Washington is among the most flood-prone US states, west of the Mississippi River. Here, the costs of flood damages exceed the financial impact of all other natural hazards.

One of the reasons for this is the recurring phenomenon of the 'Pineapple Express'. This is a persistent flow of moisture-laden cloud that flows from the tropics, near Hawaii, the source of many pineapples.

Current satellite imagery reveals just such a ribbon of cloud at the moment, while rain has been falling, sometimes heavily, for the last week.

Much has fallen on Olympic National Forest which is a natural sponge and grows on the peninsula to the south of Puget Sound.

But water flows downhill and rivers in the north of Washington State have now exceeded their normal capacity and overflowed.

In Mount Vernon, seat of Skagit County, the Skagit River crested on Friday at 10.1 metres, the highest water level since 2006.

The flood level of the river here is 8.5 metres. In 2010, authorities began building a $30m flood wall to keep downtown Mount Vernon dry - the city says the flood wall worked.

The Baker River, which is a southward-flowing tributary of the Skagit River, has overflowed.

The small town of Hamilton was flooded and evacuated for the first time in 11 years. The town's fire chief and mayor activated the emergency evacuation sirens at 8am. At noon, the evacuation became mandatory. Some had to leave Thanksgiving food uncooked.

The heavy rainfall in recent weeks has increased soil moisture to high levels across Western Washington. This has created a heightened threat of landslides.

Seattle, the state's largest city, benefits from the shelter of Both Olympic National Forest and Vancouver Island. As a result, it has a relatively low average rainfall for the month of November.

In fact, at 167mm, the effects of shelter are pretty obvious and this month, despite the now swelling rivers, Seattle has recorded 166mm of rain so far.

The next two days have the potential to deliver another 130mm of rain onto Olympic Forest, so the river levels may rise again. Much of this part of the state is put down to agriculture and as such, horse and cattle rescue efforts are likely to be an increasingly common need.

Extra reporting from Kiro7