From NOAA:

...VERY COLD AIR RETURNS TO THE PACIFIC NORTHWEST THIS WEEK...

CONFIDENCE IS HIGH IN A PERIOD OF VERY COLD CONDITIONS FOR THE
MIDDLE TO LATTER HALF OF THIS WEEK. CONSIDERABLE UNCERTAINTY
STILL EXISTS IN HOW COLD IT WILL GET. WHILE COLDER AIR WILL
ARRIVE ON TUESDAY...THE COLDEST PERIOD WILL BE WEDNESDAY THROUGH
FRIDAY...WITH THURSDAY LOOKING TO BE THE COLDEST DAY.

EXPECT TEMPERATURES WEDNESDAY THROUGH FRIDAY TO BE AT LEAST 15 TO
20 DEGREES BELOW NORMAL IN MOST AREAS. HIGHS WILL BE IN THE UPPER
20S TO MID 30S...AND LOWS WILL BE IN THE TEENS IN THE VALLEY.
EVEN COLDER CONDITIONS WILL BE FOUND IN THE CASCADES.

IN ADDITION TO THE COLD...EXPECT GUSTY EAST WINDS ACROSS THE
AREA...WITH BREEZY NORTH WINDS IN THE WILLAMETTE VALLEY.
THEREFORE...WIND CHILL READINGS WILL DIP INTO THE SINGLE DIGITS IN
THE VALLEY...BELOW ZERO IN THE GORGE...AND APPROACHING MINUS 20 IN
THE CASCADES.

CONFIDENCE IS MODERATE TO HIGH THAT A PACIFIC WEATHER SYSTEM WILL
ARRIVE ON SATURDAY THAT LOOKS TO BRING A PERIOD OF WINTRY WEATHER
TO PARTS OF THE REGION. SNOW...SLEET...AND FREEZING RAIN ARE
POSSIBLE...BUT CONFIDENCE IS LOW ON THE SPECIFICS AT THIS TIME.

RESIDENTS ON THE WEST SIDE OF THE CASCADES SHOULD BE PREPARED FOR
AN EXTENDED PERIOD OF VERY COLD CONDITIONS. THOSE PLANNING
OUTDOOR ACTIVITIES SHOULD STAY TUNED TO THE LATEST FORECASTS.