Northern California is set to see its first major storm of the season as a developing atmospheric river takes aim at the region late this week. Forecasters say the system could bring several inches of rain to the far North Coast and heavy mountain snow to the Sierra between Friday and Monday.

Meteorologists with the National Weather Service in Sacramento describe it as a “potent atmospheric river event” fueled by a deep trough from the Gulf of Alaska, which is expected to funnel a plume of subtropical moisture directly into the state.

The heaviest rainfall is forecast for Del Norte County near the Oregon border, where up to 3 inches of rain could fall. North Bay counties may see up to an inch, while the Bay Area and Central Coast are expected to receive lighter amounts, generally under half an inch.

In the Sierra, snow levels will start above 8,000 feet on Friday before dropping toward major passes like Donner by late weekend. The storm could bring hazardous travel conditions to Interstate 80 and other mountain routes as temperatures fall and winds increase.

Forecasters caution that the storm’s track could shift in the coming days, which may alter rainfall totals and snow levels. Until then, Northern California will experience calm and mild weather, with valley highs in the 70s and mountain highs in the 60s through Thursday.

This marks the first significant moisture event of the new water year and an early preview of what could be a wet late October across the region.