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Another atmospheric river event has made its way to Northern California, with a flood watch in effect through Sunday morning. 

Why it matters: This will be a milder storm than the series of blizzards that have hit the Golden State in recent weeks, delivering rain on top of snow even at mid-to-high elevations. But with a near-record snowpack, there are flood risks, Axios' Andrew Freedman and Sareen Habeshian report.

State of play: Dubbed the "Pineapple Express," the storm is drawing moist air from the subtropics near Hawaii.

Atmospheric rivers are long, narrow highways of moisture that can travel thousands of miles and are responsible for 30%-50% of the wet season precipitation along the West Coast of the U.S., according to the National Oceanic and Atmospheric Administration.

They have been unusually persistent and powerful this winter, with no signs of an impending weather pattern shift to drier conditions.

South Lake Tahoe on Thursday issued a local emergency ahead of the storm, and officials have warned all Tahoe residents to prepare for potential roof collapses and flooding, SFGATE reports.

Be smart: San Francisco's public works department is providing residents with up to 10 free sandbags per address Monday through Saturday from 8am-2pm.

Between the lines: While a recent analysis by the nonpartisan Climate Central found many areas across the country have experienced warmer winters since 1970, San Francisco has seen the opposite — average temperatures have dipped to 52.5 degrees this winter, compared to 53.6 in 1970.

Data: Climate Central; Chart: Kavya Beheraj/Axios 

The bottom line: The astonishingly thick snowpack in California has raised hopes of more abundant water supplies and a less active early fire season across the state.

But so much snow sitting at lower elevations in particular is also a concern this week.

The dramatic swing from historically dry to very wet conditions in parts of California is consistent with what climate studies show will occur more frequently and sharply in coming decades.

Another atmospheric river hits San Francisco Bay Area

A flood watch is in effect through Sunday morning.

A messy commute on Highway 280 in San Francisco on March 9. Photo: David Paul Morris/Bloomberg via Getty Images

extreme weather

Another atmospheric river event has made its way to Northern California, with a <a class="gtmContentClick" data-vars-link-text="flood watch" data-vars-click-url="https://forecast.weather.gov/MapClick.php?lat=37.777120000000025&amp;lon=-122.41965999999996#.ZApD3OzMKWA" data-vars-content-id="69e0b35f-223b-46de-b4cc-3865a41e0bd0" data-vars-headline="Another atmospheric river hits San Francisco Bay Area" data-vars-event-category="story" data-vars-sub-category="story" data-vars-item="in_content_link" href="https://forecast.weather.gov/MapClick.php?lat=37.777120000000025&amp;lon=-122.41965999999996#.ZApD3OzMKWA" target="_blank">flood watch</a> in effect through Sunday morning. Why it matters: This will be a milder storm than the <a class="gtmContentClick" data-vars-link-text="series of blizzards" data-vars-click-url="https://www.axios.com/2023/02/28/california-storms-slam-state-snow-year-records" data-vars-content-id="69e0b35f-223b-46de-b4cc-3865a41e0bd0" data-vars-headline="Another atmospheric river hits San Francisco Bay Area" data-vars-event-category="story" data-vars-sub-category="story" data-vars-item="in_content_link" href="https://www.axios.com/2023/02/28/california-storms-slam-state-snow-year-records" target="_self">series of blizzards</a> that have hit the Golden State in recent weeks, delivering rain <a class="gtmContentClick" data-vars-link-text="on top of snow" data-vars-click-url="https://www.axios.com/2023/03/02/california-storm-emergency-yosemite-snow-photos" data-vars-content-id="69e0b35f-223b-46de-b4cc-3865a41e0bd0" data-vars-headline="Another atmospheric river hits San Francisco Bay Area" data-vars-event-category="story" data-vars-sub-category="story" data-vars-item="in_content_link" href="https://www.axios.com/2023/03/02/california-storm-emergency-yosemite-snow-photos" target="_self">on top of snow</a> even at mid-to-high elevations. But with a near-record snowpack, there are flood risks, <a class="gtmContentClick" data-vars-link-text="Axios&#x27; Andrew Freedman and Sareen Habeshian report" data-vars-click-url="https://www.axios.com/2023/03/09/pineapple-express-storm-california" data-vars-content-id="69e0b35f-223b-46de-b4cc-3865a41e0bd0" data-vars-headline="Another atmospheric river hits San Francisco Bay Area" data-vars-event-category="story" data-vars-sub-category="story" data-vars-item="in_content_link" href="https://www.axios.com/2023/03/09/pineapple-express-storm-california" target="_self">Axios&#x27; Andrew Freedman and Sareen Habeshian report</a>.

Sareen Habeshian

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Another atmospheric river hits San Francisco Bay Area