A severe heat wave is spreading across the US, placing about 44 million people under heat alerts and pushing triple-digit temperatures from the Southwest through the northern plains this weekend.The National Weather Service and AccuWeather say a heat dome will trap heat for a week or longer, producing overnight lows that stay above 80 F in some coastal cities and driving readings to 111 F in Las Vegas and into the low 40s Celsius in parts of Utah and Montana.Scientists link the event to fossil-fuel driven climate change and the National Oceanic and Atmospheric Administration projects an 81% chance that El Niño will be very strong by autumn, heightening wildfire risk, public-health strain and disruption at events including a World Cup quarterfinal in Miami.