LONG BEACH — The first significant fall storm system of the season is expected to spread moderate to heavy rain across much of southwest Washington and northwest Oregon as we head into the upcoming Halloween weekend, according to the National Weather Service in Portland.
In addition to being rainy, both Friday and Saturday will be breezy. Winds blowing out of the south to southwest will increase Friday morning, then peak Friday afternoon and peak a second time Saturday afternoon. Wind gusts of 40 to 50 mph along the coast and 25 to 35 mph inland are expected. Normally these winds would not be of concern. However, in this case they could knock some trees down due to very wet soils. Any fallen trees could cause isolated power outages.
An atmospheric river of subtropical moisture is taking aim at the Pacific Northwest, and will combine with a strong Pacific jet and a slow-moving frontal zone to produce periods of heavy rain across the region. Steady rain is expected to develop across southwest Washington Friday morning, likely spreading into northwest Oregon as the frontal zone sags south during the day. Rain will become heavy at times north of Salem, especially along the coast and in the higher terrain.
The frontal zone is expected to lift slightly northward Friday night, possibly giving a brief reprieve from the heaviest rainfall before the front moves back southward across the area Saturday for another dose of heavy rain. Steady, occasionally heavy rain is expected to taper to showers from north to south as the cold front moves into southern Oregon and northern California.
At the moment it appears the heaviest rain will be focused over the Willapa Hills, south Washington Cascades, and far north Oregon Coast Range, where 5 to 9 inches of rain are possible by Sunday morning. Coastal areas, the central Oregon Coast Range, and Oregon Cascades north of Willamette Pass are expected to receive 2 to 5 inches by Sunday morning. Inland valleys will see somewhat lower totals, generally 1 to 2 inches though locally higher near the Cascade foothills.
While some rain totals from this event may be impressive, this type of event is fairly common toward the end of October and the early part of November. Given that stream flows are near annual lows, widespread significant flooding does not appear likely With this storm. However, more sensitive rivers and streams such as the Grays River in Wahkiakum County may approach flood stage.
Urban areas such as the Portland, Salem and Eugene metro areas Will likely receive enough rain to create minor urban flooding Issues, especially where storm drains are clogged due to autumn leaves.
Details on timing and location of the heavier rainfall may still Change, so the weather service recommends keeping up with the latest information at Weather.Gov/portland or your favorite media outlet.