A beast of a bomb cyclone will take shape just off the coast of the northwestern United States and western Canada later this week, and AccuWeather forecasters say it will rival, in some aspects, the intensity of strong hurricanes from the Atlantic this season. The powerful storm will bring dangerous and damaging impacts up and down the West Coast, but the precipitation it will deliver to parts of California, Oregon and Washington is greatly needed.
The storm will have some tropical origins. Energy from former Severe Tropical Storm Namtheun, which churned over the western Pacific, will join forces with a non-tropical system sitting over the northern Pacific on Wednesday, according to AccuWeather Meteorologist Randy Adkins. Rapid strengthening will result. As the storm comes together a few hundred miles off the coasts of Washington and British Columbia, its intensification could easily surpass the criteria for bombogenesis.
Meteorologists define a bomb cyclone as a rapidly strengthening storm with a central pressure that plummets by 0.71 of an inch of mercury (24 millibars) or more within 24 hours. The process is referred to as bombogenesis. As the pressure drops rapidly in the center of the storm, air rushes in to replace the vacuum created in the atmosphere and can produce damaging winds.
The central pressure of the storm is forecast to dip to approximately 28 inches of mercury (948 millibars), putting the bomb cyclone at or even below the intensity level of Hurricane Larry, which was a long-lived and intense cyclone that churned across the Atlantic in early September. At peak strength, Larry was a major Category 3 hurricane with maximum sustained winds of 125 mph and higher gusts. Its central pressure dropped as low as 28.20 inches of mercury (955 millibars).
It will not, however, come close to the strength of Hurricane Ida, which, at its peak, was a strong Category 4 storm with a minimum central pressure of 27.43 inches of mercury (929 millibars).
Damaging winds are possible from the northwestern tip of Vancouver Island, British Columbia, and the Haida Gwaii archipelago in British Columbia as the storm rapidly intensifies at midweek. In this area, wind gusts of 40-60 mph are expected with an AccuWeather Local StormMax™ of 80 mph as the storm’s associated cold front charges eastward, Adkins explained.
The monster storm will be packed with plenty of moisture and wind energy, and act as a catalyst for other storms that will target the Pacific coast of the U.S. and Canada. Each storm will have its own level of strengthening but will likely not rise to the same intensity of the offshore bomb cyclone.
As the circulation of the bomb cyclone ramps up, winds and wave action will increase over the coastal Northwest into Thursday and as far away to the north as the Aleutian Islands on Friday, where NOAA’s Ocean Prediction Center is forecasting waves as high as 48 feet.
“Strong wind gusts of 40-50 mph can also be expected for coastal sections of Washington and Oregon from Wednesday to Thursday, but with the center of the bomb cyclone forecast to remain offshore, wind damage will be relatively minor and will certainly pale in comparison to the bomb cyclone from Thanksgiving week in 2019,” Adkins added.
Conditions are likely to remain stormy in the coastal Northwest into next week and are likely to expand southward through the coast of Southern California into next week.
As the pattern evolves and more storms spin southeastward across the Pacific, the rounds of rain and mountain snow will ramp up significantly. Through later next week, when the pattern will finally ease up, some west-facing slopes of the Coast Ranges and lower slopes of the Cascades and the Sierra Nevada are expected to pick up a general 6-12 inches of rain with locally higher amounts.
Snow will fall at varying elevations over the course of several days as each storm blows through. By the middle of next week, several yards of snow could pile up across the high country of the Sierra Nevada, Cascades and the Olympics.
Up to 2 feet of snow could fall over the highest elevations of the Sierra Nevada Mountains Sunday into Monday alone, and snow totals through next Wednesday could top 6 feet in some places.
The snow that accumulates in the mountains may provide bonus runoff into streams this fall or next spring, should snowpack remain in place through the winter. Significant drought caused several of California’s critical reservoirs to reach historic lows this year.
AccuWeather Meteorologist Alex DaSilva said the storm has the potential to “be a tremendous shot in the arm due to the long-term drought in the region.” As of Oct. 12, 2021, 46% of California was in exceptional drought, the highest level of drought severity, according to data from the U.S. Drought Monitor.
The unfolding stormy pattern will knock temperatures down and may also effectively shut down the wildfire season in much of California, just as other storms earlier this autumn and late this summer have done in the coastal Northwest, DaSilva added.
While all of the storms will have their benefits, one of the coming storms, in particular, could be especially problematic.
The biggest storm in the bunch, in terms of rain and mountain snow, is forecast to roll ashore from Sunday to Tuesday and is likely to target California in general and bring soaking rain as far south as coastal areas in Southern California and spotty showers to some of the deserts.
“Storms prior to the ‘big one’ early next week will prime the landscape and set the stage for quick runoff with the big storm carrying the potential for enough rain to cause widespread flash flooding and mudslides, especially in, but not limited to the burn scar areas in Northern California, Oregon and Washington,” AccuWeather Senior Meteorologist Joe Lundberg said.
More than half of all of the rain in the week-long pattern and the majority of the high-country snow in California may fall during that single storm early next week.
People with homes built along the hillsides in recent burn scar locations will need to be extra vigilant with the stormy pattern unfolding into early next week. As the rounds of rain soak the ground, the topsoil can become progressively unstable.
Forecasters also warn motorists never to attempt to drive through flooded roads. The reasons include, but are not limited to, the risk that the water may be deeper than it appears, water may still be rising and road surfaces can wash away beneath floodwaters.
Large waves are likely to sweep southward along the coast of Southern California later this week to next week and are likely to be a boon to surfers, but large breakers and strong rip currents will pose dangers to bathers.
The stormy pattern will bring rain and hazards to some communities along the West Coast, particularly in California, and it could be a bellwether of what’s to come this winter.
“The pattern unfolding this week to next week may be one of the biggest series of storms for the rainy season for California, but there is still potential for a couple of bigger storms over the winter,” AccuWeather Lead Long-Range Forecaster Paul Pastelok said.
A precursor to the stormy pattern will bring a batch of rain and high-country snow from British Columbia to part of Northern California and northern Idaho into Wednesday evening.
This mild-mannered storm, in comparison to the coming storms, is slated to bring a general 0.50 of an inch to 2 inches of rain in the western portions of Washington, Oregon and Northern California with a few inches of snow above pass level in the Cascades and over the peaks in the Olympics.