Mudslides the size of the one at Oso, Wa. are not common, that area got A LOT of rain, (even by Washington standards) almost a foot, this March. It is unstable ground and that had been noted in surveys done more than a decade ago, I certainly wouldn't have chosen to live there if for no other reason than it's proximity to the river, which are prone to seasonal flooding. During the Winter and Spring rain is common and small local mudslides happen frequently even within Seattle as it's built on rather hilly ground. The railroad that travels north along Puget Sound between Seattle and Everett is used in part for Commute service, during the rainy season there are often days of no service due to slides wiping out the track.......
Since you're planning to be here in late Summer/early Fall I'd say you're safe from these events, the West (best) Coast is well worth the trip just for the scenery - You may find find that Clifford Chance needs to open a Seattle branch office after you visit
