Let me show some upper level (500 hPa, around 18,000 ft) forecast maps that document the development of this beast. Today was a fine day, with a modest ridge over us (see map for 5 PM today-Tuesday). This is the last day of warmth for a while.
On Saturday at 5 PM, the infernal beast is now over the entire West Coast.
Precipitation? You bet. For the next 72 hours, wet offshore and modest rain over our local terrain.
The next 72 h (ending 5 AM Monday) is far wetter, with several inches over the mountains, lots of moisture over eastern Washington, and substantial rains over northern CA.