Many people think that heatstroke is only a danger in more southern climates: Georgia, Alabama, Texas, Arizona.
The reality is that hot-car deaths happen even in mild weather.
Check out this map of where fatal cases of heatstroke in vehicles have occured this year so far:
Even on mild days of 70 degree weather, a closed car can very quickly heat up to fatal levels: there have been 22 deaths so far in an area of the country (NJ/NY/PA area) that most would consider moderate-climate.
See the interactive map here to read more of the individual stories.