I believe it took place in California and the heatwave was being caused by a comet or meteor. I remember the main guy was a doctor (but had stopped practicing for some reason) until he had to help in the makeshift hospital. The main woman had an ex-husband who was a mechanic who robbed a bank with another girl he worked with. Kidnapped his ex-wife and the doctor had to save her. They had to hide in the sewer system until the heat wave passed and it was safe to go back above ground. Hope someone can help -- I'd like to see this movie again.