A powerful storm system off the coast of the Northwest U.S. — described as a "bomb cyclone" — is expected to pummel the region with high winds, excessive rain and snow for days.
The National Weather Service has issued flood watches and various weather alerts are in effect with rain totals of up to 15 inches possible.
The potentially record-breaking storm, churning about 300 miles off the coast of Washington, strengthened so quickly Tuesday that it developed into a rare bomb cyclone.
The term "bomb cyclone," or bombogenesis, applies to a quickly intensifying storm that undergoes a rapid pressure drop over a 24-hour period, leading to hurricane-force wind gusts, according to the National Oceanic and Atmospheric Administration. Hazards of a bomb cyclone include: High, intense winds that can cause power outages; blizzard conditions with heavy, blowing snow and whiteout conditions; and rainfall on snow that can cause river flooding.
So far, winds from the storm have produced numerous power outages and reports of tree damage, and are expected to create blizzard conditions throughout the Cascades.
While the wind is are expected to gradually subside by midday Wednesday as the low pressure system swings away from the region, an atmospheric river in its wake is expected to bring nearly a month's worth of rain and mountain snow to the region through Thursday.
"A continuous plume of ample atmospheric moisture content entering northern California this morning is forecast to linger through the end of the week and lead to extreme rainfall totals," said the NWS Weather Prediction Center. "Over 10 inches of rainfall across the northern California coast and inland mountain ranges are likely to increase the threat of life-threatening flash flooding, rock slides, and debris flows."
Meantime, a separate area of low pressure is forecast to develop and rapidly strengthen off the Northwest coast on Friday. Forecasters say this storm will amplify the atmospheric river streaming into northern California, exacerbating the flooding threat.
Residents and visitors traveling between northern California and Washington are advised to check road conditions before venturing out, listen to advice from local officials, review emergency plans, and have multiple ways of receiving warnings.
Southern California may see rain starting Saturday, becoming heavier on Sunday with average totals of half an inch to one inch. The weekend forecast is complex due to multiple low-pressure systems off the coast, with the potential for more rain beyond the weekend.
Elsewhere, a slow-moving and gradually weakening low pressure system just north of the Minnesota-North Dakota border is causing heavy snow and blizzard-like conditions throughout the northern Plains. The greatest snowfall amounts, 4-6 inches, are forecast across North Dakota, eastern South Dakota, and northwest Minnesota.
In the Northeast, an extended period of dry and tranquil weather is coming to an end as the system exiting the northern Plains will slide eastward and produce a chance for heavy precipitation in the form of both rain and snow. After the cold front deepens on Thursday and "probabilities for at least 4 inches of snow by Friday night are high (70-90%) across northeast Pennsylvania and the southern Catskill mountains of New York," according to the NWS.
"Impactful snowfall is also likely to be experienced throughout the Allegheny mountains of West Virginia, Maryland, and Pennsylvania through the end of the week due to a longer duration favorable upslope snow setup," the Weather Prediction Center said. "Total snowfall amounts in these higher elevations could add up to a foot."