(CNN) The Pacific Northwest is known for rain, but not this much.
So far this year, Seattle has seen nearly 6.5" of rain. With an additional 2-4 inches forecast, the city could receive roughly 25% of its annual rainfall by January 15 from the atmospheric river that's drenching the region.
"This soggy start could propel Seattle to its single wettest start of any year on record. The previous wettest Jan 1-15 period occurred in January 1956, when nearly 7 inches fell in the first half the month," says CNN meteorologist Pedram Javaheri.
Atmospheric rivers are long, narrow regions in the atmosphere -- like rivers in the sky -- that transport water vapor, according to the National Oceanic and Atmospheric Administration.
This atmospheric river event is being classified as a Category 5 -- the highest level -- from the Center for Western Weather and Water Extremes. The flooding potential is huge, with roughly 15 million people under some sort of watch or advisory. Parts of western Washington could see 300% of normal rainfall, according to the Weather Prediction Center.
"Soil saturation levels are ranging from 75-95% along the western third of the region right now. It won't take much rainfall to lead to surface flooding," says Javaheri.
Check the forecast across the country
There's also plenty of warmth in the mid-levels of the atmosphere during this particular atmospheric river event. This will raise snow levels on elevations above 6,000 feet across much of the Cascades, with rain falling below that level.
"This will further exacerbate flooding concerns as heavy rain falls atop abundant snow. The threat for rapid melting, increased runoff and downstream river flooding is something everyone in western Oregon and Washington should be on alert for," says Javaheri.
Higher elevations in Washington could see 2 inches to 4 inches of rain during the next 24 hours, increasing the flooding and landslide risk.
Rainfall at lower elevations should be between 1 and 2 inches during that time.
In Portland, Oregon, the National Weather Service Office has forecast up to 7 inches of rain in the higher terrain and up to 2 inches for the lowlands through Wednesday morning. Along with that, there's a high-wind warning in effect.
Wind warnings extend along the whole Oregon coastline, where gusts could reach up to 75 mph. This raises concerns for downed trees, power outages and possible hazards along Interstate 5.
The impacts from the record-setting wildfires are also raising the threats of flooding for Oregon. According to the Oregon Department of Forestry, more than 1 million acres burned. These burn scars that remain make the flood threat even greater, due to charred ground with no vegetation to soak up the rainwater. This enhances the possibility of flash flooding and landslides due to the loose terrain.
The weather phenomenon causing all of this rain is called an atmospheric river. They basically are rivers of moisture high in the atmosphere. They carry abundant moisture from tropical regions and release it in other areas in the form of rain or snow.
According to NOAA: "These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River."
Not all atmospheric rivers are bad. Many times, they carry very beneficial rain to areas that need it. Many areas along the West Coast will get 30%-50% of their annual rainfall in just a few such events.
However, with a stronger event, it can lead to dangerous flooding and life-threatening landslides. The atmospheric river that stretches from Hawaii to the West Coast has been coined the "Pineapple Express" and is one of the most well-known atmospheric rivers.
A December 2019 study published by researchers at Scripps Institution of Oceanography suggests that during the last 40 years, atmospheric rivers have posed a $1.1 billion flood hazard for each of those years along the West Coast.
This particular atmospheric river has a 2,700-mile span with a bull's-eye on the Northwest. That is equivalent to the distance from Seattle to Miami as the crow flies, and the moisture it's carrying could possibly put January 2021 in the record books.
This atmospheric river event is so significant that hurricane hunters will be flying through it, dropping buoys.
"In order to really understand how significant an atmospheric river event will be, you need to get close to it. Hurricane hunters fly inside of them and collect valuable data that forecasters then use to determine how a particular atmospheric river will affect the pacific coast region," says CNN meteorologist Allison Chinchar.
This is their second flight of the season and something they do regularly.
"The Hurricane Hunters began flying in atmospheric rivers for the first time in February 2016 and have been sending out 6-12 flights each January through March ever since. In of itself, it's not that unusual that they are flying this event, though it's a popular misconception that once the hurricane season winds die, so does their role," says Javaheri.