Atmospheric river brings renewed threat of debris flows and flash floods near burn scars in California

A strong atmospheric river (AR) will impact northern California, bringing heavy rain to the northern Sierra Nevada, coastal ranges, interior foothills, and upslope areas starting Sunday, February 2.

The AR has already transported tropical moisture from near Hawaii to northern California. This moisture is being funneled into a narrow corridor due to a high-pressure system off southern California and a cold low-pressure system near Vancouver Island.

Moisture levels are significantly higher than usual, with Precipitable Water Values (PWATs) ranging from 25 – 32 mm (1 – 1.25 inches), approximately four times the normal amounts for the area.

In the 24 hours leading to 02:00 local time (LT), northern California received 25 – 100 mm (1 – 4 inches) of rain, particularly in the Sierra Nevada region around Lake Tahoe.

Satellite images show that the topsoil is significantly more saturated than the previous day, suggesting that additional rainfall will likely lead to runoff rather than absorption into the ground.

There is a risk of flash flooding and debris flows around burn scars, similar to the recent events that affected the region at the end of January.

Excessive rainfall will continue into early next week. Most of northern California is expected to receive three-day rain totals of 100 – 175 mm (4 – 7 inches), while the Sierra Nevada could see 250 – 380 mm (10 – 15 inches).

On Sunday, rainfall will continue through the morning at rates of 6 – 19 mm (0.25 – 0.75 inches) per hour. A strong weather system will move toward the coast for 12 hours after 02:00 LT on Sunday, bringing heavy rain to far northern California for the next four hours.

The rain will gradually shift southward toward Point Arena and the northern Sacramento Valley, with localized rainfall rates potentially reaching up to 19 mm (0.75 inches) per hour. Along the coast, rainfall rates will decrease through early Sunday morning, tapering off from north to south.

By 14:00 LT, much of the northern Sierra Nevada is expected to receive 75 – 150 mm (3 – 6 inches) of rain, while the northern Coastal Ranges may see 50 – 100 mm (2 – 4 inches). The Sacramento Valley, particularly along the I-5 corridor and just east of the coastal ranges, is likely to receive lower amounts.

According to the National Weather Service (NWS), the AR could bring 100 – 200 mm (4 – 8 inches) of rain totals across the affected regions, with higher isolated totals through Monday, February 3. The NWS has issued a slight risk (level 2 of 4) of excessive rainfall due to flooding concerns.

Read more about the recent debris flows in Los Angeles:

References:

¹ Short Range Forecast Discussion – NWS/WPC – February 2, 2025

² Mesoscale Precipitation Discussion – NWS/WPC – February 2, 2025

Rishav Kothari

I am an Assistant Editor and Severe Weather & Science Journalist at The Watchers, specializing in real-time severe weather coverage, geophysical event reporting, and research-driven scientific analysis. You can reach me at rishav(at)watchers(.)news.

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