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Water Year 2021 Characterized by Persistent Dry Weather and Worsening Drought in California

• Total precipitation has been well below normal throughout much of California during water year (WY) 2021
• In some regions, drier than normal conditions extend back to the start of WY 2020
• Drought has expanded and intensified across the state, and current water storage levels are below normal in many
reservoirs
• Below-normal snowpack in the Sierra Nevada may limit water resource availability as summer approaches
• The abnormally dry conditions were driven by a lack of landfalling atmospheric rivers (ARs) and persistent
ridging/blocking over the Northeast Pacific Ocean
• Drought is expected to continue through spring 2021, thereby increasing the threat of significant wildfire activity in
summer 2021

California Weighs Changes for New Water Rights Permits in Response to a Warmer and Drier Climate

As California’s seasons become warmer and drier, state officials are pondering whether the water rights permitting system needs revising to better reflect the reality of climate change’s effect on the timing and volume of the state’s water supply. A report for the State Water Resources Control Board recommends tailoring new water rights permits to California’s increasingly volatile hydrology. And it warns that the increasingly whiplash nature of California’s changing climate could require existing rights holders to curtail diversions more often and in more watersheds — or open opportunities to grab more water in climate-induced floods.

Landfalling Atmospheric Rivers Increase in First Quarter of Water Year 2021 Over 2020

A Scripps Center for Western Weather and Water Extremes report shows that more atmospheric rivers have made landfall over the U.S. West Coast in the first four months of Water Year 2021 compared to the previous period in 2020.

Atmospheric Rivers-CW3E-Scripps-Rain

Landfalling Atmospheric Rivers Increase in First Quarter of Water Year 2021 Over 2020

A Scripps Center for Western Weather and Water Extremes report shows that more atmospheric rivers have made landfall over the U.S. West Coast in the first four months of Water Year 2021 compared to the previous period in 2020.

The four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, six more than the first four months of Water Year 2020, according to the CW3E quarter year summary released February 11.

Key takeaways from the CW3E update on atmospheric rivers

• The first four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, 6 more than the first four months of Water Year 2020.
• Water Year 2021 also experienced more than twice as many strong or greater ARs (14) compared to Water Year 2020 (6).
• While Water Year 2021 saw more ARs in its first 4 months compared to Water Year 2020, a large majority of those ARs were
strongest over the Pacific Northwest (Oregon and Washington).
• The average landfall latitude thus far in Water Year 2021 was 45.9°N compared to 43.9°N during the first half of Water Year 2020.
• The lack of landfalling ARs over Southern California and Baja California has resulted in much drier conditions over the southwestern U.S. compared to WY 2020.

CW3E-Atmospheric Rivers-Water Year 2021

The four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, six more than the first four months of Water Year 2020. Graphic: Center for Western Weather and Water Extremes

Distribution of Landfalling Atmospheric Rivers over the U.S. West Coast During Water Year 2021: Quarter Year Summary

Atmospheric River-NOAA-WNN-CW3E

A partnership with the San Diego County Water Authority and the Scripps Institution of Oceanography at UC San Diego seeks to optimize water management to better predict atmospheric rivers before, during, and after those seasonal storms.

In August 2020, Scripps Center for Western Weather and Water Extremes (CW3E) launched the Water Affiliates Group, which brings together cutting-edge science and hands-on water industry experience to enhance reservoir operations in light of the changing climate. The Water Authority has a long-running alliance with Scripps and is among six founding water agencies statewide.

CW3E and its partners will share and support best practices in forecast-informed reservoir operations, increase research around atmospheric rivers and droughts, and develop strategies for mitigating flood risk and increasing water supply reliability.

Atmospheric Rivers-CW3E-water supply

The above image, from the EOS article, is a depiction of an atmospheric river, interacting with West Coast mountains and a midlatitude cyclone over the northeast Pacific on 5 February 2015. This image provides an example of approximate locations of associated tropical moisture exports and a warm conveyor belt (WCB). Graphic: Adapted from NOAA/ESRL Physical Sciences Division via CW3E

High-Tech Forecasting Model Scores Scientific Win at Lake Mendocino, Showing Promise for Western Reservoirs

Sandbars are spreading across rain-starved Lake Mendocino, the reservoir near Ukiah that is 35 feet lower than it was a year ago, a grim wintertime sight for the second major source of water for more than 655,000 people in Sonoma, Mendocino and Marin counties. But the situation would be considerably worse without the payoff from a six-year, $50 million project applying high-tech weather forecasting to management of the reservoir behind Coyote Valley Dam built on the East Fork of the Russian River in 1958.

California’s Rainy Season Starting Nearly a Month Later Than it Did 60 Years Ago

The start of California’s annual rainy season has been pushed back from November to December, prolonging the state’s increasingly destructive wildfire season by nearly a month, according to new research. The study cannot confirm the shift is connected to climate change, but the results are consistent with climate models that predict drier autumns for California in a warming climate, according to the authors.

California’s Sierra Snowpack Lags Despite Recent Big Storms

Recent storms have boosted California’s vital Sierra Nevada snowpack but not enough to fully compensate for a dry start to winter and residents should use water wisely, a state official said Wednesday after the season’s latest measurements.

What Are These Atmospheric Rivers that Bring Heavy Rain and Snow to California?

An atmospheric river — a plume of moisture that has been likened to a river in the sky — has brought heavy precipitation to the Central Coast. Now it will sag southward and bring rain and mountain snow to Southern California on Thursday night into Friday morning, the National Weather Service said.

Scientists Use New Methods to Better Forecast Atmospheric Rivers

Earlier this year, the Center for Western Weather and Water Extremes launched a new sub-seasonal to seasonal forecast product to better predict the influence atmospheric rivers will have on the Western United States. Better and more accurate forecasting tools for atmospheric rivers are critical for a number of community uses, including water management, agriculture, insurance and commodities trading, to name a few.

The demand for better atmospheric forecasting tools has facilitated the development of the new S2S forecasting products launched by CW3E this year.

From Droughts to Flooding, Here’s How California is Trying to Better Understand Atmospheric Rivers

California weather is rarely average. Historically, the state has well above or well below average rain and snow. One of the keys to prepare for these wild swings is a better understanding of atmospheric rivers. The Center for Western Weather and Water Extremes or CW3E is at the heart of this research.