Let's start with the bad news.

As wildfires continue to rage across Oregon, Portland's air quality remains the worst of any major city in the world—though it's dipped in and out of the top spot over the past few days, it's currently sitting at No. 1 on the Air Quality Index. And it's looking like it won't be any better on Sunday.

The good news? Rain is on the way.

According to the National Weather Service, the Willamette Valley is likely to see precipitation as early as Monday, with the chances increasing throughout the day and into Tuesday.

But when the skies do open up, don't expect an immediate improvement in air quality. How much the rain will help in that regard depends on how much there is and how the fires react to it, says Will Ahue, a meteorologist with the National Weather Service's Portland office.

"You know when you put out a campfire and dump a whole bunch of water on it and it produces more smoke than you were thinking? That could be the scenario that happens," he says.

Wind conditions are another issue: The strong east winds that initially exacerbated the fires blew smoke hundreds of miles offshore, and though they've since subsided, westerly winds are now bringing that smoke back inland. They'll eventually push the smoke out toward the Cascades but could make things worse before they get better.

Given those variables, Ahue says Portland's air quality may not begin to show significant improvement until later in the week.