation in WY 1617 (64.1% above normal) and WY 2223 (41.2% above normal).  In WY 1718 and WY 2122, the recorded precipitation was 25.8% and 28% below normal, respectively.  The same climate forcing resulted in much different outcomes, but the CAP forecast verified each year.

This duality reinforces the sentiment echoed by many meteorologists to “prepare for the extremes,” indicating that while we may not know what to expect, the outcome will likely be pronounced. State Climatologist Dr. Michael Anderson asserts, “While there is still a lot of uncertainty around how La Niña could impact the state this year, we know we can count on it to include extreme conditions.” ²

AccuWeather meteorologists suggest that powerful, moisture-laden systems – atmospheric rivers – could “drench the West Coast,” hinting at a potentially wet winter. They believe that this year's weaker La Niña may allow for more varied atmospheric influences.  For example, in the winter of 2022-23, nearly 40 atmospheric rivers affected California despite a La Niña in place.

Adding another layer to the uncertainty, seasonal model output from the National Weather Service and the European Center for Medium-Range Weather Forecasts (ECMWF or Euro, right) indicate that California may receive below-average precipitation between October and December.³ 

Understandably, this patchwork of predictions complicates decision-making for stakeholders seeking clear guidance. Our California Annual Precipitation (CAP) forecast offers a beacon of clarity amidst this uncertainty. CAP projects the total water year (WY) precipitation amount at 90-110% of normal values. Yet, it's crucial to note that this forecast doesn’t rule out the occurrence of significant or “extreme” weather events throughout the year. Instead, our approach projects where the water year will end, not the twists and turns along the way. 

Most traditional forecasts rely on long-term physical models that simulate complex atmospheric dynamics. In contrast, the CAP statistical model captures the effects of these dynamics without needing to model every individual component.  Our methodology synthesizes the influence of various atmospheric phenomena, providing stakeholders with a clearer and more actionable expectation.  

As terms like La Niña and atmospheric rivers lead us toward specific conclusions, the CAP model reveals that this water year isn’t a snake, a rope, or a tree—it’s a very normal-looking elephant. In a landscape where ‘extremes’ often dominate the conversation, predicting ‘normal’ feels remarkably extreme.

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¹ _{NOAA Climate Prediction Center. (2023). Precipitation outlook: October 2024 [Map].} ^{https://www.cpc.ncep.noaa.gov/products/predictions/long_range/lead01/off01_prcp.gif}

² _{California Department of Water Resources. (2023, September 24). California prepares for extreme weather swings as new water year approaches. https://water.ca.gov/News/News-Releases/2024/Sep-24/California-Prepares-for-Extreme-Weather-Swings-as-New-Water-Year-Approaches}

³ _{ABC10. (2023, October 2). The water year is over, a new one begins. What it might look like. ABC10.} ^{https://www.abc10.com/article/news/local/the-water-year-is-over-new-one-begins-what-it-might-look-like/103-fd46c5c4-aae0-4b42-8ced-66763f8a0466}