But first, let's get oriented. The location is found in the western Cascade foothills, an area of often very heavy precipitation produced by air being forced to move upward by the terrain (area map below with oval in the slide area)
And here is a closer-inn view.
A major issue was the extraordinary precipitation of the past month. Here is a map for the entire western U.S., showing the departure from average for this period: the north Cascades stick out like a sore thumb, with anomalies of over 12 inches more than normal!
Percentage of average for the past month? The north Cascades were above 300% of normal!
Or looking at the departure from normal in inches--big values in the North Cascades.
There were some very wet conditions earlier in the week. Since there are not that many observations in the immediate area of the slide, lets look at the "storm-total" precipitation from the Camano Island radar encompassing the period from March 18th at 10:33 AM through March 21st at 3:36 AM. The area in question got hit hard with at least 3-5 inches.
A nearby weather observing location in the hills behind the slide, Finney Creek, has had 24.2 inches of rain this month. This satellite image shows the location of the slide (red circle and Finney Creek observing site (red balloon symbols).
So we had a very wet month, topped off very heavy precipitation earlier in the week.