Pineapple Express brings flooding on Sunday


Fri December 10, 2010

Pineapple Express brings flooding on Sunday

UW/Dpt. of Atmospheric Sciences / (
Forecast from the U.W. showing an "atmospheric river" bringing wet clouds from the tropical Pacific (in the lower left) to the Northwest coast

A big rainstorm is headed to western Washington this weekend.  Forecasters say Saturday should start out pleasant, with the rainstorm hitting south Puget Sound in the late morning, and the Seattle area by around noon.

It’s not supposed to be as bad as devastating storms a few years ago, but flooding is likely on some rivers. 

There are a few rivers in western Washington that flood regularly – such as the Skokomish and the Tolt.  This year, you can add one more to that list, a section of the Puyallup River in eastern Pierce County.

"We have a damaged levy system up near Orting.  Flood-stage is a lot lower than it used to be," and it could swamp some residential areas there, says Ted Buehner of the National Weather Service.  The lower Green River valley, where residents are nervously watching the Howard Hanson Dam,  should escape any floods.   The flood potential on other rivers is lower, and it depends on exactly where the rain ends up falling. 

Here’s what’s happening.  A weather system nicknamed the "Pineapple Express" is bringing a narrow channel of super-wet clouds, almost like a river in the sky, from Hawaii to the northwest.  Atmospheric scientist Cliff Mass, of the University of Washington, says it’s just warm enough to hit the Olympic and Cascade mountains with a ton of rain -- instead of snow.  Still, it won’t be enough to  set any records. 

"This is not going to be like November, 2006, where the damage was extraordinary," says Mass.

Instead, it should be more like a typical wet winter storm.  It looks like temperatures will drop lower on Sunday night.  In the mountains, that means rain will turn back into snow by Monday -- above 2,000 feet.