An atmospheric river and early-season bomb cyclone will hit the Pacific Northwest

Parts of the Pacific Northwest are about to be hit with the first powerful storm series of the coming winter season, an impact that will bring some whiplash for the region as it goes from severe drought to flash flooding.

Rainfall will peak on Monday, and by Tuesday, 1 to 3 inches will be common in western Washington state, Oregon and into Northern California, mainly to the north of the San Francisco Bay Area. Even higher rainfall totals are a good bet around coastal Canada.

The main culprit is a storm that will rapidly intensify to the south of the Gulf of Alaska. It is forecast to reach “bomb cyclone” status over the weekend before unleashing fury on the Pacific Northwest into next week. Additional rounds of rain are also probable later in the week.

Reaching ‘bomb cyclone’ status

Bomb cyclones are characterized by a drop in central pressure of at least 24 millibars over 24 hours at 60 degrees latitude. There are some variations in the pressure drop required depending on latitude, but 24 millibars is usually considered the baseline.

Friday morning’s European weather model, for example, takes the storm from about 996 millibars late Saturday to 963 millibars late Sunday. That easily reaches the criteria, given a drop of 33 millibars over that day-long period. Other weather models indicate similar strengthening.

While the bomb cyclone is well offshore as it explodes in intensity, it will help to gather a potent atmospheric river on its southern flank before flinging it at the West Coast. An atmospheric river is a long and narrow channel of jet-stream transported wind, clouds and rain that moves water vapor out of the tropics, according to the National Oceanic and Atmospheric Administration.

Atmospheric river followed by more rain

Atmospheric rivers along the West Coast are rated on a 1 to 5 scale, with 5 being most powerful, by the Center for Western Weather and Water Extremes at the University of California at San Diego.

The upcoming atmospheric river is currently forecast to reach Category 4 along portions of the southern Oregon coastline, with Category 3 conditions for most of the rest of the Pacific Northwest and coastal Canada. Forecasts previously rose as high as Category 5, but limiting factors like the system’s quick movement may help keep it from rising to that intensity as it comes ashore.

Late Sunday into Monday night currently look like the peak of the rainfall associated with the atmospheric river, although additional rounds of moderate to heavy activity should also rotate through northwest Washington and adjacent Canada into at least the middle of next week.

Advertisement

Rainfall rates of up to 1 to 2 inches in six hours near the heart of the atmospheric river may lead to some flooding Monday around the Oregon and California border region, especially near any fresh scars from wildfires.

Fortunately, the heavy stream of rainfall doesn’t seem like it will last long in most spots. Totals of 1 to 3 inches should be common from the Cascades and to the west by Tuesday.

Additional showers and occasional waves of heavier rain in the wake of the atmospheric river will lead to significant totals in some locations. Seattle is on track to see its wettest days of 2023 during the stretch.

By the end of next week, when drier weather should try to resume, rainfall tallies may be around 3 inches or more in Seattle, with up to 6 inches or more in higher elevations like Olympic National Park and on Vancouver Island.

Easing drought and fire concerns

Despite some concern over strong and erratic winds ahead of the storm, wet weather is good news, as it will alleviate drought and associated wildfire activity. Although the United States is seeing one of its tamest fire seasons in recent times, the Pacific Northwest has experienced many of the country’s blazes this year.

Advertisement

Large fires ongoing in southwest Oregon and northwest California have burned more than 250,000 acres and are among the biggest in the country. They have been sending smoke into San Francisco and surrounding areas this week.

As of Thursday’s update, the weekly report from U.S. Drought Monitor had roughly 75 percent of Washington and Oregon under drought conditions, with about half of each state under severe drought or worse.

While this round of rainfall seems likely to disrupt fire season north of California, probably bringing it to a close in many spots, the expected weather pattern ahead favors a resumption of drier than normal conditions. Both the one month and three-month outlooks from the Climate Prediction Center show as much, especially for western Washington.

With a developing El Niño, a warming of Pacific Ocean waters that influences global weather patterns, the outlook into winter may also favor warmer than normal and drier than normal conditions. El Niño typically displaces the main zone of West Coast cool-season precipitation into California rather than the Pacific Northwest.

ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZMSmrdOhnKtnYmV%2FdHuPcmZral%2BWwa670qmfnqqZmHqztdWeqWavkai1qrrGraanZZ%2Bnsqi7zWaZqKWSYrC6r8uopZ5n