The first powerful cold front moves through the Northwest Saturday afternoon through early Sunday morning, with an initial round of heavy rainfall and high winds in Washington, Oregon, northern Idaho and northwest Montana.
Another powerful impulse will arrive later Sunday into early Monday with additional high winds and heavy rain. Interestingly enough, this second round appears to have received an injection of remnant moisture and energy from the former western Pacific Typhoon Pabuk.
That said, periods of showers, wind and lowering snow levels will linger Monday into at least Tuesday, as pulses of energy rotate around the bullish upper-level jet stream dip into the Northwest.
Let's break down the forecast impacts of heavy rain and high winds, then explore how unusually strong this storm is for late September.
Heavy Rain/Flooding Threat
Latest Radar
Storm Total Rainfall Forecast
By early Monday, rainfall totals from 5-8 inches are possible along the southwest-facing slopes of the Olympics and Cascades of western Washington and western Oregon. Rainfall totals over five inches are also expected along the coastal ranges of Oregon.
General three-inch-plus totals are expected along the Interstate 5 corridor of Washington and Oregon, including Seattle, Portland, and Eugene.
Parts of northeast Washington, the Idaho Panhandle and northwest Montana may pick up 1-3 inches of rain, as well, with locally higher amounts.
This rain will likely send some rivers into flood stage this weekend, including the flood-prone Skokomish River north of Olympia. Fortunately, as the National Weather Service in Portland, Ore. notes, pre-storm river levels are running low, since we're coming out of the summer dry season.
However, NWS-Portland also notes with leaves still on trees, the potential exists for clogged storm drains and urban flooding. Mudslides and rockslides are also possible in a few areas.
Snow levels will also fall through the period, possibly as low as 4,500 feet in the Cascades and 5,500 feet in the far northern Rockies (northern Idaho/northwest Montana) early next week.
High Wind Threat
High Wind Alerts
The combination of deep low pressure diving southeastward from the Gulf of Alaska and a powerful jet stream punching in will lead to a large area of high winds not solely at the Pacific Northwest coast, but also well inland as far east as the lee slopes of the northern Rockies.
The strongest winds will accompany each frontal system's arrival. Here are some forecast wind gusts by region:
- Washington/Oregon coast: Gusts over 60 mph, possibly reaching 80 mph Sunday along Ore. coastal headlands through late Sunday
- I-5 corridor: Occasional gusts over 40 mph through late Sunday
- Eastern Wash./eastern Ore./northern Idaho: Gusts from 50-60 mph particularly late Sunday/early Monday
- Western Montana: Gusts to 75 mph along Rocky Mountain Front, 50+ mph gusts elsewhere through Monday
Overall, the combination of heavy rain and strong winds will lead to some downed tree limbs, weaker trees, and powerlines, leading to some power outages. Dangerous crosswinds will also be a concern particularly on north-south roads.
In addition, expect seas to build to 20-25 feet near the Washington and Oregon coast by Sunday.
How Unusual For September?
Seattle's Wet Ramp Up
We're in a transition zone between the summer dry season and the wet season of late fall and winter.
The jet stream eventually begins to aim at the Northwest more often in the fall. When this occurs, a succession of storms can march into the West Coast, delivering rain, wind, and mountain snow.
Excessive rain can occur when one storm taps a so-called atmospheric river of moisture originating from the subtropics, or, when a series of storms delivers moderate rainfall totals over a period of a week or more.
This is quite an exceptional storm for late September, as the NWS office in Portland, Ore. noted by leading off its Saturday morning forecast with "A rare major September storm system."
By the time this storm parade ends, Seattle may set its record wettest September. The current record is 5.95 inches in 1978.
Prior to the rain ramping up Saturday, Seattle had already measured over three inches this month. As you can see in the graph above, an average September manages just under an inch and a half. Seattle may pick up that much rain from just one round of the weekend storm parade!
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