When Will It End? Sooner for the Northeast, Later for Elsewhere.

The air quality in the Northeast and Mid-Atlantic should begin to improve on Thursday, according to a New York Times analysis of forecast models, as the dense mass of smoke that gripped those areas the day before becomes a more widespread haze that will cover most of the East Coast as far south as Florida.

The layer of wildfire smoke from Canada that covered the New York area on Wednesday was gradually moving south, to Philadelphia and Washington, but conditions were expected to improve over the course of Thursday.

There was, however, a possibility that places like New York could see deteriorating conditions if smoke that had drifted off the coast gets pushed back inland by a sea breeze. Regardless, the smoke should not be at the level seen on Wednesday, forecasters said.

Moving into Thursday evening, a smaller, dense plume of smoke could move across Ontario before settling into western Pennsylvania and moving west into Ohio overnight into Friday.

Noxious wildfire smoke that has engulfed New York City and other parts of the Northeast this week after drifting down from Canada was forecast to spread farther south on Thursday, enveloping millions more Americans in unusually dangerous pollution.

As about 150 fires burned in Quebec alone on Thursday, the pollution in the United States was expected to extend well beyond New York — to Boston, Philadelphia, Washington and other cities.

By Friday, the worst pollution is expected to move west, away from the Northeast, as a stagnant lower pressure system that has been sending the smoke southward this week changes direction, the National Weather Service said. But as long as the fires in Canada continue, it said, “the smoke may simply be directed towards other areas of the U.S.”

However, the further into the future you go, the less confident forecasters are in predicting the effects and density of the smoke.

That is partly because the high-resolution computer models get refreshed once an hour and are distributed a little less than a day in advance. Also, the models can’t predict how much smoke the wildfires will produce over the next several days.

This weekend, a storm system that has persisted off the Northeast all week and directed the smoke south should begin to move away, clearing the air in the region.