For those curious, these are the criteria used in SE MI for determining whether extreme heat warrants a heat advisory or an extreme heat warning:
- Excessive Heat Watch – Issued when the Heat Index (HI) may be at least 105° within the next 2 to 3 days.
- Heat Advisory – Issued when the HI is expected to be at least 100° for 3 hours or more. Can be issued with lower criteria with successive days heat.
- Excessive Heat Warning – Issued when the HI is expected to be at least 105° for 3 hours or more. Can be issued with lower criteria with successive days heat.
Source: Heat Awareness Day 2012 information, which also includes some other useful heat-related information.
(Thanks to @ryanvis for pointing me to the relevant document)