Heavy rain events have repeatedly pummeled the northern Lynn Canal in recent weeks. These events are called atmospheric rivers, and meteorologists are studying them and  working on a classification system. Just in the last month, South East Alaska has seen the arrival of five or six atmospheric river events. Formerly called plumes of moisture or the Pineapple Express, these events have recently been recognized as  a global phenomena, and scientists have settled on a definition 

According to a NASA website the term atmospheric river is used to indicate narrow, elongated corridors of concentrated moisture transport associated with extratropical cyclones. Atmospheric rivers are the largest transport mechanisms of freshwater on Earth.  This moisture transport occurs under particular combinations of wind, temperature, and pressure conditions. Based on satellite observations, an atmospheric river is greater than 1,250 miles long, less than 600 miles wide, and averages 1.8 miles in depth. Typical atmospheric river conditions last around 20 hours over an area on the coastline. 

As Aaron Jacobs, Senior Service Hydrologist at NOAA in Juneau explains:

“They usually produce heavy precipitations, very warm temperatures, so when you add the snow melt, that can produce a lot of runoff, and flooding”

These events may not generally occur more frequently but they may become more intense due to a warming atmosphere.

“Some of the research shows that we may not see more  atmospheric river events, but we may see more intense events, the warmer the atmosphere, the more moisture it can handle”

However, in the last month, these events have occurred back to back, increasing their effects on the landscape.  And, as they  melt the snow already present on land, atmospheric rivers can even add more water to streams and rivers, contributing  to flooding. Meteorologists are working on a 1 to 5 scale to rate the severity of those events.

“We are in the works of trying to come up with an atmospheric river scale, so that when you hear that word, you know what kind of impact to expect, similar to hurricanes, when you have category 1, 3 or 5, and there is a certain amount of impact that takes place”

With their origins in the warm southern air, atmospheric rivers are unlikely to fall as heavy snow.

” Ahead of it, you could see snow,  before that warm air moves into the region, you could have precipitations that would fall as snow, and once the warm front moves through, you would have rain on top of that snow, and we see how much runoff you can have from melting all that snow.”

 

You can find the full interview with NOAA Senior  Service Hydrologist Aaron Jacobs below.