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Trio of storms to bring autumn's first significant rainfall, possible flooding

FILE photo of rainy day in Seattle (Photo: Tim Durkan){p}{/p}
FILE photo of rainy day in Seattle (Photo: Tim Durkan)

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SEATTLE -- The weeks and months of dry weather are about to become a distant memory as the pendulum finally shifts to our traditional late October stormy pattern this week.

A series of three storms due in this week promises to bring not only the most rain we've had since a storm in early June, but also our first generally windy day of the autumn.

The wind doesn't look like too much of a bother, but the rainfall could cause some flooding in some of the mountain-fed rivers and urban flooding where those pesky leaves have clogged storm drains.

The first storm is the appetizer and it begins to reach the coast Monday night, spreading its rain into the Puget Sound region Tuesday morning. This is just a ho-hum rainy and breezy day with gusts to about 25-35 mph in the lowlands and a few inches of rain in the Olympics.

A stronger storm comes in on Wednesday -- this one is wetter and windier as it's part of an "atmospheric river" -- a fancier term for Pineapple Express. Rain will redevelop Wednesday morning and it'll keep raining rather heavily into Thursday morning. In the lowlands and Seattle area, forecast charts expect 1-3" of rain by Thursday morning (although the Olympic Rain Shadow will keep rainfall totals significantly lighter in the North Sound lowlands.) The Euro model is predicting about 2.2 inches of rain in the Seattle area over the two-day period meaning some likely flooded streets in urban areas due to clogged storm drains.

But in the mountains, more significant rainfall is expected. River weather forecasters with the National Weather Service are expecting about 6-9 inches of rain in the Olympics with 3-6 inches of rain likely in the North Cascades (lesser amounts forecast for the Central and Southern Cascades at this point). With relatively high snow levels, this will fall as rain across much of the elevations and will run off into the rivers. However, since it's the first significant rain of the season, the rivers are low enough that it should be able to handle much of this storm - maybe some flooding on the Skokomish, which threatens to flood if someone leaves a hose on for 10 minutes.

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As for wind, it'll be stronger that Tuesday's storm, but still nothing too major in the grand scheme of Northwest autumn weather -- gusts 30-40 mph late Wednesday into Thursday, maybe some gusts to 45 mph. As always, the first storm of the season has greater potential for damage than subsequent storms because the trees still have their leaves, which work like sails in catching the wind. Plus after our record dry summer, there's a lot of dead branches that will get blown off and potentially affect power lines. Call it nature's pruning. So I'd expect some scattered power outages that may seem like more than we'd normally get for such an event later in the season.

The rain will taper to showers by Thursday afternoon or evening, and by this time we should still be in good shape, but on the edge, with high rivers but likely not flooding, and some minor, sporadic wind damage that should be quickly repaired.

Friday will feature scattered showers amid a few sunbreaks, but we're not done yet. Forecast models are predicting a third storm over the weekend -- another atmospheric river -- that could rival the Wednesday/Thursday storm as far as rainfall and wind. Only this time, the rivers will be running much higher and now we have potential for some river flooding, especially along the Olympics and North-Cascade-fed rivers. So those of you who live in the river flood plains need to start preparing now.

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Some forecast charts suggest about 2-2.5 inches of new rain in the Seattle area over the weekend to where we could have 5-6 inches of total rain over the next 7 days.

(Although the American GFS model is going a bit drier in the lowlands, buying a bit more extensive rain shadow.)

So make sure those storm drains are clear and prepare for some extended morning and evening commutes this week!

What Seattle is doing to clear 66 billion leaves:

While you can do your part to keep storm drains clear, Seattle Public Utilities will be out in force trying to clear as many leaves as possible.

The city's new Vactor Truck -- essentially a massive wet vac on wheels -- can suck up 14 tons of fallen leaves and enough water to fill a family swimming pool through its powerful vacuum.

Just ask Randall Dickey, a 20-year veteran with Seattle Public Utilities.

"We have enough power to suck up bricks — which we do at times," he said.

By the end of every summer, Randy and his teammates have cleaned most of the city’s 85,000 stormwater inlets in preparation for the long, wet Seattle winter.

Then come the first winds and rains of the fall season, which Dickey says can rip upwards of 66 billion leaves from the branches of the city’s estimated 3.3 million hardwood trees.

Many of those leaves eventually wind up in storm drains where they can cause backups and street flooding. When this happens, SPU officials say there just aren’t enough work crews and half-million-dollar Vactor trucks in the city to go around, so that's where you can come in:

"Adopt a local street drain and rake debris away from it to keep if from clogging and causing street flooding," SPU officials suggest. "Only rake the leaves/litter on top of drains; don’t attempt to clean inside. If you find a lot of debris inside the drain, call your local public utility. In Seattle, you can report a plugged drain at (206) 386-1800."

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Officials also suggest wearing bright colors and having a spotter watch for traffic -- and try to rake the leaves from the curbside. If you feel it's too unsafe, call your local public utility. And they advise never remove a storm drain grate or manhole cover.

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