The Bay Area and Northern California woke up Wednesday to the arrival of a storm, fueled by a strong atmospheric river and intensified by a so-called “bomb cyclone” off the coast of the Pacific Northwest. The storm is expected to bring significant impacts, including heavy rainfall, damaging winds and flooding.

The most impacts are expected in the North Bay and northeastern part of the state. The National Weather Service has issued a flood warning for Marin, Napa and Sonoma counties until 4 a.m. Saturday as 2 to 12 inches of rain is expected. The North Bay, San Francisco Bayshore and the Pacific Coast are also under a wind advisory for gusts of up to 70 to 80 km per hour until early Thursday.

In addition, a high surf advisory was in effect Wednesday morning through Friday morning as the weather service said breaking waves of 15 to 20 feet were possible.

KPIX First warning again: Current conditions, warnings, maps for your region

“Big event” for the North Bay

Rain totals will be highest in the North Bay and localized flooding is likely in low-lying areas.

“Here’s what we know: This will be a significant event for the North Bay,” the Weather Service said Wednesday in its forecast discussion for the San Francisco area. “There is now a 90% chance that Santa Rosa will record more than 5″ by the end of the weekend and a 10% chance of exceeding 12″ by the end of the weekend. Normal for the entire month of November is 3.25”.

Pacific Gas and Electric said its crews are preparing for outages caused by the intense winds expected in the region.

“With the high winds, we can expect to see tree limbs snapping and coming into contact with power lines, or tree lines falling into power lines and causing power outages,” said PG&E spokesman Paul Moreno. “We have ensured we have the personnel and resources such as materials, poles and wires to meet this challenge.”

Atmospheric river + bomb cyclone

The atmospheric river – a long stretch of heavy moisture from the tropics – flows into Northern California as a polar air mass collides with a tropical air mass in the Pacific Ocean. The two air masses cause atmospheric pressure to drop rapidly, creating a bomb cyclogenesis or bomb cyclone, which strengthens the storm and increases winds.

Atmospheric rivers can produce heavy rain and snow, and strong ARs can transport water vapor approximately equal to 7.5 to 15 times the average flow of the Mississippi River. When the atmospheric river and the bomb cyclone coincidea major weather event is expected.

North of the Bay Area, the storm is expected to be particularly treacherous, while storm effects will diminish further south. The Weather Service’s Eureka office has issued a series of storm and flood warnings, warnings and advisories throughout Northern California to the Oregon border. Meanwhile, a Winter Storm Warning was issued for counties along the northern Sierra Nevada.

Precipitation expected into the weekend

On Thursday, the Weather Service said there could be a slight lull in the storm ahead of another intensified cyclone on Friday, which would push another round of heavier rain through the region. The rain will be accompanied by a cold air mass and increased instability, transitioning from steady rainfall to showers and possible thunderstorms late Friday through Saturday.

While the heaviest rainfall will fall over the weekend, the chance of additional light rain will remain through Wednesday. The Weather Service said that since it is still early in the water year, the largest rivers are still at high capacity and are not expected to overflow. However, flooding of smaller creeks and streams, as well as flooding of towns and roads, are all expected and could have significant impacts.