The National Weather Service has issued an excessive heat warning for the Phoenix area, with temperatures projected to hit 114 degrees Fahrenheit (45.5 Celsius).
The service says the warning runs from Thursday morning through Friday evening as the risk rises for potentially fatal heat-related illnesses like heat stroke and heat exhaustion for people spending time outside in the broiling sun. Afternoon hours will be especially dangerous.
Forecasters recommend that people stay home or seek shelter in air conditioned public places such as libraries, community centers, shopping malls, as well as special cooling and hydrating stations.
There were 155 heat-associated deaths in Phoenix's Maricopa County in 2017, the highest annual number ever recorded.
The weather agency uses a complicated formula to declare an excessive heat warning, including close to record-breaking temperatures.