A long Air Stagnation Advisory is in effect through Thursday morning. 

The advisory is in place for most of eastern Washington. 

National Weather Service Meteorologist Rocco Pelatti says it's being caused by a temperature inversion. 

"The temperature is increasing with height," said Pelatti. "It kind of acts as a lid, or cap, so just anything that's emission or things, they're just not dispersing. They're all kind of collecting near the surface. And that's why things are stagnating." 

Pelatti says the temperature inversion is squashing all pollutants close to ground level, which will continue for the next several days. 

He said the extended length of the advisory is connected to the El Nino winter pattern currently hitting the Northwest. 

'El Nino usually is a split flow pattern, and you tend to have some type of blocking high," Pelatti said. "So, this is kind of some type of blocking high pattern that's not really allowing storm systems to move through. When you have weather systems moving through, you tend to increase ventilation and mixing." 

Pelatti says a weather system is moving through the region toward the end of the week, which is why the Air Stagnation Advisory will end Thursday. 

He said a strong atmospheric river will impact the weekend, bringing significant snow to the mountain passes by Sunday. 

The system will add ventilation and clean out things out. Pelatti said stagnation actually started over the Thanksgiving weekend. 

Over the longer haul this winter, the El Nino pattern will bring in systems from the southwest, which will be accompanied by warmer air than last year's La Nina winter with colder air coming from the northwest and the North Pole.  

There were also a lot of storm systems moving through Washinton last year. 

El Niño events tend to bring warmer and wetter winters to Washington. So far, most of the rain has been on the state’s west side. 

The air stagnation will bring the heaviest pollution to places like Wenatchee, which sits next to the foothills of the Cascades. 

Pelatti said the temperature inversion bringing the air stagnation is being hit with a low-level easterly flow of wind that’s pushing everything into a wall that is the mountain range. 

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