The western parts of the US are bracing for a series of storms that will bring rain, snow, and more rain to millions of people in the upcoming days, initiating a slushy start to December.
The National Weather Service (NWS) has issued several alerts for Seattle and surrounding areas, including a flood watch, a gale warning, and a small craft advisory. The heavy precipitation is predicted to trigger flooding and landslides and disrupt travel.
A long-duration atmospheric river event
Spencer Platt/Getty Images
The storms are caused by an atmospheric river, which is a long and narrow band of moisture that transports water vapor from the tropics to higher latitudes.
Atmospheric rivers can produce heavy rainfall and snowfall over a large area, especially when they encounter mountain ranges.
According to the NWS, the atmospheric river event that is affecting the Pacific Northwest is expected to last for several days, with total rainfall forecast to range from 3 to 7 inches over the lowlands to upwards of 10 to 12 inches over the Cascades and 10 to 15 inches over the Olympics.
The excessive rainfall from the atmospheric river event will cause excessive runoff that may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations.
Flooding may also occur in poor drainage and urban areas.
Extensive street flooding and flooding of creeks and rivers are possible. The NWS advises people to monitor later forecasts and be alert for possible flood warnings.
Those living in areas prone to flooding should be prepared to take action should flooding develop.
The storms will also bring strong winds to the region, which will cause hazardous seas and reduce visibility for mariners.
The NWS has issued a gale warning for Puget Sound and Hood Canal until 1 PM PST Saturday, with southwest winds 25 to 35 knots.
A small craft advisory is also in effect from 1 PM Saturday to 1 AM PST Sunday, with south winds 15 to 25 knots.
The NWS warns that operating a vessel in gale conditions requires experience and properly equipped vessels, and recommends that mariners without the proper experience seek safe harbor prior to the onset of gale conditions.
As storms continue to pound the region this weekend and early next week, temperatures are forecast to rise, which will raise snow levels.
The combination of heavy rain and melting snow could cause river flooding.
The NWS predicts that snow levels will rise to 6000 feet by Saturday afternoon, then lower to 4000 feet by Sunday morning, and then rise again to 6000 feet by Monday afternoon.
This means that the higher elevations will see a mix of rain and snow, while the lower elevations will see mostly rain.
The NWS advises travelers to be prepared for winter driving conditions and possible road closures in the mountains.
Impacts and preparedness
The series of winter storms will have significant impacts on the region, including power outages, flooding, tree damage, and road closures.
The Seattle Office of Emergency Management has issued an alert for residents and visitors, urging them to take precautions and prepare for the storm. Some of the tips include:
Have flashlights, batteries, and a battery-powered radio ready in case of power outages.
Avoid driving through flooded roads and report any flooding or landslides to 911.
Stay away from downed power lines and report them to Seattle City Light at 206-684-3000.
Secure outdoor furniture and garbage cans that could become projectiles in high winds.
Have an emergency kit and a plan for yourself and your family in case you need to evacuate or shelter in place.
The Seattle Office of Emergency Management also encourages people to sign up for AlertSeattle, the official emergency notification system for the City of Seattle, to receive real-time information about emergencies and how to stay safe.
