A stream of moisture from near Hawaii will impact the Pacific Northwest Sunday through Tuesday, bringing heavy rain along its path and the possibility of river flooding early next week.
However, present indications are that the rainfall will be spread out over enough of Washington and British Columbia that drier periods will prevent major river flooding. But rivers will rise and urban flooding will occur with the heavier downpours.
Rain will impact the Pacific Northwest starting Saturday, though this area of rain -- heavy at times -- will be out ahead of the "firehose" of moisture that will stream from near Hawaii straight to the Northwest.
On Saturday, a half-inch of rain (on average) could fall in many lowland locations with a couple of inches in the mountains below the snow level of 2,500 feet.
This will not cause significant flooding problems.
It will be pretty breezy Saturday, though, especially early in the day.
On Sunday, all forecast models have the core of the very heavy precipitation from the atmospheric river -- or "Pineapple Express" -- over southern British Columbia and the very north Washington Cascades.
This will be torrential mountain rainfall of 5-10 inches but will miss the headwaters of most of our river systems of Western Washington.
If this rainfall were to shift southward, we will be dealing with significant river flooding early next week in Western Washington.
Fortunately, this is looking less probable.
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For Sunday, most lowland areas of Western Washington will receive mainly light rain. Some southern sections might receive no rain at all.
The core of heavy rain will shift south into Western Washington on Monday afternoon into early Tuesday morning.
This will bring the heaviest rain of a half-inch to 1 inch in the lowlands, 1-4 inches in the Cascades and Olympics, and higher amounts locally, especially in the North Cascades.
Snow levels Sunday and Monday will be over 6,000 feet at times.
River flooding is possible by Tuesday for some rivers in Western Washington, but major flooding looks unlikely.
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