Valley could see soaking rains, potential flooding


Valley flooding and heavy mountain snowfall a growing concern

PORTLAND, Ore. (KOIN) — Heavy rain and bountiful snow will take aim at the Pacific Northwest later in the week.

Forecast models have been hinting at a strong atmospheric river developing by early Thursday. This weather event could bring soaking rains and potential flooding to the valley and coast. Meanwhile, heavy snow may bring travel headaches across the Oregon Cascades.

Rain Potential

With snow levels sitting above 5,000 feet by late this week, expect only rain in the valley during this event. Forecast models have been pushing impressive and somewhat extreme amounts of rain to the coast and valley from Thursday through Sunday.

The latest runs of both long-range weather models, the GFS and ECMWF, show rain totals close to 5 inches or more from Wednesday night through Sunday morning. The ECMWF shows totals closer to 7 inches. However, confidence in forecast rain totals across the region still remains low.

The event still lies 4 to 5 days out and rain amounts can alter and decrease. Cues for heavy rain late this week are still apparent, so coastal and valley flooding remains a growing concern.

The first round of rain will arrive in Portland by Wednesday morning. Forecast models call for the heaviest rounds of rain to start Thursday night and go through Friday afternoon.

How normal are these soaking rain events?

We are currently in the heart of the rainy season. According to climate statistics from the National Weather Service, about 90 percent of the annual rainfall in Portland occurs between mid-October and mid-May. December is the second rainiest month of the year with 5.63 inches on average.

So far Portland remains in a rainfall deficit of 6.62 inches from October 1st through now. After a soaker of a September, October and November both delivered little rainfall to the region, causing the current shortage to our water year.

So far this December, Portland is also running in a rainfall deficit. The rain event forecast late this week will bring much-needed moisture to our parched region.

Mountain Snowfall Potential

The atmospheric river will also bring another round of significant snow to the Cascades. With snow levels near 3,000 feet on Wednesday night, heavy snow may fall along the local mountain passes.

Confidence is low for snow totals, as well as the timing of when snow levels may rise on Thursday. If you are traveling over the passes on Wednesday, preparing for winter driving conditions is advised.

Continue to monitor the forecast with our KOIN 6 Weather team as we head towards the end of the week.

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