The city on Sunday recorded a low of 20 degrees F (-6.7 C), which broke a record from 1948 by two degrees, the NWS was reported as saying.
Snowfall in the city has meanwhile beaten records set in 1979, when there was 179 inches (4.6 meters) on the ground in Seattle.
The University of California also reported that 193.7 inches (4.9 meters) of snow had fallen so far this December and that more was expected.
Emergency situations have meanwhile been declared in Oregon and Seattle, where shelters have been opened for those in need.
Temperatures in western Washington and Oregon are not forecast to rise above freezing for days, the NWS has warned.
More than 5,000 customers were reportedly without power on Monday in western Washington and Oregon, as a result of the record breaking cold.
— Seattle Sounders FC (@SoundersFC) December 26, 2021
Images and video shared online hasshowed Seattle’s famous International Fountain frozen, with locals sledding down it – and in view of the Space Needle.
Streets have been made almost impossible to drive down.
People have been advised to wrap-up warmly to avoid hypothermia, with temperatures along the West Coast usually more mild at this time of year.
Additional reporting by The Associated Press.