November Is The Wettest Of Them All

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Out of all the months in the year, November is the wettest of them all. As a result, November is the number one month for river flooding. The primary reason beyond being the wettest month of the year, there is usually little mountain snowpack to soak up those warmer rainy days when snow levels rise.

The North Sound 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 – 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 do bring an onslot 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 6000 and 8000 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 2 inches of rain in the lowlands and up to 10 inches of rain 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 US 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 North Sound 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.

Snohomish near Snohomish – 33.50 feet on Nov 25, 1990, Flood Stage is 25 feet.

Pilchuck near Snohomish – 20.39 feet on Jan 8, 2009, Flood Stage is 18 feet.

Stillaguamish near Arlington – 21.16 feet on Dec 12, 2010, Flood Stage is 14 feet.

Skagit near Concrete – 42.21 feet on Oct 21, 2003, Flood Stage is 28 feet.

Skagit near Mt Vernon – 37.37 feet on Nov 25, 1990, Flood Stage is 28 feet.

You can monitor North Sound river forecasts via the National Weather Service website at this link.  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 North Sound river valleys.