Moisture continues to be transported into the Pacific Northwest on Saturday along a narrow band of horizontal winds. When this transportation of moisture over the Eastern Pacific into land from the warmer subtropics is aided by a strong low pressure system, it can often cause flooding in Western United States. The highly anomalous events come from the warmer/moister area near the subtropics because this allows for higher moisture content than if they originated from further north. This highly above-normal rainfall as a result of strong winds and high moisture content is called an atmospheric river because of the narrow region in the common setup between high pressure to the south and low pressure to the north. This allows for fast moisture flow into land, in this case, along the Western United States. In some cases, the high and low pressure systems can stall out and allow for a long-lasting continuous flow of moisture into land along the low level jet stream. In other extreme cases, a developing low pressure system with strong low level winds can also augment the event by allowing for large amounts of rainfall into an area in a short period of time.
For this week’s event in particular, the original rainfall event total forecast for the period that the moisture plume would flow into land was for as much as 15 inches in portions of the Pacific Northwest especially along the western slopes of mountains where orographic lift would occur. Orographic lift is caused by air being forced upward over mountains because it, essentially, has nowhere else to go and that causes lift which causes more condensation and enforces heavier precipitation especially along a strong low level jet.
As a result of the flow of moisture into the mountains, many rivers flowing out of the mountains into lower areas flooded. Rivers like the Snoqualmie, Snoquomish and Skokomish, all near the western coast of Washington, rose into moderate flood stage during the event. But they have all crested on Saturday. According to their public information statement, the National Weather Service in Seattle reported Cape Elizabeth, Washington had the highest 24 rainfall total of 8.09 inches. It is in the ideal spot to have the most rain in an event like this, right along the western coast of Washington.
But the rainfall is not done yet either as another strong low pressure system slams into western Canada early next week allowing its winds to carry more moisture into a similar area in the Pacific Northwest. Early model projections have 5 to possibly 8 more inches into the region. Even though this does look to be a slightly weaker event, some of the rivers are still swelled and will have little time to recover before the next event. This does not bode well for the next event and more flood warnings could be issued by the National Weather Service in similar regions as this past event.