The National Weather Service warns that a heat dome is pushing temperatures into the 90s and triple digits across the eastern US, exposing more than 250M Americans to dangerous heat and prompting Level 4 health alerts in some areas.Philadelphia canceled its America 250 parade and Amtrak canceled more than 20 Northeast trains as parades, fireworks and fairs are postponed, moved or called off and officials set up cooling and hydration stations to protect attendees.Climate scientists say the scale and intensity are virtually impossible without fossil fuel-driven warming, and forecasts of severe thunderstorms raise risks of flash flooding and storm damage on top of dangerously high temperatures.