Why such warmth? Strong and warm southwesterly flow has moved in aloft associated with the atmospheric river moving in off the ocean.
The time-height vertical cross section (height in the Y direction, time in the X direction--in UTC/GMT) over Seattle Tacoma Airport shows the situation. The red lines are temperature. At 850 hPa (around 5000 ft) the winds were 35 knots, with a temperature of 6C. The freezing level is at roughly 7500 ft.
The radar at around 2 PM shows the precipitation band approaching the coast and a strong suggestion of rain shadow in the lee of the Olympics (NE side).
The rain will move in later in the afternoon.
Finally, let me end with a most amazing satellite image (visible). You see the long cloud band stretching from the Northwest way back into the Pacific? That is the cloud signature of the atmospheric river....one of the LONGEST atmospheric rivers I have ever seen!