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October 20, 2021 – By now you’ve heard of the atmospheric rivers (ARs) forecast to bring much-needed rain (and snow) to Northern California over the next seven days. The strongest of these ARs is forecasted to make landfall on Saturday, with heavy rains through Tuesday.

Here’s a 90-second video explainer on what atmospheric rivers are.

Atmospheric rivers are long, narrow bands in the atmosphere that transport water vapor, like a river in the sky. When an atmospheric river makes landfall, it often releases this water vapor as rain or snow. Atmospheric rivers are responsible for up to half of California’s annual precipitation, and can cause flood damages averaging $1.1 billion annually throughout the West. Learn more about how atmospheric rivers form, and a scale to categorize their intensity and impacts, in this animation.

The Center for Western Weather and Water Extremes (CW3E) based at the Scripps Institution of Oceanography at UC San Diego just published their latest outlook:

Multiple Atmospheric Rivers to Bring Heavy Precipitation to Northern California

  • A series of landfalling atmospheric rivers (ARs) will impact the western US this week into early next week
  • AR 4/AR 5 conditions (based on the Ralph et al. 2019 AR Scale) are expected in coastal southern Oregon in association with the first and second ARs today through Friday
  • The strongest AR is forecasted to make landfall across Central and Northern California on Saturday, potentially bringing AR 4/AR 5 conditions to the San Francisco Bay Area
  • Inland penetration of this AR may bring AR 2/AR 3 conditions to portions of the interior western US
  • Yet another landfalling AR is forecasted to impact the US West Coast on 26–27 Oct
  • The first two ARs are forecasted to produce 2–5 inches of rainfall in portions of Northern California and southern Oregon
  • The fourth AR is forecasted to bring widespread precipitation to much of the western US, with the heaviest precipitation amounts in Northern California
  • Significant snowfall accumulations are also possible in the Sierra Nevada in association with the fourth AR
  • Portions of Northern California may receive more than 10 inches of total precipitation over the next 7 days

Feather, Yuba, Bear and American Watershed forecast

The 10-day watershed precipitation forecast shows precipitation totals of over 200 millimeters, or 7.87 inches.

10-day Watershed Precipitation Forecasts from the Center For Western Weather and Water Extremes (CW3E)

Atmospheric river storms now provide half of California’s annual precipitation. “ARs are strengthening and becoming even bigger contributors to the annual [precipitation] total, and California’s topography is ideally aligned to extract increasingly heavy precipitation from strengthening ARs,” the Scripps study, ‘Precipitation regime change in Western North America: The role of Atmospheric Rivers’ states.

Prepare for the storms

  • Have sufficient food and water for yourself and your pets to last for several days.
  • Pick up any prescription medication you may need before the storm hits.
  • Charge your electronic devices and check your flashlight and radio batteries.
  • Have repair supplies available.
  • Be aware of changing conditions and monitor your local media for weather alerts and updates.
  • If you are able, check on your neighbors.

Sandbag locations

Sandbags are available throughout locations in the Foothills. Bring your own shovel, sand and bags are provided for free.

Nevada County

  • Next to the Nevada County Warehouse on the corner of Highway 49 and East Broad Street in Nevada City
  • Penn Valley Fire Protection District located at 10513 Spenceville Road in Penn Valley
  • North San Juan Community Hall located at 10057 Reservoir Street in North San Juan
  • Higgins Fire Department, 10106 Combie Road​, Auburn

Residents must bring their own shovel (and gloves!) to fill the sandbags with sand. Please, only take what you plan to use for protection of your personal property.

Reminders for severe weather preparedness:

If you are operating a vehicle, please review the following tips:

  • Avoid areas subject to sudden flooding
  • Do not attempt to drive (or walk) across running water more that a few inches deep
  • Avoid driving in water that downed electrical or power lines have fallen in
  • Avoid hydroplaning by driving slowly and keeping your tires inflated correctly

Don’t forget to accomplish the following if possible

  • Clear rain gutters
  • Repair roof leaks
  • Cut away tree branches that could fall on your house

Rock and mudslides on mountain roadways are likely. Reduce your speed, turn on your headlights and be aware of your surroundings.

Travel could be very difficult with some roads covered by water. Turn around, do not drown.