More than half the US was under excessive heat warnings on Friday amid a brutal heatwave – which experts warned could become the “new normal” because of the climate crisis.

Forecasters warned of “very dangerous heat” as temperatures rose to 100F (37.7C) – particularly in urban areas where the mass of buildings creates “heat islands”. Alerts were in place in states as far reaching as New Mexico and Maine.

The worst of the heatwave is expected to be felt in Washington DC, Richmond, Virginia and Raleigh, North Carolina, where air temperatures of about 100F are predicted on Saturday and Sunday, with a heat index – which is closer to what the temperature actually feels like – of 110 to 115F (43.3 to 46.1C).

A heat “dome” of hot and humid weather is roasting the midwest. The high humidity makes it more difficult for the human body to cool itself down by sweating, because perspiration does not evaporate easily in the moist air.

Organisers of the New York City Triathlon cancelled Sunday’s event, a day after Bill de Blasio, the city mayor, urged them to do so out of concern for racers, spectators and staff. Many horse races are also cancelled. De Blasio announced the cancellation of Ozy Fest, a music and discussion event due for Central Park on Saturday and Sunday, and a moon landing anniversary celebration meant for Times Square.

The National Weather Service said 167 million people are under a heat advisory or excessive heat warning.

David Roth, a National Weather Service meteorologist, said: “It’s very dangerous heat. It’s not just how hot it’s going to get in the day – overnight temperatures are only falling into the mid 70s to low 80s. So the big problem, of course, is going to be urban areas because of the ‘heat islands’ that you get.”

De Blasio declared a “local emergency due to the extreme heat” and issued an executive order from 9am Friday to 11.59pm Sunday that orders tall office buildings to raise their thermostats to 78F to conserve energy, and encouraged residents to raise theirs.

Utility giant Con Edison said it has put 4,000 workers on extra shifts following last weekend’s power outage that plunged much of Manhattan into darkness.

June was the hottest month in history – a record that could be broken by July.

A new study by the Union of Concerned Scientists found the number of extremely hot days in the US could more than double as the impact of the climate crisis is felt.

Experts warned that heatwaves are likely to become common as global heating increases their strength and likelihood, with their impact felt by millions of Americans and by agriculture.

Deke Arndt, chief of the monitoring branch for National Centres for Environmental Information, said: “That’s what’s happening. And indeed, if that’s what plays out over this coming several days, that kind of reinforces that statistical pattern.”

He added: “The long-term increase in these dimensions of heatwaves is directly related to global warming.”