November is wettest month, officials warn about flooding

Out of all the months in the year, November is the wettest of them all, making it the No. 1 month for river flooding.

The primary reason — beyond being the wettest month of the year —  is there’s usually little mountain snowpack to soak up those warmer rainy days when snow levels rise.

Rainy season is in a month, are you ready with flood insurance?

The flood season runs from the latter part of October into March. Most heavy rain events that produce flooding are the result of an atmospheric river – aka – the Pineapple Express.

Atmospheric rivers bring the bulk of middle-latitude rainfall in both the Northern and Southern Hemispheres. Many atmospheric rivers do not produce river flooding, but some bring an onslaught of heavy rain over a period of a few days along with elevated snow mountain levels, resulting in swelling rivers.

Last year, an atmospheric river event in November produced heavy rainfall and significant flooding in Whatcom and Skagit Counties, adding to a long list of such major flood events over the years.

Some of the more prolific atmospheric rivers in recent years sparked the floods of November 1986, November 1990, November 1995, November 2006, January 2009, and November 2015. Total rain amounts in the Cascades reached as high as 30 to 40 inches of rain.

Snow levels also rose to between 6,000 and 8,000 feet for these warm wet events. To track atmospheric river episodes, follow this link where the plumes of subtropical moisture can be monitored. That includes the next atmospheric river event to spread into the North Sound late this week.

More heavy rain amounts of up to two inches of rain in the lowlands and up to 10 inches in parts of the Cascades will likely swell rivers. These rains will also aggravate the threat of landslides and flash flooding in the Bolt Creek wildfire area along the U.S. 2 corridor.

This warm wet period should not reach the all-time record floods of past years. Here is a list of those all-time records on selected Western Washington rivers.

Skykomish near Gold Bar – 24.51 feet on Nov. 6, 2006, flood stage is 15 feet.

Snohomish near Monroe – 25.30 feet on Nov. 25, 1990, flood stage is 15 feet.

Stillaguamish near Arlington – 21.16 feet on Dec. 12, 2010, flood stage is 14 feet.

Skagit near Mt Vernon – 37.37 feet on Nov. 25, 1990, flood stage is 28 feet.

Snoqualmie near Carnation – 62.1 feet on Jan. 8, 2007, flood stage is 54 feet.

Puyallup near Puyallup – 34.1 feet on Dec. 18, 1917, flood stage is 26.2 feet.

Chehalis near Grand Mound – 147.2 feet on Dec. 4, 2007, flood stage is 141.0 feet.

Skokomish near Potlatch – 18.1 feet on Dec. 3, 2007, flood stage is 16.5 feet.

King County Executive Dow Constantine released a warning to Puget Sound residents to be aware of the dangers of flooding the La Niña weather event can bring on, encouraging those in high-risk areas to be prepared.

“Climate change is already increasing our odds of seeing more frequent and more intense flooding, and a third La Niña weather pattern only adds more certainty to predictions of an active flood season,” Executive Constantine said. “I urge everyone who lives, works, or travels through flood-prone areas to take steps now to be prepared.”

You can monitor river forecasts via the National Weather Service website. Once soils reach the saturation point, these kinds of heavy rain events can also trigger landslides. Given the incoming rain late this week, it may be worthwhile to keep an eye on the rivers if you live near or commute through any of these river valleys.

Follow Ted Buehner, the KIRO FM news meteorologist on Twitter