NORTH SOUND, NOV. 19: November is historically the wettest and most flood-prone month of the year. One key reason is the mountains do not yet have a snowpack to help soak up any heavy rain periods. That rainfall simply runs off into the rivers.
About halfway through this month, the recent soggy weather pattern sure looks like November will retain its wettest of the year image. A number of Pacific weather systems have marched through the North Sound so far this month, and the latest weather outlook until December looks to have more wet weather in store.
Other than historically not having a mountain snowpack in place in November as a key reason for rising rivers, another big player involved with flooding is The Pineapple Express, a term coined by a National Weather Service Seattle forecaster back in the 1980s. He saw a moisture stream on Eastern Pacific weather satellite imagery that started near the Hawaiian Islands and extended into the Pacific Northwest.
The proper pineapple express term is an atmospheric river, a surge of subtropical moisture transported north into the mid-latitudes. An atmospheric river usually has an abundant amount of warm moist air and can produce a lot of precipitation. This feature is actually quite common around the globe and helps produce the bulk of mid-latitude annual rainfall.
Some of the wettest atmospheric rivers over the years have produced as much as 40 inches of rain in just a few days in the mountains. These heavy rainfall amounts often result in significant and widespread flooding of North Sound rivers and streams. If soils are already saturated, this heavy rainfall can also lead to landslides.
The warm air flow also means freezing levels rise – well above 6,000 feet. Later in the season, the rain falls into a mountain snowpack and is usually absorbed like a sponge. Studies have revealed that only up to 15 percent of mountain snow melts into river runoff, spoiling the myth that snowmelt contributes a great deal to flooding rivers. The biggest contributor to flooding rivers by far is heavy amounts of rainfall.
So, when you learn that an atmospheric river – aka – pineapple express – is developing, a lot of rain and the likelihood of river flooding and possible landslides lie ahead, your opportunity to get better prepared in advance.
