An unusual group of storm systems battering the Pacific Northwest has halted dozens of flights and knocked out power for thousands, hitting Seattle with as much snowfall in one day as it usually receives in a year, according to the National Weather Service.

Gov. Jay Inslee of Washington declared a state of emergency on Friday.

“Everyone in our state needs to focus on preparing for the snow and staying safe,” he said in a statement. “Weather forecasters predict this may be a storm unlike one we’ve seen in many years. I encourage everyone to stay off the roads if possible and plan ahead if you must travel.”

In one day, 7.2 inches of snow were recorded at Seattle-Tacoma International Airport. “Our average snowfall is around seven inches for the season,” said Jeff Michalski, a meteorologist with the weather service.

“It is a lot of snow at once to deal with across the region,” Mr. Michalski said. “There is another system that is going to come in tomorrow.”