Throw those rain slickers on and slip into those mud boots if you’re in California, because it’s going to be a muddy, windy, rainy few days. A weather phenomenon called an “atmospheric river” is snaking its way through the golden state.

An atmospheric river is a slender, transient column of condensed water vapor from the tropics located up in the atmosphere – “like a river in the sky,” according to the National Oceanic and Atmospheric Administration (NOAA). When the “river” makes it to land, it’s usually in the form of heavy rain or snowfall.

This current event is also known as a “Pineapple Express” because the moisture originated near Hawaii and traveled to the Western US. Typically, atmospheric rivers are around 1,600 kilometers (1,000 miles) long and 400-600 kilometers (250 to 375 miles) wide. They can carry water vapor 7.5-15 times the average water flow at the mouth of the Mississippi river.

While this sounds doom and gloom, it’s not all bad news. As long as everyone is safe during the event, the rainfall can provide much-needed water and snowpack to areas like the Sierra. 

“Not all atmospheric rivers cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply,” notes NOAA. “Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks – particularly in the western United States.”

Like most weather phenomena, it’s a double-edged sword. The atmospheric river can provide water to regions where it’s needed but it could also cost lives and result in injuries, as witnessed a couple days ago when passengers on a flight from Southern California to Seattle were hit with severe turbulence due to the storm and were forced to make an emergency landing.

Just this month, a new scale to categorize atmospheric rivers was created by researchers at UC San Diego in the Bulletin of the American Meteorological Society. They have assigned these weather events on a Category 1 to Category 5 scale, from “weak” to “exceptional” respectively. The criteria is based on the amount of water vapor and its duration in a single location.

This storm is a Category 3, according to CBS San Francisco. This deems it a “strong” event with a balance of beneficial and hazardous outcomes. An example the scientists provide of a previous Category 3 event was the atmospheric river on October 14, 2016, that lasted for 36 hours and produced 5-10 inches of rain. In good news, it helped refill reservoirs after a drought. In not so good news, it caused some rivers to rise too much. 

It should also be noted that atmospheric rivers “move with the weather and are present somewhere on Earth at any given time,” according to NOAA.

For this event, the National Weather Service reported wallops of rain and wind, with a region in the San Bernardino Mountains receiving an incredible 9.4 inches over 48 hours. Thousands in Northern California lost power and flood warnings are in effect for many regions. 

In other words, stay safe out there.