Extreme Heat

Extreme Heat Information


Excessive heat in the City of McKinney is a very common occurrence for the summer months of June through September, but can be recorded as early as April and late into November. Extreme heat is generally defined as temperatures that hover 10 degrees or more above the average high temperature for the region and last for several weeks. Located near the urban Dallas / Fort Worth Metroplex, McKinney is at a greater risk from the effects of prolonged heat.
The Heat Index is what is known as the apparent temperature. It is a measure of how hot it actually feels when relative humidity is added to the actual temperature. Air quality conditions may also deteriorate due to trapped pollutants, such as ozone from the atmosphere or carbon monoxide from vehicles.

Watch Versus Warning


  • Excessive Heat Outlook: Issued when the potential exists for an excessive heat event in the next 3 to 7 days.
  • Excessive Heat Watch: Issued when conditions are favorable for an excessive heat event in the next 12 to 48 hours. A watch is used when the risk of an event has increased, but the timing and occurrence is still uncertain.
  • Excessive Heat Warning / Advisory: Issued when an excessive heat event is expected within the next 36 hours. These are used when an event is occurring, is imminent or has a very high probability of occurring. The warning is used for conditions posing a threat to life or property, while the advisory is used for less serious conditions that cause significant discomfort or inconvenience and if caution is not taken, could lead to a threat to life or property.
  • Ozone Health Advisory: Issued when ozone levels in outdoor air are expected to be greater than 0.08 parts per million when averaged over an eight-hour period.