Without such timely warnings accompanied by effective local responses, extreme temperatures are more than uncomfortable.
Health threats posed by extreme heat are likely to grow as climate changes.
The IPCC points to several studies suggesting a consequent increase in mortality rates. Since then, the system has evolved and been applied to Washington, DC, and Phoenix, Arizona. If forecast 48 hours ahead of time, the system recommends issuing a health watch. With 24 hours before onset of the potentially dangerous conditions, the system advises that authorities issue a health alert. If the model predicts that the oppressive air mass threatens to elevate mortality rates above normal, the National Weather Service may decide to issue an excessive heat warning.