Forecasting how wildfire smoke will behave has always been tricky, and even cutting-edge computer weather models are not able to produce definitive answers when the conditions are shifting so quickly.

Meteorologists are taking their cues from the smoke itself, the fires on the ground, incoming winds high in the atmosphere and thunderstorms — any one of which might throw a wrench into their forecasts.

Some days, each element is safely predictable. On other days, each one of those elements is itself chaotic, making it even harder for forecasters to know how they’ll behave together.

Which brings us to this weekend.

The stubborn weather pattern that has brought the steady stream of smoke across Canada and the United States for days will start to shift on Friday. How, exactly, it does that will make the next couple of days even more of a tossup than usual.

Challenge 1: Predicting the strength of the wildfires

The first and most challenging variable to predict is just how much smoke will pour into the sky from the wildfires themselves.

Smoke concentrations on Tuesday were much thicker than in the days just before. The weather then had favored rapidly growing wildfires that burned hot enough to create their own weather patterns and pump dense smoke into the atmosphere.

And that’s what happened. Wildfires on Wednesday and Thursday continued to send a steady stream of smoke from Minnesota and Ontario. Strong winds high in the atmosphere pushed the plume across the Great Lakes and into the Northeast, where at times it combined with smoke from wildfires farther to the north.

On Friday, a shift in the weather will push much of the smoke out of Minnesota, Wisconsin and Michigan, keeping the worst of it away from major cities.

But that relief isn’t likely to last long. Another shift on Saturday will pull the smoke back over the same areas that have endured the worst air quality this week. Forecasters warned that shifting winds will worsen air quality once again and limit visibility on lakes and roads across the Upper Midwest.

This shifting pattern also brings the chance of thunderstorms over the active wildfire areas. While rain sounds like a good thing, these thunderstorms aren’t likely to drop enough water to put out the fires, and their erratic, strong winds could actually intensify the flames.

The northward push of air will also bring a return of warm temperatures, and heat advisories have been issued for portions of the Midwest.

As the smoke returns, the chance of rain also comes back Sunday and again, it’s likely to be less helpful than you might assume. By early next week, critical fire danger could return to the exact areas where the fires are burning now. That means more fires could spark, and the ones that are already burning could get a push to grow larger.

Challenge 2: Predicting severe storms

On Friday morning, the Northeast was still enduring the latest plume, with steady streams of smoke moving into the region. Between Thursday and Friday, the worst of the air pollution pushed south through New York City, and it then settled over Washington, D.C., early in the day.

But that relief in New York may be brief. As the next weather system moves through on Saturday, bringing the chance of severe thunderstorms, the latest plume in the Northeast is likely to break off from the main atmospheric flow and drift back north of New York City.

A risk of severe storms is expected across the Northeast on Saturday, with the potential for damaging winds, hail and even an isolated tornado.

While the thunderstorms could help knock some of the smoke out of the air, the smoke itself complicates the forecast: It can block sunlight, keeping temperatures cooler at the surface and preventing storms from rising and intensifying.

Whatever smoke isn’t knocked down by rain could disperse into a lighter haze or clear out completely Saturday night.

Looming to the west, however, is another potential plume. Whether it moves east will depend entirely on how the weather pattern shifts Sunday and the intensity of the fires over the next few days.