Forecasters: Rain might impact search but not flood risk
Rain on Tuesday and Wednesday could worsen conditions for landslide search-and-rescue work near the Stillaguamish River, but the chances of a flash flood seem to have lessened, according to the National Weather Service.
Seattle Times staff reporter
Rain forecast for Snohomish County on Tuesday and Wednesday could make mudslide search-and-rescue operations more difficult but shouldn’t affect the Stillaguamish River levels, the National Weather Service said Monday.
As a precaution, a flash-flood watch remains in effect for Snohomish County through Tuesday afternoon.
Half an inch to an inch of rain is forecast for Tuesday and Wednesday, according to weather-service meteorologist Johnny Burg. The rain will add moisture to the already unstable ground.
“This will make for messy conditions for Oso and the debris dam area,” Burg said.
Saturday’s landslide blocked the North Fork of the Stillaguamish River near Oso, which led to concerns about flooding upstream of the blockage and flash flooding downstream.
A pool of water formed behind the blockage created minor flooding, the U.S. Geological Survey said. The pool’s depth was 20-30 feet by Monday afternoon.
A new river channel was created on the north end of the landslide blockage, according to weather-service hydrologist Brent Bower. But the blockage is unstable and there could be a rapid rise of water downstream.
“There’s no way to know for sure,” Bower said. “It’s going really slowly and orderly right now, but we don’t know for sure that it will always do that.”
The greatest threat of flooding is in the area immediately downstream of the debris blockage, the weather service said. The river gauge level downstream rose to about 4 feet Monday. Immediately after the slide, the level was “essentially zero,” which suggested that the river flow was completely blocked, Bower said.
The North Fork of the Stillaguamish River should be avoided, the weather service said. A flash-flood watch means conditions are favorable for flooding, but it is not imminent.
Though it shouldn’t be considered a potential source for a “catastrophic flash flood,” as it was during the weekend, the river is still a concern, Bower said.
“The blockage is huge, but pieces of it might go,” he said. “There are too many unknowns to say there is no threat at all.”
Paige Cornwell: 206-464-2530 or firstname.lastname@example.org