By Luke Stone, Forecaster Posted 2 months ago September 24, 2023
Bomb Cyclone to Bring Drought and Wildfire Relief, High Elevation Snow, Rain, and Wind to the Pacific Northwest
As of Sunday morning at 9 AM, the low-pressure system off the coast of British Columbia is already undergoing the rapid intensification process characteristic of a bomb cyclone. Read more about bomb cyclones here.
The storm will continue to deepen through Monday morning, with impacts as early as midday today (Sunday). Atmospheric rivers along the West Coast are rated on a scale from 1 to 5, which characterizes the strength and impacts of atmospheric rivers based on their water vapor content and duration, as described below.
- AR5: Exceptional
- AR4: Extreme
- AR3: Strong
- AR2: Moderate
- AR1: Weak
The associated atmospheric river may reach category 4 in Oregon and Washington, and category 3 in northern California and British Columbia.
Image: Atmospheric river scale forecast for northern California through British Columbia.
Image: Plume of atmospheric river moisture stretching across the North Pacific and impacting the Pacific Northwest during the early part of the week.
The atmospheric river will make landfall today, first bringing rain to Oregon this morning. Throughout the day, rain will spread north into Washington, and into British Columbia overnight. Peak rainfall is expected from Sunday night through Monday, while moderate to heavy rain will continue off and on through the middle of the week.
By midweek, 1-3 inches of rain is expected for Seattle and Portland, while coastal and mountain areas from northern California through British Columbia will see 2 - 5 inches, with isolated higher amounts. Some of the heaviest rain will fall on Victoria Island in British Columbia. As the upper-level storm slowly moves inland, rain showers will continue through the latter part of the week.
Despite the antecedent drought conditions, minor flooding is still possible in northern California and southern Oregon. Ahead of this storm, the 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.
Image: Current drought severity across the northwestern US and British Columbia, with a large area of D# Extreme Drought conditions.
Debris flows on fresh burn scars present an additional concern in this region. Overall, the rain will benefit the Pacific Northwest, alleviating drought and extinguishing wildfires. Some of the worst and largest fires this season have burned in Washington, Oregon, and Northern California. Poor air quality has plagued much of the region, and this rain event should, at least temporarily, put an end to that.
While the fronts remain offshore at this time, the tight pressure gradient associated with the low-pressure system will result in strong winds from northern California to British Columbia. In Washington, coastal areas could see gusts up to 50 mph. Gusts up to 70 mph are possible on the coast as well as inland in southern Oregon. In northern California, gusts up to 70 mph are possible, with some 100 mph gusts possible at higher elevations.
Snow will not be the most impactful aspect of this storm, but some areas will see solid accumulations. Despite high snow levels, upper elevations in the coastal mountains of British Columbia and some of the stratovolcanoes in Washington, Oregon, and northern California should see accumulating snow.
Snow levels will start around 7k feet in British Columbia, lowering to around 6k by Wednesday. Several inches are possible above 6k with higher amounts at the peaks. In Washington, snow levels will start around 8k and lower to around 6k by Wednesday. Accumulations will mainly occur on the stratovolcanoes; Baker, Rainier, Glacier Peak, and Adams, with the deepest snow on Mount Rainier. Even the stratovolcanoes in Oregon and northern California could see several inches of snow above 7k feet.
The atmospheric river-fueled impending bomb cyclone will bring a myriad of challenges and potential hazards to the Pacific Northwest this weekend and early next week. While damaging winds, minor flooding, and possible debris flows are anticipated, the overall impact of the storm is expected to be beneficial for the region. Alleviating drought conditions, extinguishment of wildfires, and improving air quality are welcome results of this storm.
This storm will remain offshore, preventing the most severe impacts from reaching the coast. Check out the current satellite image of the bomb cyclone off the coast of the Western US.
As the bomb cyclone approaches, residents in the affected areas should regularly check local weather forecasts and alerts for updates on the storm's progression. By staying informed, taking precautionary measures, and adhering to safety advice, residents can mitigate risks and navigate through the challenges posed by this formidable storm system.