'Once A Decade' Bomb Cyclone-Fueled Atmospheric River: Severe Bay Area Impacts

SAN FRANCISCO — The season's first atmospheric river is poised to hammer the Bay Area with high winds and heavy rain, and forecasters predict most areas will pick up more than a month's worth of precipitation in just a few days.

According to the National Weather Service, nearly everyone in the Bay Area will log an inch or two of rain by week's end, while the North Bay faces a potentially historic dousing.

"The [ensemble model] shows a 90% chance that Santa Rosa will see more than 4 [inches] of rain this week," NWS Bay Area said Tuesday. "There is some spread between the floor and ceiling, with a 10% chance of Santa Rosa getting over 10 [inches] from Wednesday to Friday. If the high end verifies, that would approach the highest 3-day total ever recorded at Santa Rosa."

A flood watch will be in effect for Sonoma, Napa and Marin counties starting at 4 a.m. Wednesday.

"Flooding is the main concern, though with a lot of capacity left in the large rivers, most impacts will be to smaller creeks and streams and low-lying, flood-prone urban areas," NWS Bay area said in a Tuesday briefing. "Strong winds, combined with the heavy rain could also cause some tree damage and related impacts."

The multi-day soaking is being fueled by an unusually powerful "bomb cyclone" centered hundreds of miles away and headed for the Washington and Oregon coastlines. Also called bombogenesis, the phenomenon occurs when a midlatitude cyclone quickly intensifies, with air pressure dropping at least 18 millibars over 24 hours.

As of Tuesday morning, the Ocean Prediction Center was forecasting a 68 millibar drop in pressure, which would surpass the benchmark for a so-called "triple bomb" and could produce hurricane-force winds over the ocean.

"In fact, if the forecast holds, 943 [millibars] is approaching the lowest pressure ever analyzed in this part of the ocean and may produce historic wind speeds," the weather service said. "The Medford, Ore. office has issued a very rare hurricane-force wind warning for coastal waters from Tuesday morning through Wednesday morning. In their [forecast discussion], Medford notes this type of storm comes around once a decade or so."

While the worst impacts will stay considerably north of the Bay Area, the San Francisco Chronicle notes the cyclone spinning in place for several days will continue to influence a long-duration atmospheric river in NorCal.

Forecasters expect the first round of rain will begin to unfold across the region Wednesday morning.

"By the time the sun comes up Wednesday (or when the clouds become a slightly lighter shade of grey) the North Bay will be dealing with heavy rain, which will taper off with decreasing latitude, and only light rain is expected south of San Jose or Santa Cruz," the weather service said. "It looks nasty all day with strong wind and heavy rain continuing. This is especially true north of the Golden Gate, but Wednesday will be an impactful day across much of the Bay Area."

A wind advisory will be in place from Tuesday night to Thursday morning for San Francisco, the San Francisco Bay Shoreline, the Peninsula coast, and Marin, Sonoma and Napa counties.

A "brief reprieve" forecast for most of Thursday will still feature plenty of rain in the North Bay as winds taper off to just below advisory levels. The break will be short-lived as the cyclone to the north weakens, and a cold front takes hold Friday, setting up for another big soaker with potential thunderstorms.

"The cold front will finally push through Friday with another push of heavier rain and stronger wind ahead of frontal passage and stronger showers and potential thunderstorms during and after frontal passage," the weather service said. "For areas south of San Francisco, this will likely bring the heaviest rainfall of the week. The cumulative effects of this rain will start to be felt along the North Bay creeks and streams."

Forecasters expect conditions will begin to wind down over the weekend but caution persistent, lighter rain may linger well into next week. Extended outlooks from the Climate Prediction Center favor wetter, colder weather impacting Thanksgiving travel across much of the continental United States.