UPDATE 2-Smoky haze parked over U.S. East Coast, with relief seen as days away

(Adds more details on conditions in Washington, D.C., event cancellations in paragraphs 4, 6-8, 15)

By Tyler Clifford

NEW YORK, June 8 (Reuters) - A smoky yellow haze generated by hundreds of Canadian wildfires hovered over a large swath of the United States on Thursday, causing breathing problems and flight disruptions and threatening to linger until the weekend and beyond.

The U.S. National Weather Service extended air quality alerts for another day for the East Coast from New England to South Carolina, as well as parts of the Midwest, including Ohio, Indiana and Michigan.

Health officials in more than a dozen states have warned millions of residents that spending time outdoors could cause respiratory issues due to the high levels of fine particulates in the atmosphere.

As the smoke pushed southward, conditions were expected to improve in the Northeast on Thursday while worsening for residents in Pennsylvania, Maryland and Washington, D.C. In New York, a faint smell of burning wood lingered, even as patches of blue opened up in the morning skies.

The haze and low visibility prompted aviation officials to halt incoming flights to major airports in New York and Philadelphia from the Northeast, Mid-Atlantic region and Ohio for a second day. All flights bound for the airport in Newark, New Jersey, a major New York-area airport, were delayed.

In Washington, the National Zoo was closed, while the Nationals baseball team postponed its afternoon home game. A live concert and an open air public movie screening scheduled for Thursday evening in Franklin Park was also canceled.

The White House also postponed its Pride Month event, which had been expected to be the largest such White House celebration for LGBTQ+ people in history. It was rescheduled for Saturday when the air quality was expected to improve.

Across the entire affected region, many schools once again called off outdoor activities, including sports practices and recess. And horse racing was scratched at a track on New York's Long Island that is set to host the prestigious Belmont Stakes this weekend.

It was the worst case of wildfire smoke blanketing the U.S. Northeast in more than 20 years, according to private forecasting service AccuWeather.

Smoky conditions are likely to persist until Sunday, when a new storm system shifts the direction of prevailing winds and brings a chance of rainfall in parts of the country nearing drought conditions, National Weather Service meteorologist Peter Mullinax said.

"We're finally going to start to see more relief by the time we hit the early part of next week when we start to see those southerly winds come in and push that smoke more farther north and out into the Atlantic," Mullinax said.

Canadian cities including Ottawa, Toronto and Montreal have also experienced smoke in recent days, as blazes rage in both the east and west of the country.

MOST POLLUTED CITY

With an "unhealthy" Air Quality Index reading of 178, New York City's air on Thursday was again more polluted than any major city in the world, topping cities such as Dhaka and Hanoi that are fixtures on the global bad-air list compiled by IQAir, a Swiss technology company.

The U.S. Air Quality Index (AQI) measures five major pollutants, including particulate matter produced by fires. The higher the reading, the more polluted the air. Readings over 100 are classified as "unhealthy" while those exceeding 300 are "hazardous."

Several readings over 300 were recorded in Washington on Thursday morning, prompting many people to wear n95 masks outside as a thick layer of white smoke veiled the capital. The smog, which reeked of ash, grew denser as the day wore on, erasing the top of the Washington Monument from view.

"This problem is likely to continue or worsen through Friday," Washington Mayor Muriel Bowser said in a tweet. "We urge residents and visitors to follow precautions."

Large areas of Michigan are also under red flag warnings due to dangerous fire weather conditions in both of the state's peninsulas, according to the weather agency.

Forest fires continued to burn across Canada on Thursday, sending more smoke across the U.S. border.

The country is enduring its worst-ever start to wildfire season. Thousands of Canadians have been forced from their homes and about 3.8 million hectares (9.4 million acres) have already burned, roughly 15 times the 10-year average, according to federal Minister of Emergency Preparedness Bill Blair.

(Reporting by Tyler Clifford Additional reporting by Kanishka Singh, Gabriella Borter, Trevor Hunnicutt and Brad Brooks Writing by Joseph Ax Editing by Jonathan Oatis and Frances Kerry)